a 1 1 1 ob o 3j,sMb tro | Algorithms for Building

Management and Control Systems —

Hot Deck/Cold Deck/Supply Air Reset,

Day/Night Setback, Ventilation

Purging, and Hot and Chilled Water

Reset

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Building Equipment Division

Washington, DC 20234

March 1984

Sponsored by:

Office of Buildings and Community Systems

U.S. Department of Energy

U.S. Navy Civil Engineering Laboratory

U.S. Department of Defense

,U56 ! k % 4 . |

34-23451

1334

NATIONAL BUREAU

OF STANDARDS

LIBRARY

NBSIR 84-2846

CONTROL ALGORITHMS FOR BUILDING

MANAGEMENT AND CONTROL SYSTEMS - HOT DECK/COLD DECK/SUPPLY AIR RESET,

DAY/NIGHT SETBACK, VENTILATION

PURGING, AND HOT AND CHILLED WATER

RESET

William B. May, Jr.

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Building Equipment Division

Washington, DC 20234

March 1984

Sponsored by:

Office of Buildings and Community Systems

U.S. Department of Energy

U.S. Navy Civil Engineering Laboratory

U.S. Department of Defense

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

ABSTRACT

Software is an important component of building management and control

systems (BMCS). This report describes concepts, algorithms, and software used

in BMCS components developed in the NBS building systems and controls

laboratory. The basic concepts, considerations and general algorithms for

hot deck/cold deck supply air setpoint reset, day/night thermostat and

ventilation setback, ventilation purging, and hot/chilled water supply

setpoint reset are presented. Reset is the changing of a setpoint on a Heating, Ventilating and Air Conditioning (HVAC) system controlled by a feedback controller to match the system output to the system load. Setback is

the changing of HVAC system operation to reduce energy use during unoccupied

periods. Purging is the use of outdoor air during unoccupied periods to reduce

mechanical conditioning requirements. Specific implementations of the

algorithms in software on an actual BMCS are presented as examples.

KEY WORDS: air handling unit control; building (energy) management and control

systems (EMCS.BMCS); chilled water reset; computer control; control

algorithms; control software; day/night setback; energy management; hot water

reset; heating, ventilating and air conditioning (HVAC); outside air reset;

supply air setpoint reset; ventilation purging.

iii

TABLE OF CONTENTS

Page

1 . INTRODUCTION .. 1

2. THE AIR HANDLING UNIT AND BUILDING ZONE SYSTEM 2

3. SUPPLY AIR SETPOINT RESET CONTROL 5 3.1 Reset Algorithm for Single Path Systems Based on Outside

Air Temperature 5 3.2 Reset Algorithm for Dual Path Systems Based on Outside Air

Temperature 11

3.3 Setpoint Reset Algorithms based on Zone Requirements 12

4. DAY/NIGHT SETBACK 20

4.1 Local Space Equipment 20

4.2 Space Temperature and Ventilation Setback 22

4.3 Day/Night Setback Actions 23

4.4 Basic Setback Algorithm. 25

5. VENTILATION PURGING 27

5.1 Basic Concepts.... 27

5.2 Basic Algorithm for Ventilation Purge Control 30

5.3 Ventilation Purging Algorithm Components 30

5.4 Ventilation Purge Algorithm Timing. 35

6. HOT WATER AND CHILLED WATER SUPPLY TEMPERATURE RESET 36

6.1 Outside Air Temperature Supply Water Reset 36

6.2 Demand Supply Water Temperature Reset 36

6.3 Possible Demand Supply Water Reset Load Determination Algorithm.. 38

6.4 Hot Water Reset Setpoint Selection Algorithm.... 40

6.5 Chilled Water Reset Setpoint Selection Algorithm 42

7. EXAMPLES OF ACTUAL CONTROL ALGORITHMS 43

7.1 Example Field Interface Device Software Architecture 43

7.2 Basic Air Handling Unit Control Task 45

7.3 Example Outside Air Supply Air Reset Algorithm 46

7.4 Example Zone Demand Supply Air Reset Algorithm.. 48

7.5 Example of Day/Night Setback Algorithms 51

7.6 Example Ventilation Purging Algorithm 52

8. CONCLUSIONS 56

9. REFERENCES 57

APPENDIX A - Outside Air Supply Air Reset Control Routines 58

APPENDIX B - Zone Demand Supply Air Reset Control Routines for

Constant Volume Reheat System 60

APPENDIX C - Day/Night Setback Control Routines 6 2 APPENDIX D - Ventilation Purging Control Routines 6 5

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LIST OF FIGURES

Dual duct air handling unit and multiple zone system

Steady-state room temperature versus outside air

temperature for various supply air temperatures

Required supply air setpoint temperatures versus outside air

temperature for winter and summer room temperatures

Supply air temperature versus outside air temperature for two

extreme zones served by an air handling unit

Supply air temperature versus outside air temperature for an an air handling unit....'

Outside air reset algorithm for supply air temperature setpoint

in single path (deck) air handling systems

Supply air temperature versus outside air temperature for two

decks of a dual path system before application of limits

Block diagram of feedback system for supply air reset based on zone requirements

Generalized zone heating equipment output versus supply air

setpoint for a given room condition

Supply air temperature setpoint reset algorithm using feedback

from measurement of local zone conditioning requirements

Algorithm for day/night setback from normal to setback state....

Day/night setback algorithm for return to normal state from

setback.

Space, supply and outside air temperatures during an unoccupied

period with ventilation purging

Basic ventilation purging algorithm

Algorithm to determine if the building space requires cooling

for the ventilation purging algorithm

Algorithm to determine if outside air can provide cooling used

for the ventilation purging algorithm

vi
17 Algorithm to determine current load of air handling unit coil

for supply water reset 39

18 Algorithm to select supply water setpoint temperature for

supply water reset 41

19 Example of outside air supply air reset schedule specified by

three points 48

LIST OF TABLES

Number Page

1 Supply air reset criteria for various system types 15

2 Space zone thermostat types 21

3 Possible information transmitted between parts of a supply water

reset algorithm 37

4 NBS FID task table 44

5 NBS FID outside air supply air reset parameter table 47

6 NBS FID zone demand supply air reset parameter table 50

7 Parameters used by the ventilation purging algorithm 54

vii

1. INTRODUCTION

A computer-based building management and control system (BMCS) relies heavily

on computer software to utilize efficiently the heating, ventilating, and air

conditioning (HVAC) equipment in the building. HVAC control software is

available in proprietary or system dependent packages, usually supplied with a BMCS system supplied by a particular vendor. However, if listings of the

software source are not supplied, the BMCS owner or HVAC designer will not

know if the BMCS system can meet the HVAC design specifications. Even if the

control algorithms used are known, there is only a small amount of HVAC

control algorithm software in the public domain to use for baseline comparison

purposes. This report describes basic concepts and non-proprietary algorithms

used in control software that was developed at the National Bureau of

Standards (NBS) for use in the NBS building management and controls

laboratory

.

BMCS control strategies fall into at least two categories. Direct control (or

Direct Digital Control, DDC) refers to strategies and algorithms that control

the building equipment directly without the use of conventional pneumatic or electronic local analog control, implementing lower level functions such as closed loop control of valves, dampers, and actuators. Supervisory or management strategies control the building in a broader sense, managing

equipment systems by methods such as selecting setpoints, and choosing optimum

operating times as a function of variables including electrical demand, time,

weather conditions, occupancy, and heating and cooling requirements.

The many functions of modern building management and control systems are

usually distributed among a number of computer processors. Control algorithms

are usually intended for operation at a particular level of the system. For a large BMCS, the tri-services specification [1] defines four levels in a hierarchy. These are the central level (CCU), the communication controller

level (CCC), the distributed control level (FID), and the data gathering and

conditioning level(MUX). The distributed control level is occupied by

equipment given the name FID for field interface device, or RPU for remote

processing unit. It is at this level that the software described in this

report is intended to operate.

This report discusses several types of supervisory control strategies. These

are day/night thermostat and ventilation setback, ventilation purging, hot or chilled water supply temperature setpoint reset, multizone/dual duct hot

deck/cold deck setpoint reset, and reheat/VAV supply air reset. These are all

strategies that may be applied to control of air handling units serving

building core or perimeter spaces in commercial or industrial buildings.

Algorithms for these strategies are discussed in several chapters of thus

report, and actual algorithms and software developed for the NBS controls

laboratory BMCS system are presented in a separate chapter.

1
2. THE AIR HANDLING UNIT AND BUILDING ZONE SYSTEM

The strategies discussed in this report are primarily intended for control of

HVAC systems that utilize central air handling units to supply ventilation

and/or conditioned air to the spaces in a building. Although day/night

setback is applicable in any type of building, and supply water reset is

applicable to any building with central boilers or chillers, all control

strategies are discussed in the context of a building with one or more central

air handling units.

An air handling unit can be one of several different types. The types of

units currently in use will be described to define the terminology used in

this report. One type of unit, the dual duct system, is shown in Figure 1 with much of the detail of an actual air handler omitted. Two ducts allow air

to enter the unit, one carrying return air from the building spaces, the other

bringing in outside air for ventilation or ’free” conditioning. Dampers

control the relative amounts of each air source. The dual duct unit has two

paths or "decks" containing heat exchangers, and two supply ducts leading from

the air handler to one or more building zones. The heat exchangers in each

deck are either for heating or cooling, and a preheat coil used to avoid

freezing of the other coils is shown just after the return-outside air mixing

area. The cooling coils might be chilled water coils, refrigerant or direct

expansion (DX) coils, or air washers. The heating coils could be steam or hot

water heat exchangers. Two coils may be used in each deck to allow optional

humidity control by overcooling to produce condensation, and reheating. In

actual installations there may be more, fewer, or none of these coils in a deck. The upper deck is called the "hot deck", and is intended to produce

warm air for the building spaces. The lower deck is called the "cold deck"

and is intended to provide cool air. In a dual duct system air from hot and

cold decks is sent through separate ducts to building spaces and is mixed at

the space to maintain a desired room temperature.

Damper Return air fan

Relief ^ air ^

Return air

figure 1. dual duct air handling unit and multiple zone system

2
The dual duct air handler design can be converted to other types of air

handlers by omitting one of the supply ducts or one of the decks. If the

mixing for each zone is performed at the air handling unit and the mixed air

is transported to each zone in a single duct* the system becomes a multizone

system. If the cold deck is eliminated, so that only a single deck with a supply fan is used, this would represent a single or multiple zone constant

volume system, or, if the fans and zone supply boxes could vary the volume of

air delivered to the zones, this configuration would be a variable air volume

system (VAV). If reheat coils were added to the individual zones, the system

would be a reheat system, either VAV or constant volume. Complete

descriptions of system types are available in the ASHRAE handbook [2].

In the central air handler and multiple building zone system, there are two

important types of control or process variables which must be maintained at

desired values. The first of these is the space temperature (and humidity) in

the zones. The other variable is the temperature (and humidity) of the air

stream or streams leaving the air handling unit. For a dual deck system,

there is a cold deck setpoint temperature and a hot deck setpoint. For

systems with a single deck such as reheat, single zone, or single deck VAV,

there is a single supply air temperature.

The primary criterion for selection of the zone temperature setpoint is the

comfort of the building occupants, or if the building is not occupied, the

requirements of materials or machines located in the building. The zone

setpoint is often not a single temperature/humidity but a range of

temperatures and humidities, within which the comfort level is considered

acceptable. This is the "zero energy band' concept which is useful for

conserving energy when space loads swing from heating to cooling within a relatively short time span [3]. If the space condition is floating within the

comfort band, no mechanical conditioning of the space is permitted.

The selection of supply air setpoint(s) is more complicated than the selection

of a space setpoint. If a single zone is being supplied by an air handler,

the supply air setpoint is the temperature (and humidity) of the supply air

which will allow the space temperature (and humidity) for that zone to be

maintained. This air condition is determined by the space heating or cooling

requirement and the air volume flow rate entering the zone. If two or more

zones are being supplied, it is to be expected that the proper entering air

condition will vary among the zones. A building may be designed to have equal

entering conditions for all zones, even though zone requirements differ, by

adjusting the design air flow to each zone. The design air flow distribution

is probably not ideal for all zone loading conditions, resulting in unequal

entering air condition requirements at certain times. The supply air condition

must then be chosen to allow the zone with the highest or lowest entering air

temperature and humidity requirement (depending on the specific system) to be

satisfied, because otherwise some zones will not be able to maintain a comfortable state.

The simplest control of supply air temperature is to use a constant hot deck,

cold deck, or supply air temperature so that all zones are satisfied for all

reasonable load conditions. For dual deck systems, mixing of hot and cold air

streams, and for reheat systems, reheating of cool air at the zone, will allow

any zone to be supplied with air at the proper temperature. However, when air

3
is cooled and reheated, this uses more energy than if the air had simply been

cooled or heated to the correct tempeature for a zone. Therefore it is

important to select the proper supply air temperatures to minimize energy

waste

.

The least complex control system for a central air handler and zone system

uses independent feedback control loops to maintain each zone temperature (by

reheat coil control, or mixing damper control), and an independent system to

maintain supply air temperatures. In this case the zone and supply air

controls are loosely coupled, since neither control loop is connected to the

other. An example of this is a system with "outside air reset" control of

supply air temperature. The selection of supply air setpoints is based on the

outside air temperature, not the actual space conditions. With tightly

coupled controls, the system controlling supply air temperatures takes into

account the actual conditions in the space zones. An example of this is the

selection of supply air setpoints based on actual measurement of individual

zone loads.

Two of the strategies discussed in this report control the setpoints for the zone and supply air temperatures. Day/Night thermostat setback changes the

zone setpoints. Hot deck/cold deck/supply air reset changes the supply air

setpoints. The other strategies discussed in the report are concerned with

two other types of process or control variables. These are the ventilation

air rate and the hot or chilled water supply temperatures for heat exchanger

coils. Day/night ventilation setback changes the setpoint for minimum

ventilation air rate. Hot water/chilled water reset changes the setpoint for

the hot or chilled water supplied to the coils in the air handling unit. The

remaining strategy, ventilation purging, makes a temporary change in the

ventilation air setpoint to improve building operation economy.

4
3. SUPPLY AIR SETPOINT RESET CONTROL

Throughout this chapter, supply air setpoint will be assumed to be either a dry bulb temperature setpoint, or a combination of dry bulb and humidity (wet

bulb) setpoints. If only the dry bulb temperature is controlled, under

certain conditions the space humidity may approach an unacceptable state if

the dry bulb setpoint is reset to a high value during cooling operation. If

this is possible, it is desirable to have a humidity override included in a setpoint reset algorithm. A humidity override would use data from one or more

humidity or wet bulb temperature sensors in the supply air stream or the

building zones. If the humidity level became unacceptable, further reset of

the supply air temperature would be prevented.

Depending on the specific system, an air handling unit may have one or two

supply air conditions that must be maintained under closed loop control. In

simple VAV or reheat systems there will be one supply air condition which is

selected to match the space loading. For multizone or dual duct systems,

there is a hot deck and a cold deck supply air setpoint. These must be

selected both to match the space loading in the zones supplied, and to

minimize the difference between the two supply conditions to avoid excessive

reheating and recooling.

The use of fixed design setpoints for air supply temperatures is an easy

approach, but can result in energy loss through reheating or recooling of air,

either by mixing of hot and cold deck air streams which are widely separated

in temperature, or by terminal reheat of excessively cold supply air. Since

conditions change throughout the year and even during the day, the energy lost

will vary with time.

Reset is defined as a method of changing the setpoint of a system to

minimize system energy consumption by matching system output to the system

load. Reset control methods may be categorized by the means used to select a setpoint temperature for different load conditions. Some control algorithms

might use outdoor air temperature as a load indicator, while others might

directly or indirectly measure actual space heating and cooling requirements.

3.1 Reset Algorithm for Single Path Systems Based on Outside Air Temperature

The least complicated method of supply air reset is outside air temperature

reset. The basis of this method is the assumption that the heating or cooling

requirements of a zone can be considered a function of a single variable, the

outside air temperature. In reality, zone requirements are dependent on other

factors such as solar gain, wind conditions which affect air infiltration

rates, internal heat generation rates, and latent heat loads. However, the

outside air reset method is not intended to exactly match supply air setpoints

to system requirements. The reset schedule used is an estimate based on

calculations for the best setpoint temperature at various design conditions

where solar gain, infiltration, and internal loads are specified in addition

to outside air temperature. If the design conditions selected are an accurate

estimate of the conditions encountered in actual operation, then the outside

air reset method will yield acceptable results, and savings over the use ot a fixed setpoint.

5
If the infiltration, internal heat generation rates, and maximum solar gains

are relatively constant in a given zone from day to day, the required dry bulb

temperature for air entering the zone to maintain zone conditions can be

assumed to be approximately a linear function of outside air temperature. The

curves in figure 2 were generated using a simple model of a zone with an

internal heat source, a gain or loss to the outside air, and a constant volume

supply air source with a variable room entering dry bulb temperature. Latent

loads were not included. A more complicated zone model could be used to

generate more accurate curves.

figure 2. Steady-state room temperature versus outside air temperature for

various supply air temperatures.

Figure 2 shows that for a fixed supply air temperature, the steady state room

temperature will vary with outside temperature, as would be expected. The

curves for different supply temperatures can be considered to be parallel,

indicating that figure 2 can be described by:

T

room A * Toa + B * T

ga + C. ( 1 ) where A,B, and C are constants, T Qa is outside air temperature, and T

ga

is

supply air temperature. The value of the constants will change with the

characteristics of the zone. If the zone were highly insulated, the curves in

figure 2 would be nearly horizontal. If the internal heat generation were

increased, all of the curves in the figure would be translated upward. If

equation (1) is solved for the supply air setpoint temperature:

X =d*t +E*T + F, sa room oa * ( 2 ) where D,E, and F are constants derived from A,B, and C. If a constant room

temperature is desired, the supply air temperature is a linear function of

outside temperature. If different room temperatures are required for cooling

and heating seasons, the supply air temperature can be described by one

equation for T

ga

versus T Qa for outside temperatures above a balance

outside air temperature (summer) and another equation for conditions below the

balance (winter). An example of summer and winter curves is shown in figure

3. Obviously, if two room temperatures are used, at some point during the

year there must be a switch from winter to summer, or summer to winter

operation, since the two lines in figure 3 have no common point. The switch

from winter to summer operation would ideally occur when the cost of providing

air at the summer setpoint is less than the cost of supplying air at the

winter setpoint.

6
LU £. OC

>- o <

-J CL DC Q-hUJ

9- UJ QH

CO

A

Winter

(Low room

temp.)

Summer

(High room

temp.)

(Balance)

OUTSIDE AIR TEMPERATURE

figure 3. Required supply air setpoint temperatures versus outside air

temperature for winter and summer room temperatures.

If the building zones are under zero energy band control, then the space

temperature will be allowed to vary from the winter setpoint to the summer

setpoint throughout the year. When it is cold outside, the space temperature

will move to the winter setpoint. When it is hot outside, the space

temperature will then move to the summer setpoint. When outside conditions

are moderate, the space temperature will be somewhere between the summer and

winter setpoints. This might be represented by the dashed line on figure 3,

between the two setpoint lines, which would prevent the abrupt change from

the winter to the summer setpoint at the balance temperature.

It will be assumed that, for either the heating or cooling season, one

unbroken curve will allow selection of the room entering air temperature for

any outside air temperature. There are two additional factors which

complicate the situation. The first factor is that due to air temperature

rise through the supply fan, duct leakage, and thermal losses and gains, the

temperature entering the air handling unit supply fan will not be the same as the air entering the zone. The sensor used to control supply air temperature

is usually placed at the supply fan inlet, while the temperature of the air

entering the zone is usually used in zone calculations. Therefore the actual

supply air temperature used will be different than the desired zone inlet

temperature. The change in air temperature can be estimated using standard

methods [2].

A second factor complicating outside air reset is that unless the system is a single zone system, each zone will have a different equation for desired

supply air temperature versus outside air temperature. The less well designed

the system, the more diversity there will be between different zones served by

the same air handler. Figure 4 an illustrates an extreme case of this

situation. In this figure, two curves are shown, representing two zones out

of several supplied by an air handler. One of the zones, 1, might be an

interior zone, well insulated from the outside air. It requires a relatively

constant supply air temperature. The other zone, 2, might be a perimeter

zone, coupled to the outside air temperature. It requires a lower supply

temperature in warmer weather than in cooler weather.

7
figure 4. Supply air temperature versus outside air temperature for two

extreme zones served by an air handling unit.

Deciding on which curve from figure 4 to use for supply setpoint selection is

somewhat dependent on the system type. For a single path (or deck) system*

the system type is either terminal reheat or VAV. For a reheat system, the

lowest supply air temperature should be selected, since air to the other zones

can be reheated. This would indicate that curve 1 in figure 4 should be used

at lower outside temperatures and curve 2 should be used for higher outside

temperatures. For a VAV system without reheat, however, the situation is

different. In the summer, the setpoint should be selected to satisfy the zone with the highest percentage of maximum zone cooling load, since the air to zones with lower percentages of maximum load can be throttled. In the winter,

the setpoint should be selected to satisfy the zone with the highest percent

of maximum heating load, since the air to zones with lesser loads can be

throttled

.

When reset is applied to supply air setpoints, the use of setpoints which are

too high or too low should be avoided. In summer, a setpoint which is too low

may cause condensation problems or cold drafts. A summer setpoint which is

too high under humid conditions may not reduce humidity sufficiently to

provide comfort. In such a case it might be desirable to have even the zone

of greatest cooling load require some reheat or mixing of hot and cold air to

reduce humidity. Alternatively, if comfort is not of primary importance, it

may be desirable to use a supply temperature which will not satisfy the worst

zone, thus causing some occupant discomfort in this zone, in order to minimize

reheating in other zones.

For a VAV system without reheat, the use of supply air reset may not result in

a net reduction of energy cost. Setpoint reset in VAV systems without reheat

would only save some energy used by chillers and boilers in the plant, but not

energy wasted by reheating or mixing of air streams, since these effects do

not occur with a VAV system. However, in a VAV system with reset, more fan

energy might be used to condition a space with a reset setpoint if the system

is equipped with supply fan air volume reduction equipment, since with a reset

supply temperature the fan must run at higher volume and consume more,

electrical power, given the same space load.

Based on the previous observations, it is assumed that a plot of supply air

setpoint temperature versus outside air temperature can be drawn using four

straight lines, as in figure 5. This can be done by plotting four curves for

8
the zones with extreme loads* for winter and for summer, as in figures 3 and

4, and using rules for the system type, as described above, to draw a single

curve which replaces the four curves. Alternatively, separate curves could be

developed for winter and summer, and a manual change from the use of one curve

to the other could be used.

figure 5. Supply air temperature versus outside air temperature for an an air

handl ing unit.

At high or low outside air temperatures, the setpoint would be constant to

avoid extremes of temperature. The two middle regions of the curve might have

different slopes, to allow use of the lowest curve like those in figure 4.

The selection of the specific curves for any system must be performed by a person familiar with the building design. A program used at the central level

could be used to assist a building operator in the selection of the proper

values for a particular building. The points describing curves like those in

figure 5 could then be used by the FID software during periodic execution of a supply air reset algorithm.

A general algorithm to perform outside air reset is shown in figure 6. This

algorithm is intended to implement the reset schedule of figure 5. The

outside air temperature is determined from an analog input point to the BMCS

system. There may be several setpoints which are being reset by the same

algorithm. Any or all of these setpoints may be designated as fixed if, for

example, the building operator wants to hold the setpoint for a particular air

handler constant, with no reset. If the setpoint is fixed, no reset is

performed. If the reset is to be performed either a table or equation is used

to determine the setpoint temperature. The equation or table is called the

reset schedule. It is possible and may be desirable to have a different reset

schedule for each setpoint to be reset. Once the new setpoint is determined

from the schedule, it is substituted for the old setpoint. If there are more setpoints to reset, the same steps are repeated for the other setpoints.

Otherwise an exit is made from the setpoint reset algorithm. The algorithm i 1 intended to be periodically executed at some interval, which could be anywher*

from once per day to once every minute.

The drawback to the use of an outside air supply air reset algorithm is that

it is essentially open loop control, without feedback to the control sy: t. -i i allow the system to sense whether all zones are being adequately condit icin'C

without waste. The success of outside air reset depends heavily on t h • correct determination of the reset schedule, which is in turn based on prop*

selection of design conditions and estimation of the response oi th< i i’ n

9
zones at those conditions. It also necessary to change the reset schedule

when there is a change in season. The advantage to outside air reset is that

it is easy to implement in software* and requires only an outdoor air sensor.

An example of an actual outside air reset algorithm implemented in a real BMCS

system for a constant volume reheat air handling system is described in

section 7.2. Listing of the software is given in Appendix A.

(

BEGIK )

determine oi tempei

itside air

rature

use schedule (equation or table) to determine supply

air setpoint

figure 6. Outside air reset algorithm for supply air temperature setpoint in

single path(deck) air handling systems.

10
3.2 Reset Algorithm fox Dual Path S ystems Based on Q.p£.sjLdg idx. Tempera

When an air handling system is of the dual path or dual deck type (multizone

or dual duct), there are two supply air setpoints be concerned with. One air

handler path supplies cool air with a setpoint lower than the other path,

which supplies warm air. It is possible for each setpoint to be reset

independently of the other, or one to be reset while the other is held fixed.

The reason for having two decks in a dual path system is to allow diversity of

loads in the multiple zones served by a dual duct or multizone air handling

unit. If all zones had identical loads, they could all be supplied with air

at the same temperature and the same flow rate. In such a limiting case, both

decks of the air handler would have the same setpoint. If a building is

designed with zones of different heating and cooling requirements, the same supply air setpoints may be used for all zones by using different air flow

rates for different zones. However, the heating and cooling requirements of

the zones will diverge from the design values when internal, solar,

transmission, and air infiltration loads become different from assumed design

values. Under these circumstances the hot and cold deck setpoints must be

different

.

Ideally, at least one zone with a cooling load should be using 100 percent

cold air from the cold deck and no warm air from the hot deck. Similarly, at

least one zone with a heating load should be using 100 percent warm air and no cold air. In dual path systems without any supply air setpoint reset,

however, the setpoints for the decks are usually fixed throughout the year or the season, resulting in conditions where all zones are using a mixture of

warm and cold air. This situation results in energy waste due to reheating or recooling of air.

The basic strategy for dual path supply air reset is to set the hot deck

temperature to cause a particular zone, the one which currently has the hot

deck air making up the greatest percentage of the total air flow into the

zone, to use only a small amount of cold deck air. Therefore the hot deck

temperature will be the entering air temperature required to keep this zone at

a comfortable temperature. Similarly, the cold deck temperature is set to cause a different zone, the one which currently has the cold deck air making

up the greatest percentage of the total air flow into the zone, to use only a small amount of hot deck air. The cold deck temperature is then the proper

entering air temperature for th'is zone. The same assumptions apply to selection of a proper inlet temperature to a zone as were made in the case of single path systems in section 3.1.

Basically zones are assumed to require a supply temperature which is a linear

function of outside air temperature in order to maintain a desired room

temperature without reheating or recooling supply air. Curves of supply

temperature versus outside air temperature can be drawn as in figure 4. For .i single path system, a single curve was chosen which fit the syster.,

requirements. For a dual path system, two curves must be chosen, one for th.

hot deck and one for the cold deck. The simplest solution is to select the

hot deck curve to follow the highest points of the two curves in figure 4,

to select the cold deck curve to follow the lowest points of the curvi

figure 4. This is illustrated in figure 7.

11
figure 7. Supply air temperature versus outside air temperature for two

decks of a dual path system before application of limits.

As with single path systems, there are limits to deck setpoints when they are reset. The same restrictions apply as with single path systems with the

additional problem that a hot deck setpoint may be set to a temperature

colder than the equipment in the deck will allow, or a cold deck setpoint may

be set to a temperature which is warmer than the equipment will allow. An

example of this is a case where there is no heating apparatus in the cold deck

and a setpoint above the air temperature entering the cold deck is specified.

As discussed in section 3.1, the outside air supply air reset method does not

provide feedback to the control system as to the correctness of the setpoint

choice. An example of this is that in dual path systems, if an incorrect

schedule produces excessively low cold and hot deck setpoints, the building

will become too cold and perhaps be using excessive mechanical cooling. If

reheat coils exist, reheating will keep the building comfortable, but cause energy waste.

The schedule depicted in figure 7 must be modified to provide limits to the

deck setpoints, in the same way that the schedule in figure 5 for single path

systems was created by applying limits to the schedule derived from figure 4.

The outside air supply air reset algorithm in figure 6 can be used for dual

path systems, if the cold deck and hot deck setpoints are treated as separate

setpoints with individual setpoint reset schedules. As with the single path

systems, these schedules must be changed with the seasons, if a change in room temperature is required when a new season begins.

3.3 Setpoint Reset Algorithms based aa Zone Requ jrepeht?

Use of outside air temperaure to reset supply air setpoints has the inherent

drawback that a simplified model of the HVAC system is used to represent the

system and control actions are made based on the predicted response of the

model rather than the actual response of the system. The control is open

loop, without feedback. The alternative to the outside air reset method is to

actually measure the requirements of the zones served by the air handling

unit, and reset the setpoint until the zones with the worst loads are just

satisfied. This results in closed loop control since there is a feedback loop

from measurements at the local zone temperature control equipment to the

controller determining the setback of supply air temperature. Figure 8 is a

12
schematic block diagram showing a possible configuration of the complete

system. An existing inner feedback loop controls the supply air based on a setpoint input. This setpoint input is based on the zone equipment output.

This diagram implies that the system is continous in time, where in an actual

system the setpoint would be changed only at discrete time intervals and held

at the new value until changed again (a ’sample and hold’).

minimum

local zone

figure 8. Block diagram of feedback system for supply air reset based on zone requirements

There are two separate components to an algorithm for supply air reset on zone demand. First, the method of creating a feedback loop from the local zones to

the setpoint controller must be decided upon, and the nature of the

information to be determined from the feedback must be decided. Once the

feedback loop has been arranged, a method must be decided upon for determining

what the correct setpoint should be based on the feedback information.

There are three possible methods of determining the actual requirements of

zones. All are based on instrumenting the building zones to some extent and

this is a disadvantage of using a zone demand supply air reset algorithm.

Outside air reset requires only the use of an outside temperature sensor. A

complete implementation of load demand reset would involve at least one sensor

in each zone. It should also be possible to select representative zones for

instrumentation, taking into account the possible shifting of large loads

between the zones of a building during the day or year. The sensor to be used

in the zones depends on what type of local equipment and controls are used to

control zone temperature. At this time, it is assumed that the local controls

are not digitally controlled by the same controller that is controlling the

air handler supply air temperature. The local controls will probably be some

type of thermostat, either mechanical, electronic, or possibly digital.

Eventually, most control systems may utilize some sort of microprocessor

controlled thermostat with digital communication capabilities, or the local

control may be taken over by the FID controller.

Local control mechanisms will either use on-off control, modulating control,

or be non-existent. If local control is of the on-off type, a switch closure

sensor can be installed in the zones to monitor the status of the ono::

control. If this sensor is sampled by the controller, an approximate

summation of the total on-time of the local conditioning equipment can be

obtained. An example of this might be a thermal switch on a reheat coil or a contact on a wall thermostat.

13
If local control is of the modulating type, such as a VAV damper, modulating

reheat valve, or dual duct mixing damper, the local sensor should be able to

measure the position of the actuator. This sensor could be a potentiometer or similar device attached to the valve or damper, producing a change in

electrical resistance with movement of the valve or damper. The measurement

of damper position could be either averaged over a time period, sampled for a peak damper position, or used as a discrete measurement of instantaneous local

equipment output.

If local control does not exist, or attachment of sensors to the local

equipment is not feasible, an alternative is to directly measure the space

temperature (and humidity) in each zone. In any zone there will be

oscillations in temperature as conditioning equipment is modulated or turned

on or off, and these oscillations can be monitored with temperature sensors.

If the supply air temperature to the zone is exactly matched to the space

requirements, the temperature oscillations will be minimal. If the the supply

air temperature is incorrect, however, the local temperature will oscillate,

if local conditioning equipment is active, or drift outside of allowable

limits, if there is no local control of conditioning equipment. This

information, then, can be used to alter, if necessary, the supply air

setpoint

.

When the feedback loop from the local zones has been implemented, information

about heating and cooling in all instrumented local zones will be available.

This information will be in the form of a value such as a total on-time of

local conditioning equipment, or an instantaneous, average or peak value for

local equipment output, or a value for space temperature oscillation frequency

or rate of change. Some method must then be applied to use this information

to determine the proper setpoint for the supply air.

The control of the supply air setpoint at a particular time is usually based

on information from one of the zones served by the air handler. Which zone

this is will probably vary seasonally or even from hour to hour due to changes

in solar and internal gains, and will differ from system to system. Table 1 summarizes types of air handler systems, the zone used as a basis for reset,

and a general method of selecting a setpoint. The basis for selecting the

critical zone is not the absolute zone heating or cooling load, but is the

ratio of the energy currently removed or delivered at a zone to the the total

possible cooling or heating capacity available for a zone. This will be called

the greatest percentage of maximum capacity. For a reheat system the zone with

the largest percentage of maximum capacity will be the zone with the least use

of local reheat. For dual path systems, the critical zone is the one with the

greatest percentage of maximum heating capacity when setting hot deck supply

temperature and the zone with the greatest percentage of cooling capacity when

setting cold deck supply temperature.

For VAV systems, the situation is more complex. To minimize fan usage in a VAV system, the setpoint in the cooling season should be lowered, and based on the damper position in the zone of the smallest percentage of the maximum

supply air flow. However, t q minimize chiller energy in the cooling season,

higher setpoints are required, and selection is based on the zone with the

greatest percentage of maximum supply air flow. The reverse is true for the

heating season. If local heating and cooling equipment is available, the

14
selection of the zone is complicated by factors such as whether some zones are cooling and others are heating, and the cost of providing heating or cooling.

TABLE 1. Supply air reset criteria for various system types

system type I zone choice I method

+ +

1. Mix hot and cold air | I a. hot deck I Largest hot deck | Lower setpoint if hot deck damper

Idamper opening |not fully open. Raise if full open.

I I b. cold deck |Max. cold deck iRaise setpoint if cold deck damper

Idamper opening I not fully open. Lower if full open.

I I 2. Reheat ISmallest amount IRaise setpoint to minimize reheat.

I of reheat(small |Lower setpoint if reheat almost

1% of max. cap.) I never on.

I I 3 . Packaged or unitary | I equip. and central air! I a. all zones heating Ismallest % of IRaise setpoint to minimize heating.

Imax. local heat I

b. all zones cooling Ismallest % of |Lower setpoint

Imax . local cool . I to minimize cooling.

4.

c. Some zones cooling | Smallest % of IRaise setpoint to minimize heating

Some zones heating Imax cooling if | if winter. Lower setpoint to

Iwinter, smallest Iminimize cooling if summer. Or

1% of max. heat ladjust setpoint to minimize

I if summer, (or (expensive local conditioning.

I zone with most I

I expensive local I

I conditioning) |

I I VAV (non- dumping) | | a. Minimum fan energy | lowest % of max | lower setpoint to minimize air flow

I supply air flow I if cooling or raise setpoint to

Iminimize air flow if heating.

I I b. Minimum supply airlLargest % of maxlraise setpoint to maximize air flow

Conditioning | supply air flow I if cooling or lower setpoint to

Imaximize air flow if heating

Table 1 gives strategies for determining how to correct the current setpoini

to minimize energy waste. For systems with local zone equipment which can

only reheat the incoming air, the strategy is simply to minimize reheating <>l air which has been overcooled. In systems with the capability to both locally

heat and cool air entering the zones, the strategies are complicat'd bv

factors such as whether the zone has a cooling or heating load, how expensive

15
it is to lower or raise the setpoint of the supply air, and whether the zone

temperature is allowed to float between limits (zero energy band). Two levels

of strategies will be discussed. One level will assume that the information

from a selected zone is sufficient to determine a new setpoint. This will be

adequate for many systems with simple zone controls without zero energy band

control. A second level is to include other factors and zero energy band zone

control. A general algorithm for the lower level will be developed first.

The relationship between the supply air setpoint and the information from a zone currently being used to determine the supply air setpoint can usually be

plotted in the form shown in figure 9. Figure 9 is an example for a zone with

a reheat coil. The important point about this figure is that when the supply

air setpoint is below the best value to maintain the zone condition, the zone equipment will have some measurable average output as an attempt is made to

reheat the air to a better temperature. As the supply air setpoint approaches

the best value for this room condition, the zone equipment output will

approach zero. If the supply air setpoint is made too high, the zone

equipment output will remain at zero. However, the zone will probably be

uncomfortably warm. When the supply air setpoint is too low, the reheat coil

will remain on. Continuous running of the reheat coil indicates that the room

is too cold, but gives no indication of the error in the supply air setpoint.

For a simple system with local reheat and a constant space temperature, a practical approach to control of supply air setpoint is to assume that the

best minimum value for zone equipment output is at some level slightly greater

than zero, and to attempt control of the supply air setpoint to hold the zone equipment output at this level. If the setpoint is controlled to keep zone

equipment output at zero, and the relationship of figure 9 is applicable, it

is possible to determine that the setpoint is too low (local equipment on),

but not if it is too high. By selecting the desired value of zone equipment

output to be a small positive value, indicated as the control point in figure

9, then if the zone equipment output is zero, the supply air setpoint is known

to be too high.

figure 9. Generalized zone heating equipment output versus supply air setpoint

for a given room condition.

16
If the zone equipment is under modulating control and has both local heating

and cooling capability, such as with heat pumps, an output versus supply air

setpoint curve for this zone would have a similar shape compared to the curve

in figure 9, but instead of remaining at zero output with increasing setpoint.

There would be an increase in cooling output. The equipment would reach a maximum cooling output and stay there as supply air setpoint were increased.

In this case, it is possible to determine if the setpoint is too high, and

also the approximate error in the setpoint. A minimum value of zone equipment

output must be specified for the heating and cooling output. The supply air

setpoint must then be controlled to keep the output within the minimum values.

For a dual duct or multizone system, with two supply air setpoints, the local

equipment is usually a modulating mixing box, providing a mix of warm and cool

air. A plot of output versus supply air temperature for one deck would show

the use of all warm air at low supply temperatures and all cool air at high

supply temperatures. In this case the desired supply air setpoint is one which

causes the mixing box to use air from only one duct (see table 1). Therefore

the desired zone output is when warm or cool air flow is a maximum for the

zone. The same principles apply, however, as in the case shown in figure 9,

and therefore the control point should actually be when the warm or cool air

flow is slightly below maximum.

Once a desired value for the local equipment output for the zone identified as the critical is selected, the actual zone equipment output can be measured,

and compared to the desired value to determine how much the current output is

in error. In the simplest case, a gain factor can be applied to the error to

produce a correction to the current setpoint. For example, if a reheat coil

is observed to operate 20 percent of the time, and the desired minimum is 5 percent, the error is 15 percent. If the gain is 0.033 degrees C/percent on- time, the correction to the setpoint is 0.5 degrees C. If the current

setpoint is 15.0 degrees, the new setpoint would be 15.5 degrees. If the

reheat coil in the zone is not operating at all, the error would be -5

percent. The setpoint correction would then be -0.165 degrees.

Unfortunately, the setpoint might really be several degrees too high, but it

is impossible to determine this. Thus, the response of the setpoint

controller in this case might be slow when the setpoint is too high, although

the setpoint would eventually arrive at the correct value.

More complicated algorithms could be developed to relate the supply air

setpoint to the zone equipment output error, such as adding integration of

past error to the algorithm (a PI controller). It is beyond the scope of this

report to develop such algorithms, however.

Figure 10 depicts a general algorithm for supply air reset based on zone

requirements. This algorithm is intended for periodic execution at some rate

which must be selected based on how many measurements are required to obtain

an accurate picture of what is occurring in the zones. Only after a sufficient number of samples have been obtained from the zones will the

algorithm determine what the new setpoint should be. In figure 10, this is

shown as an early exit from the algorithm if not enough samples have been

collected. Each time a new set of samples is taken, the new set is either

added to or averaged with the previous measurements and a count of the nuu In i of samples is incremented. If a measurement of modulating control out. j ut in

17
the zones is being made, one sample may be sufficient to determine a setpoint,

and the sample rate should be the rate at which the system is likely to

require a new setpoint (a rate of once every fifteen minutes may be

acceptable). If a measurement of on-off control output is being made,

however, one measurement will not be sufficient. If, for example, twenty

measurements are made on an on-off controller, this will allow the maximum

resolution of on-time to be 5%. If the setpoint should be adjusted every

fifteen minutes, a sample time of 45 seconds would be required.

figure 10. Supply air temperature setpoint reset algorithm using feedback

from measurement of local zone conditioning requirements.

18
After sufficient samples have been taken from the zones, the sample counter is

reset, and the algorithm is ready to determine a new setpoint. A

determination of which zone to use as a basis for the new setpoint is made,

based on the type of the air handler system and the season. Rules for this

were discussed previously. Once a single value for local zone activity has

been determined, this can be used as input to equations to predict what a new

setpoint should be and the setpoint can be altered. The equations used can be

multiplication of an error in zone activity by a simple proportional or gain

factor to produce a desired change or setpoint. More complex equations are

certainly possible.

An example supply air setpoint reset algorithm based on the procedure in

figure 10 is described in section 7.3. The system controlled in the example

is a constant volume, terminal reheat system, with two zones. Actual software

listings for the algorithm are included in appendix B. The basic algorithm is

also applicable to dual duct systems, if the zone determination step is

expanded to include a selection of separate zones for the hot deck and cold

deck setpoints, and the setpoint equations are expanded for hot and cold deck

setpoints

.

If the zones in a building are under zero energy band control, more

complicated algorithms than the one in figure 10 may be necessary to optimize

building energy use. If, for example, the temperatures of the zones in a building were all floating between limits, but none were at the limits, and if

mechanical equipment were being used to cool the supply air, energy would be

wasted. If the supply air setpoint were adjusted to move the space

temperature closer to the high limit, mechanical cooling would be reduced, and

energy would be saved. More sophisticated instrumentation would be required

for this application, such as sensors for measurement of space temperature or thermostats with analog or digitized analog output.

19
4. DAY/NIGHT SETBACK

Most commercial buildings are not occupied 24 hours a day* and do not require

space conditions to be within a comfort zone when unoccupied. When the

occupants leave a building, usually at night or on weekends, the space

conditions can be allowed to drift outside of the usual setpoints for space

temperature and humidity. If weather conditions are not severe, the HVAC

equipment can be turned off completely. In many cases, however, weather

conditions might result in space temperatures becoming so low or high as to

either cause damage to the building (e.g. freezing, condensation, high

humidity), or make it impossible to recover to a comfortable temperature

before building occupation. In such cases the HVAC equipment must be

activated to keep the space at a minimum or maximum temperature or humidity

condition. The temperatures and humidity values at the minimum or maximum

conditions replace the normal occupied period space setpoints. This strategy

is commonly called "setback”. Setback is usually associated with a lowering

of space temperature setpoint. If the space temperature setpoint is raised,

such as during the cooling season, this may be referred to as "setup.” The

modifier "day/night” is also added to the term "setback,* to imply that the

setback occurs at night. However, some buildings may have occupancy schedules

not related to day and night, such as theatres and churches.

Space comfort condition specification also includes a minimum ventilation

rate. This is commonly set at a minimum value for all conditions and is not

under closed loop control. If the ventilation rate is placed under control,

then the setpoint of minimum ventilation air can be setback, just as the

temperature can be setback. Usually the ventilation level is setback to zero

by closing the outside air dampers during unoccupied periods.

4.1 Local Space Equipment

Day/night setback is usually accomplished throught the control of local

heating and cooling equipment located in the zones of a building, since this

is the equipment which directly determines the space temperature. Although

the basic concept of day/night setback is simple, the design of a general

setback algorithm is complicated by the number of different system types and

local equipment configurations. Day/Night setback is applicable to any

building in which space conditions may be allowed to drift out of normal

ranges at any part of the day. However this discussion will be limited to

non-residential buildings which contain a central air handling system

providing ventilation air and possibly conditioned air to the building spaces.

Table 1 in chapter 3 gave a listing of four generalized air system types which

may serve a building zone. These types may be mixed in any particular

building, with one type of system serving, for example, a perimeter zone and

another type serving an interior zone. The actual control of local space

conditions is usually accomplished by a separate controller device located in or adjacent to the zone. This device will be referred to as a thermostat,

although more complex controllers such as those used in VAV systems might not

usually be referred to by this term. Table 2 is a listing of the possible

types of thermostats which might be used to control zone temperature.

20
TABLE 2. Space zone thermostat types

thermostat type I system type I comments

1. On-off heating or cooling only1 reheat, package | Setup and setback not possible at

lAC or HP, | same time. Manual choice of heating

1 radiation lor cooling equipment control.

1 1

2. On-off heating and

cooling

1 1

1 package AC or HP | Setup and setback not possible at

I

| same time. Controls both heat and

1 Icool equipment at same time. Subject

I

| to cycling from heating to cooling.

1 1 3. On-off zero energy

band heating and

cooling

.

1 1

1 package AC or HP I Setup and setback possible at same

1 I time. Gap between use of heating and

|

| cooling equipment.

1 ! 4. Analog heating or cooling only

IVAV, modulating I Setup and setback not possible at

1 reheat, hot/cold I same time. Manual choice of heating

lair mixing lor cooling equipment control.

I | Produces variable output.

1 | 5. Analog heating and

cooling

8 1 Ihot/cold air I Setup and setback not possible at

Imixing system, | same time. Produces variable output

|4 pipe fan coils | on one or two output lines,

land radiation

6. Analog zero energy

band heating and

cooling

l l |4 pipe fan coils I Setup and setback possible at same

I Itime. Gap between heating and

1 | cooling signals.

1 1

1 1

In table 2, the thermostats are one of two basic types, either on-off or analog. On-off thermostats produce either no output signal or an output

signal of a fixed level. The output can be used to turn on or off equipment

which is to be either fully on or off. Analog output thermostats can be used

to modulate the output of zone local equipment. The output of such a thermostat varies with the difference between the setpoint and the measured

space condition. In addition, thermostats can be classified as either zero

energy band or conventional. A conventional thermostat is used to maintain a space at a fixed setpoint. A zero energy band thermostat is intended to allow

the space condition to float between limits which may be as much as 5-10

degrees G apart.

Table 2 implies that zero energy band control is not possible with certain

system types. Strictly speaking, this is true, since zero energy band control

at the zone level requires that the zone controls be able to both locally heat

and cool the zone. In a system such as a terminal reheat system, the cooling,

of the zone is provided by the temperature of the air from the air hand 1 ' i ,

21
which the reheat thermostat has no direct control over. However, it is

possible to provide effective zero energy band control for the zones if the

local thermostat can provide a signal to the air handler controller whenever

the local zone is outside of zero energy band limits. For example, with a terminal reheat system, the local thermostat could provide a signal if the

space temperature were above the upper space temperature limit. In this case

the supply air temperature could be adjusted upward until either mechanical

cooling at the plant could be turned off or until the zone temperature was at

the upper limit. If the zone temperature were below the limit, the local

reheat coil would keep the zone at the limit.

Local zone thermostats can be implemented in many types of devices. A

thermostat can be mechanical, with a bellows or bimetallic strip moving a mercury switch, or pneumatic, with a fluid filled temperature sensor producing

a fluid pressure which is converted to an air pressure, or electronic, using a sensor producing an electrical output signal. Many new thermostats are being

designed with microprocessors and digital interfaces. Most thermostats have

traditionally been set, calibrated, or programmed at the thermostat location.

Thermostats to provide day/night setback have been built with internal clocks

to switch between day/night setpoints. In order for day/night setback to

become a function controlled by a BMCS (EMCS), the local thermostats must have

the abiltity to either switch between locally programmed setpoints on the

reception of a signal from a remote controller, or to have the setpoint

directly programmed by the remote controller. Another concept is to have the

remote controller take over the function of the local thermostats.

To reduce manual reprogramming requirements for setback of local thermostats

at the transitions between cooling and heating seasons, it may be possible

with some thermostats to implement cooling setup and heating setback at the

same time. For a zero energy band thermostat, this requires that the zero

energy band be increased in size on both ends of the band.

Control of the ventilation air provided to a space is usually not implemented

locally. The criterion for acceptable space conditions is usually a minimum

quantity of outdoor air per occupant of a zone. Some systems measure either

occupant count or carbon dioxide concentration to control the amount of

ventilation air. If a zone has no occupants, no ventilation air is required.

The outdoor air quantity is varied by changing the opening of the air handler

system outside air dampers. In this report, only change of ventilation air

quantities from occupied conditions to unoccupied conditions will be

considered

.

4.2 Spacjg. Temperature and Ventilation Se tba ck

When temperature and ventilation setback are to be considered for use in a particular building zone, the characteristics of the zone will determine to

what degree setback may be used. If a zone is occupied continously, then no setback will be possible. Examples would be security guard offices or buildings with multiple shifts of workers.

If the zone is unoccupied for part of the day, but sensitive equipment or materials are located in the zone, or temperature sensitive equipment runs

22
con t inou s ly # then a limited setback may be possible. If the equipment or materials do not require ventilations but do require a certain temperature

range, it may be possible to shut off ventilation air to the zone. Examples

of this type of zone would be computer rooms, laboratories, or rooms for

storage of temperature sensitive chemicals.

If a zone is unoccupied for part of the day and contains no sensitive

equipment or materials, a setback of the zone temperature is possible. The

local controller can be setback to a temperature which is above any

temperatures where damage to the zone might occur, or setup to a temperature

which is below a temperature where heat damage might occur. Often the cooling

ability of the space is disabled since heat damage requires very high outdoor

temperatures or solar gain rates. Unless the local climate is mild, heating

equipment is not disabled since cold damage such as condensation or freezing

may occur at temperatures not that far below room temperature. A building

zone whose setpoint has been setback can maintain the minimum space

temperature by one of two methods. Many systems have a local perimeter

heating system which does not require the central air system to be operating.

Examples are terminal reheat systems with coils in the zone space (induction),

or perimeter hot water radiation coils. In such systems the air handler fans

may be completely shut off during the setback period. If a local perimeter

heating system does not exist, the air handling system must be used to provide

warm air to the space. Examples would be VAV, dual duct, or multizone systems

without local zone heating equipment. In this case the air handler fans must

either be left operating or cycled on and off to maintain the minimum space

temperature

.

A consideration in setback or setup of thermostats is that when the thermostat

setting is returned to normal before the start of the occupied period, local

heating or cooling equipment may turn on at full output. In some systems this

might be undesirable and it might be necessary to delay the return to the

normal thermostat setting until after the air handling unit has started.

4.3 Day/night Setback Actions

The use of day/night setback involves two separate procedures. These are the

transition of the building to the setback condition, which will be referred to as shutdown," and the transition of the building from the setback to the

normal condition, which will be referred to as "startup."

For shutdown the actions that may be taken are as follows:

1. thermostat setback and/or setup - this action causes the local zone

control equipment to use a minimum and/or maximum space condition as tht zone thermostat setpoint.

2. stop air handling unit - this action is used when the zone can maintain

the minimum or maximum condition without assistance from the air handler.

3. change operation of air handling unit controller - this action i: required so that the air handler controller will not attempt to control

23
the unit in the same way as it would under normal conditions. This

includes the operation of automatic supply air setpoint reset

controllers (see chapter 3).

For action 1, the control unit (FID) which manages the shutdown must either

send a signal to the local zone thermostats which will cause them to use a setback and/or setup setpoint for the zone, or somehow reprogram the zone

thermostats to use an alternate thermostat setting. The choice of whether to

force a setback or setup thermostat setting may have to be made as a function

of the season. If the local zone equipment is not capable of maintaining the

space condition without the air handling unit, then either the FID must

monitor the zone conditions and restart the air handler if the space

approaches minimum or maximum condtions, or the local zone thermostat must

provide a signal which the FID can interpret as indicating that the zone

requires that the air handler be activated.

Action 2, stopping the air handling unit, will usually consist of a number of

separate actions including stopping the supply fan, and if present, a return

fan, shutting valves to heating and chilled water coils, shutting all outside

air dampers (including minimum outside air dampers), and stopping any

auxiliary pumps or fans. If the zones require that the air handling unit

provide conditioned air during setback, action 2 would consist only of

shutting outside air dampers, and some of the non-essential air handler

support equipment.

Changing the operation of the air handling unit controller, action 3, is

required to prevent the controller from trying to maintain a supply air

temperature when there is no airflow through the unit. If the FID is also

running an algorithm to reset supply air temperature setpoint, it may be

necessary to disable this algorithm, unless the supply air setpoint is

initialized at the startup of the air handler.

For startup, the following actions may be required:

1. removal of thermostat setback and/or setup

2. start air handling unit

3. return operation of air handling unit controller to normal

For startup action 1, the control unit (FID) which manages the startup must

either send a signal to the local zone thermostats which will cause them to use the normal setpoint for the zone, or reprogram the zone thermostats to use

the normal thermostat setting. Startup action 2, starting the air handling

unit, will usually be the opposite of stopping the unit. The start operations

will include starting the supply fan, and if present, the return fan, opening

outside air dampers for minimum ventilation, and starting any auxiliary pumps

or fans. A delay between successive starts of separate pieces of electrical

equipment should be used to avoid high utility demand charges. Returning the

operation of the air handler controller to normal will cause the outside air

dampers and heating and cooling coils to open as needed. If the FID is also

running an algorithm to reset supply air temperature setpoint, it may be

necessary to initialize the supply air setpoint at the startup of the air

24
handler. Action 1 may be performed after 2 and 3 in certain situations where

local equipment might turn on unnecessarily when the space setpoint is

returned to normal.

4.4 Basic Setback Algorithm

The shutdown and startup sequences described in section 4.3 will be used

before and after an unoccupied period. These sequences can be initiated in

one of three ways. The simplest method is by a manual command to the FID from

a building operator to either startup or shutdown. The disadvantage of this

is that the operator must always be present before the sequence is to occur. Also the timing of the shutdown and startup determines the energy savings

which will result from the use of setback. A manual setback will yield

erratic savings if the operator forgets or is late in starting a sequence.

A second method is to use "time of day" control to initiate a setback

sequence[4]. This method uses an algorithm in the FID which allows a "task

to be scheduled to occur at a specific time of day on a specific day of the

week. A setback shutdown could then be scheduled to occur at, for example,

5PM, Monday through Friday, and a startup could be scheduled to occur at 6AM

Monday through Friday. The startup time would have to be scheduled to allow

enough time for the HVAC equipment to bring the building from the setback

condition back to the normal state before the building were occupied. This

time will vary depending on the severity of the weather. During the spring,

the startup might require one half-hour, while during severe winter conditions

the startup might require as long as 8 hours. To some extent the schedule can

be changed to reflect the weather, but maximum savings occur if the end of

startup recovery always coincides with building occupancy. With shutdown, the

sequence may be scheduled to start when the building occupants leave.

However, the shutdown can often be started prior to the unoccupied period and

space conditions will not deterioriate out of the comfort region until after

the building is unoccupied. To achieve the maximum savings with setback, the

setback startup and shutdown should be controlled by an "optimum start/stop

control algorithm, which makes calculations based on outside conditions and

the past behavior of the building to select the best time to begin startup and

shutdown[ 5]

.

Figure 11 represents a basic algorithm that would be used for day/night

setback shutdown and figure 12 presents the basic algorithm for startup.

These algorithms would be executed under control of one of the three methods

mentioned in the preceding paragraph. The setback actions were described in

detail on previous pages.

An example of setback algorithms based on the procedure in figures 11 and 12

is described in section 7.4. The system controlled in the example is a constant volume, terminal reheat system, with two zones. Actual software

listings for the algorithms are included in appendix C.

25
BEGIN

figure 11. algorithm for day/night setback from normal to setback state

figure 12. day/night setback algorithm for return to normal state from setback

26
5. VENTILATION PURGING

Ventilation purging is a control strategy which uses outside (ventilation) air

to reduce energy requirements and improve occupant comfort. This strategy is

intended to operate in a building which has unoccupied periods when it is

possible to allow space conditions to range outside of an envelope of

acceptable dry bulb temperatures, humidity, and fresh air requirements.

Ventilation purging is not useful in a building which must maintain comfort

conditions at all times. Purging is defined as the use of outside air to

either heat or cool a building during unoccupied hours (depending on the

anticipated loads) if the outside air is at the correct conditions relative to

the inside air. Ventilation purging is usually used as a cooling strategy.

During the cooling season, cool outside air in the early morning (e.g. 4AM)

can be used to precool a building if the outside air is cooler than the air

currently in the building. This precooling can reduce the cooling capacity

required for occupied period startup.

5.1 Basic Concepts

The ventilation purging concepts discussed here are assumed to apply to the use of purging as a cooling strategy. The usefulness of ventilation purging

to provide inexpensive cooling during an unoccupied period depends on the

characteristics of the particular building in which the strategy is to be

used. Two characteristics are of primary influence. These are the building

thermal mass and the unoccupied period internal gains. The ideal application

for ventilation purging is a building with high thermal mass and low

unoccupied internal gains, located in an area where the outdoor temperatures

during the unoccupied period are lower than the temperatures during the

occupied period. If outside air is used to purge such a building at night,

then because the internal gains are low and entering air is relatively cool,

the building mass will approach the outside air temperatures much more quickly

than if no purging were used. This results in interior temperatures near the

lower end of the comfort range by the time the building is to become occupied

again. The high mass keeps building temperatures down for a relatively long

period with subsequent reduced mechanical cooling equipment loads compared to

days where purging is not used.

If a building has high internal gains during the unoccupied period,

ventilation purging is still beneficial, but if the purging period is not

timed correctly, savings can be negated. Timing considerations will be

discussed in section 5.3.

Ventilation purging will not be desirable if the outside air temperature does

not drop appreciably during the night, or if the building has very low thermal

mass and low unoccupied internal gains. If the outside air temperature is not

lower than the internal temperature, purging the building with outside air

will actually increase cooling loads. If the building has low thermal mass

and low internal gains during the unoccupied period, the interior temperature s will normally follow the outside temperature changes closely and ventilation

purging will have no useful effect.

27
In order to make use of ventilation purging, the building air handlers must be

capable of a "purge cycle." A purge cycle usually begins with an air handling

unit in an off state with all heat exchanger water and steam valves fully off

and outside air dampers closed. This state would have been entered when the

building became unoccupied. To begin the purge, the air handling unit

controller must start the air handling unit supply fan, and possibly the

return fan (if one exists), and fully open the outside air dampers. Outside

air will then flow into the building space, and air in the building spaces

will be exhausted to the outside. In some systems such as VAV systems, the

zone supply air boxes may have to be forced to a fully open condition. To end

the purge, the air handling unit fans will be stopped and the outside air

dampers closed. Throughout the purge, any heating or cooling coils will not

be allowed to operate.

An example of the temperature response of a building during a purge cycle is

useful to illustrate concepts. Figure 13 is a graph of the average interior

temperature, the outside air temperature, and the air handler supply air

temperature for a simulated building during the course of an unoccupied

period. The simulation is for a single constant volume air handler serving

two zones with terminal reheat. The figure can be divided into six time

periods when the air handling unit is in different states. At the start of

the figure, the air handler is in the "normal" state and the building is

occupied. The space temperature is at approximately 24 C. The fan is on and

the supply air temperature is below 15 C due to mechanical cooling. During

the next period, the fan is off, but the space is still occupied. Due to

internal gains, the space temperature rises. The supply air temperature is

shown as approaching the space temperature, but this is not significant since

there is no air flow through the air handler. The building is unoccupied

during the next time period (III in figure 13) and this causes internal gains

to be lower and the space temperature to fall. During this period the outside

air temperature drops to a lower value. In period IV, the ventilation purge

takes place. The fan is activated and the supply air temperature quickly

reaches the outside air temperature. The room temperature drops due to the

purging. In region V the purging has stopped and due to internal gains the

space temperature rises slightly. Finally, in period VI, the air handling

unit is started and placed in a normal mode in preparation for the next

occupied period.

Two important quantities should be observed in figure 13. One is the smallest

difference between room temperature and outside temperature that the purging

can achieve (marked as A - n

in figure 13). The magnitude of this differential

depends on the amount of unoccupied internal gain and the heating of the air

due to fan temperature rise and duct gain as it passes through the air

handling unit. As this minimum is approached, the room temperature will not

decrease unless the outside temperature drops. The purging has no effect at

this point. The other important item is the time required for the system to

arrive at the minimum differential between room and outside air temperatures.

This is the purging time, and it depends on factors such as the indoor and

outdoor temperatures, the building thermal mass, the supply air flow rate, and

the internal gain.

28
TEMPERATURE,

TIME

figure 13. Space, supply and outside air temperatures during an unoccupied

period with ventilation purging

Three types of measured data are required to implement a ventilation purging

algorithm in a BMCS. The current state of the air handler used for purging

must be known. In addition, measurements of outdoor air temperature (and

humidity) and indoor air temperature (and humidity) must be made. The

measurement of outdoor temperature is straightforward, but the determination

of indoor temperature can be difficult. In a single zone small building, the

indoor temperature can be easily determined from a single measurement.

However, if the building is large, with multiple zones, and diverse internal

and solar gains, each zone may be at a different temperature, particularly

when the air handling units are off during the unoccupied period. Two

alternatives to indoor temperature determination in this case are computing an average indoor temperature from a number of individual readings, or using

selected temperature readings from zones with the highest and lowest

temperatures. For determining when a purge is required, the highest

temperature reading might be used. For determining when a purge should b< stopped, the lowest reading might be used.

The use of ventilation purging can actually be considered an extension of tin- optimum start/stop control strategy [5]. With optimum start, the optimur tim29
to begin the start up of air handlers and mechanical cooling equipment prior

to the occupied period of a building is determined. The desired start time is

just early enough to allow the building space to be brought to a comfortable

state a short time before the building is occupied. If used in conduction

with optimum start time control, ventilation purging might be performed before

the building start-up, or as a first stage in building start-up. In this

report, the concerns involved in coordinating optimum start and ventilation

purging strategies will not be discussed. Ventilation purging will be

assumed to be an independent control technique.

5.2 Basic Algorithm fox Ventilation Purge Control

A basic algorithm for ventilation purging must determine whether ventilation

purging should be started if the building is unoccupied, or if purging is

currently taking place, whether ventilation purging should be stopped. Figure

14 shows the structure of such an algorithm. It is assumed that a mechanism

exists in the FID to periodically execute this algorithm at a selected time

interval, such as every thirty seconds. The algorithm has three possible

output actions each time it is executed. These are do nothing, start purging, or stop purging. The actual actions required to start and stop purging are

assumed to be performed by another algorithm which is controlling the air

handling unit, such as the controller described in section 7.2.

The algorithm of figure 14 has two major paths, one used if purging is

currently taking place, and the other used if purging is not currently

occurring. The determination of whether or not the air handler is purging is

assumed to be made by checking a status variable which has been set by the air

handling unit controller.

Each major path of the algorithm asks two questions to determine what action

should be taken. Both paths ask the same two questions, but the answer will

depend on which path the question was asked by. The two questions are Does

the building space require cooling?’* and "can outside air provide a cooling

effect on the building space?." The answers to these questions are determined

by using measurements of the space and outside air temperatures, along with

several parameters. Specific methods to answer the questions are discussed in

the next section.

The algorithm in figure 14 may be stated in words as follows: If the air

handling unit is off, the space requires cooling, and the outside air can

provide cooling, a purge is begun. Otherwise no purge is started. If a purge

is currently taking place, and either the space no longer needs cooling, or

the outside air cannot provide cooling, the purge is stopped. Otherwise the

purging is allowed to continue.

5.3 Ventilation Euj.&in& Al&fixithm Compone nts

The two main component questions of the ventilation purging algorithm of

figure 14 are presented in greater detail in figures 15 and 16. Detailed

methods for answering the questions are presented in the following sections.

30
4

STOP PURGE

<

END ^

figure 14. Basic ventilation purging algorithm

5.3.1 Does the space need cooling? / not currently purging

This component of the ventilation purging algorithm determines whether the

building space would benefit from ventilation purging during the unoccupied

period. A simple test is to compare the current space conditions to spac.

conditions definined as boundaries of a comfort zone. If the space condition.^

are warm or humid compared to the comfort zone conditions, then cool in* ;

* needed. If the space conditions are cooler or dryer than the comfort zon-- conditions, cooling is not needed. A threshold condition within the contort

zone must be defined, above which cooling is needed. This method will vi r>. the space conditions have reached a steady state condition.

31
figure 15. Algorithm to determine if the building space requires cooling

for the ventilation purging algorithm.

When the space becomes unoccupied, the space temperature may drop rapidly due

to causes such as low outside temperatures or the removal of a large internal

gain. Purging may not be desirable even though the space temperature is above

the cooling threshold because the space temperature may reach comfortable

temperatures without the use of purging. A second test, which determines if

the space temperature is currently rising or holding steady, should be used in

conjunction with the threshold test. An example method for determining the

space temperature trend is given in section 7.5. These two tests to determine

if the space requires cooling are shown in the left side of figure 15. If the

result of both tests is positive, then cooling is needed. If the result of

either test is negative, then cooling is not needed.

32
5.3.2 Does the space need cooling? / now purging

Once ventilation purging is in progress, the building space temperature should

fall, if the decisions made by the purge-starting path of the algorithm have

been correct. Eventually the space temperature will either level off at a value within or above the comfort zone, or, under certain conditions, at a point below the comfort zone. If the space temperature falls below the

comfort zone, then heating energy will be required to bring the temperature

back up, negating the benefits of the purge. Therefore it is important to

stop the purging if the space temperature falls too low. This is accomplished

by comparing the space condition with a lower limit. If the space condition

falls below the limit, then cooling is not needed. As long as the condition

stays above the low limit, then cooling is still needed. This test is part of

the algorithm in figure 15, and is shown on the right side of the figure.

5.3.3 Is cooling available? / not currently purging

This test is intended to determine if starting the use of ventilation purging

will provide practical cooling of the building space at the current time. As

was shown in figure 13, if ventilation purging is started under the proper

conditions, the space temperature will fall and level off at a value above the

outside air temperature. To start purging when the differential between the

space and outdoor temperatures is below the minimum differential would result

in no useful cooling effect. Unfortunately the minimum differential is not a constant for all buildings, since it depends on building internal gains,

thermal mass, and heat transfer coefficients. This differential could,

however, be considered to be a constant for a particular building. The test

for availability of cooling would then be to compare the current difference

between indoor and outdoor temperatures with a minimum differential parameter

which could be set by the building operator. Improper choice of a minimum

differential parameter would result in either waste of fan energy or missed

opportunities for useful ventilation purging.

To prevent damage to the air handling system when outdoor temperatures are

low, a test should be made to determine if the outside temperature is low

enough to cause freezing of heat exchange coils. The two tests for

determining if cooling is available are shown on the left side of figure 16.

If the outside temperature is above a safe minimum, and also the differential

between indoor and outdoor air temperatures is above a minimum, then cooling

from ventilation purging is considered available. If either of the two tests

is not passed, ventilation purging is considered unavailable.

In areas where relative humidity is high during the cooling season, it is also

important to include a comparison between indoor and outdoor relative

humidity. If outdoor air is cooler than indoor air, but the relative humidity

is high (such as during a rainstorm), ventilation purging should not be

started, since the latent cooling load will actually be increased.

33
BEGIN

purge Yes

figure 16. Algorithm to determine if outside air can provide cooling used

for the ventilation purging algorithm.

5.3.4 Is cooling available? / now purging

The purpose of this test is to determine if ventilation purging is continuing

to have a cooling effect on the building space. In general, the space

temperature will fall rapidly at the start of purging and approach a steady

value. By monitoring the trend of the space temperature, a decision to stop

purging can be made when the rate of decrease in space temperature has become

so slow that further purging will not provide additional cooling. An example

of a method for determining whether the space temperature is falling is given

in section 7.5 (the same method is used to determine if the temperature is

rising or steady). The test for continuing cooling availability is shown in

the right side of figure 16.

34
If high relative humidity is a consideration, it may also be necessary to

stop purging if the outdoor relative humidity rises above the indoor relative

humidity. The indoor humidity will approach the humidity of the outdoor air

in the same fashion as the temperature.

5.4 Ventilation Purge Algorithm Timing,

The optimum starting time for ventilation purging when all of the tests

described in section 5.3 have been passed depends to some extent on the

characteristics of the building where the algorithm is applied. At first it

will be assumed that the ventilation purge algorithm is executed periodically

for the entire unoccupied period. If ventilation purging is started as soon

as the tests of section 5.3 are passed, and the purging is stopped some time

later by the algorithm of figure 14, the space temperature could conceivably

rise and cause ventilation purging to be initiated again. Under certain

conditions, the purge cycle might take place two or more times. Unless an

attempt is being made to maintain the space at some condition with purging,

the most important result of the purging is that when the building is occupied

the space conditions are in the lower part of the comfort zone. If the purge

cycle takes place more than once, the cycles before the last one will waste

fan energy.

Excess cycling of ventilation purging is more likely to occur in a building

with high unoccupied internal gains. In a building with low internal gains,

once the space has been cooled to a minimum temperature, it will probably

remain there once purging is stopped. Thus the time-of-day that the purging

takes place is less critical. In such a case, the most important timing

consideration is that the purging take place at a time when outdoor air

enthalpy is at its lowest. This can be accomplished by time-of-day control or

by monitoring the trend of outside temperature.

To prevent excess cycling of a ventilation purge, the simplest approach is to

only begin periodic execution of the purge algorithm a relatively short time

before the startup of equipment for the occupied period. The approximate time

required to purge the building can be used as a guideline for how early the

purge algorithm should start. A time-of-day control algorithm can be used to

schedule the starting of the ventilation purge algorithm [4],

A more complicated method of causing the purge to occur just before the

startup for the occupied period requires that records of purge times, and

outside and indoor temperatures at the start of previous purges be kept. This

information can be used to predict the time that will be required to purge the

building. This method is actually a form of optimum start algorithm, and if an actual optimum start algorithm is being used for the startup of the air

handler system, the control algorithms must be carefully designed to prevent

conflicts

.

An example algorithm used in the actual NBS FID and based on the algor it hi: < : figures 14, 15, and 16 is presented in section 7.5. The system control]''! i r. the example is a constant volume, terminal reheat system, with two zone.,

thermal mass, and high internal gains. Actual listings of software for thexample are included in appendix D.

35
6. HOT WATER AND CHILLED WATER SUPPLY TEMPERATURE RESET

HVAC equipment is usually sized to meet conditions at a design peak load.

Coil water temperature setpoints are also chosen to meet the design load.

During most of the equipment operating hours, the equipment will be operating

at part-load conditions. Use of design setpoints on water loops at part-load

results in unnecessary piping loss (or gain for chilled water) and possibly

equipment inefficiencies. Setpoint reset control reduces energy consumption

by performing a reset of hot water or chilled water supply setpoints to match

the actual equipment load. Usually the supply water setpoint is constant for

an entire HVAC system, since the water will typically be supplied from boilers

or chillers in a central plant.

6.1 Outside Ml Temperature Sjjp_p_ly. WiLtgr

The goal of water supply temperature reset is to supply the various heat

exchange coils that use either hot or chilled water with water at a temperature which is just sufficient to supply the load at the coil with the

greatest demand. For example, if all chilled water coils in a system of air

handling units have their valves open somewhere between 5 and 40 percent, this

indicates that the coils are not at full load, and that the chilled water

temperature could be raised to improve chiller economy and lessen piping heat

gain. After the chilled water temperature is raised, the chilled water valves

might be open from 70 to 85 percent, which is close to the full load of the

coils. The coil with the greatest opening determines the limit for the

reset. When any of the valves in the system reaches 100 percent open, then

the chilled water temperature should be lowered, because the fully open valve

indicates that this particular coil is at or above full load, and might not be

able to provide sufficent cooling capacity for the air handling unit.

An approximate method of implementing supply water reset is to assume that the

space load is a function of the outside air temperature. As the outside air

temperature rises, chilled water temperature is adjusted downward and hot

water temperature is adjusted downward. The algorithm for outside air supply

water reset is functionally the same as the outside air supply air reset

algorithm, as discussed in section 3.1 and presented in figure 6. Because of

the similarity to outside air supply air reset, outside air supply water reset

will not be discussed further in this chapter.

6.2 Demand Supply Water Temperature Reset

A more accurate method of supply water reset than outside air temperature

reset is based on readings from instrumentation of actual heat exchanger

coils, either the valves or the coils themselves. This will be referred to as demand supply water reset. Demand supply water temperature reset control is

different from the other strategies discussed in chapters 3-5 because a demand

supply water reset algorithm cannot be placed in a single FID associated with

an air handling unit. The plant equipment for production of hot and chilled

water is likely to be controlled by a FID unit or controller which is

physically separated from the FIDs or controllers which oversee the air

handling units in a building. There may be a controller, located in a plant

FID, which maintains the hot or chilled water system at a setpoint. The

algorithm to determine what the water system setpoint should be might reside

36
in the plant FID, or at the central control level of the system. If the

demand water reset algorithm is to monitor the coil loads at the individual

air handlers, there must also be algorithms in the air handler FIDs, in

communication with the main demand supply water reset algorithm, to provide

information on valve positions or coil conditions. There are several possible

conf igurations

.

The simplest configuration of algorithm location for demand supply water reset

is to have the main algorithm at the central control level. Information on

the air handling units is obtained by the central control unit (CCU) and used

by the reset algorithm. The selected supply water temperature is then

transmitted to the control unit which is maintaining the supply water

temperatures. This configuration has the disadvantage that the central unit

is required in the system at all times. An alternative architecture might

configure the CCU as only an operator interface device, which would not

execute many control algorithms. The demand supply water reset algorithm

might then reside at the FID level in a plant FID, and obtain information from

routines in the air handling unit FIDs through the BMCS communication system.

The air handler information might be transmitted through the CCU or directly

to the plant FID, depending on what sort of communication network existed for

the system.

The demand supply water reset algorithm is therefore divided into two

physically separate parts, the actual setpoint determination and control part,

and the air handler load determination part. The air handler load

determination part is distributed among the FIDs which control air handling

equipment. The information which must be supplied by the load determination

part to the setpoint determination part depends on the sophistication of the

algorithms in the air handling unit FIDs. The highest level of information

would be a suggested supply water setpoint based on the local air handling

unit load. The setpoint determination part would then consist of selecting

the highest hot water or lowest chilled water setpoint suggested by all of the

air handler FIDs. Table 3 contains a list of several possible levels of

information that could be determined by the load determination algorithms.

The information in table 3 is organized in order of load determination

algorithm complexity with more complex algorithms required by the first item.

Conversely, as the load determination algorithm becomes more complex, the

setpoint determination part becomes simpler. The advantage of greater load

determination algorithm complexity is that the amount of information about air

handling units that must be known by the setpoint determination part of the

algorithm is minimized, making the total algorithm more flexible.

Table 3. Possible information transmitted between parts of a supply water

reset algorithm.

1. suggested water setpoint

2. suggested change in water setpoint (+ or -)

3. suggested direction of change in water setpoint (up, down, or no change)

4. average valve position or average valve position error

5. averaged binary valve position (eg: above 80% / below 80%)

6. average coil inlet and outlet temperatures

37
6.3 Possible Demand Sitpjtlx HfllLfiX &££££. Lo&A Algorithm

A possible demand water setpoint reset load determination algorithm might use

a hot water or chilled water valve' instrumented with a potentiometer to

measure absolute valve position and produce as an output a suggested increase

or decrease in supply water setpoint. The algorithm output would be used by a supply water reset setpoint selection algorithm. Figure 17 presents how such an algorithm might be structured.

The algorithm of figure 17 is intended for periodic execution at an interval

which must be selected for the system being controlled. The first part of the

algorithm is used to create an average value of valve position. If a sufficient number of samples for an average have not been taken* then the

algorithm is exited.

Once an average valve position has been determined, this valve position is

compared to a desired valve position which has been selected as a point with

good control characteristics. The desired valve position is actually a range

of possible valve positions* with lower and upper target values. If the

average valve position is determined to exceed the upper target value, this

indicates that supply water temperature should be decreased for a chilled

water valve or increased for a hot water valve. If the average valve position

is determined to be less than the lower target value, the supply water

temperature should be increased for a chilled water valve and decreased for a hot water valve. If the valve position is between the upper and lower target

values, then no change in setpoint is suggested. If a change is needed, an

optional calculation can be made to estimate the magnitude of the change to be

suggested. This value, in degrees of temperature, is combined with the

direction of change determined previously to produce a suggested supply water

setpoint for the setpoint determination part of the supply water reset

algorithm.

The calculation to determine the size of the suggested change in supply water

setpoint is performed using the error between the appropriate target value for

valve position and the average measured valve position. This error is

multiplied by a proportional factor with units such as degrees C/percent valve

position. This factor is dependent on the characteristics of the valve and

heat exchanger using the supply water, and would probably be determined

experimentally by observing a valve’s position for one water setpoint,

changing the setpoint, and noting the change in valve position.

The algorithm in figure 17 can be expanded to include more than one valve if

the FID controls more than one air handling unit, or the air handling unit has

more than one chilled water or hot water coil. The algorithm would be

duplicated if both hot and chilled water temperatures are to be reset, and the

two algorithms might run independently. Additional tests could be added to

the algorithm for a chilled water coil to ensure that the supply air humidity

was not excessively high. If the humidity was approaching a limit value, the

algorithm could suggest no change, or a lowering of setpoint, even if the

valve were almost closed. Higher chilled water temperatures would lead to a loss of any humidity control.

38
BEGIN 5

read current valve position

and average with previous values

increase supply

water temp. —— - decrease supply

water temp.

decrease supply

water temp.

1

increase supply

water temp.

calculate size of suggested change

in setpoint from valve postion error

and proportional factor

->+

END ~)

figure 17. Algorithm to determine current load of air handling unit coil

for supply water reset.

39
An additional feature that might be included in a load determination algorithm

for supply water reset would be to reserve a special value of suggested change

in supply water setpoint to indicate a suggestion that the plant equipment be

turned off. If the average valve position indicated that a valve were

completely closed, this would imply that no chilled or hot water is required.

If all valves were closed the plant equipment could be stopped.

6.4 MX, Water Reset Set point S.eleetiim Alg.OJi.thm

The setpoint selection algorithm corresponding to the load determination

algorithm of figure 17 would take the setpoint changes suggested by several

distributed load determination algorithms and use them to select a setpoint to

be used by the actual plant equipment controller. Figure 18 presents such an algorithm used to select the setpoint for a hot water heating system.

The algorithm of figure 18 would be executed at a regular interval which would

depend on the requirements of the system on which it was implemented. This

interval would not necessarily have to be the same as the execution interval

for the load determination algorithms running in the air handler control FIDs,

as long as it were not executing with a shorter interval. The algorithm would

have to somehow obtain the suggested setpoint change from each load

determination algorithm. This could be done by running the algorithm on the

CCU and using the CCU data base, or by direct communication with the air

handler FIDs.

Each FID air handler setpoint change suggestion would be compared to the

others to determine which of the suggestions had the largest numeric value.

The suggestions might include negative values (decrease temperature), zeros

(no change), and positive values. If a positive suggestion were taken, this

would result in a higher setpoint, and the heating loads at all air handlers

and other coils would be satisfied. If a negative suggestion were taken, the

setpoint would be lowered and any FIDs requesting an increase in setpoint

would have coils which would be fully open, but not meeting the load. Thus if

any positive suggestions are made, the setpoint must be increased. If only

negative suggestions are made, the setpoint can be decreased.

In order to provide a check of the instrumentation, a comparison is made

between current and previous load determination algorithm suggestions. If a setpoint change was made previously, and the current and previous suggested

setpoint changes are not zero, then a change should occur in the suggestion

value. If no change occurs this indicates an error in the valve

instrumentation or the FID operation and the system operator is notified.

If all of the load determination suggestions indicate that all valves are

shut, and were previously shut, the algorithm can either command or suggest

that the plant water heating equipment be stopped. The controller for the

plant will make the final decision about plant shutdown.

If no shutdown is indicated, the new setpoint can be calculated as the old

setpoint plus the largest setpoint change suggestion from the load

determination algorithms. This new setpoint must be checked to ensure that it

does not exceed high and low limit values.

40
figure 18o Algorithm to select supply water setpoint temperature for

supply water reset.

41
If the setpoint temperature is too high, this can cause dangerous system

pressures and temperatures. If the setpoint is too low* this might cause

problems with combustion heating equipment. If the setpoint is outside the

specified limits* it will be set to the closest limit. Once the setpoint

selection has been made and verified, it can be passed to the plant controller

which may be in a separate FID or in the same FID.

6.5 Chilled HateX Reset Se.tp.oint Selection Algorithm

The setpoint selection algorithm for a chilled water system corresponding to

the load determination algorithm of figure 17 would be very similar to the

algorithm of figure 18, with minor changes. The method of execution would be

the same, the same or similar timing of the algorithm execution would be used,

and the method of obtaining load determination algorithm suggested setpoint

changes would be the same. The major difference is that instead of using the

numerically largest setpoint change suggestion required for heating, a chilled

water algorithm would use the smallest numerical suggestion.

For a chilled water system, if a negative suggestion were taken, this would

result in a lower setpoint, and the cooling loads at all air handlers and

other coils would be satisfied. If a positive suggestion were taken, the

setpoint would be raised and any FIDs requesting an decrease in setpoint would

have coils which would be fully open, but not meeting the load. Thus if any

negative suggestions are made, the setpoint must be decreased. If only

positive suggestions are made, the setpoint can be increased.

The limits for setpoints must also be checked for the chilled water system.

High setpoints or low setpoints can damage refrigeration equipment, or cause other problems in the system.

The algorithms for hot and chilled water reset, although studied, were not

actually implemented in the N3S FID software at the time this report was

written. This is because the NBS FIDs have been initially developed as air

handling unit control FIDs rather than plant control FIDs, and therefore

complete testing of the algorithms would not have been possible. The response

of a simulated air handling system in a building to changes in chilled water

supply water were studied with the building system emulator used to test the

other algorithms in this report.

42
7. EXAMPLES OF ACTUAL CONTROL ALGORITHMS

As part of the research program in building HVAC control at NBS, a prototype

BMCS has been constructed using "off-the-shelf" components and microcomputer

boards. The system is organized into a hierarchy of three levels* central

(CCU), distributed (FID), and data gathering (MUX). Custom software has been

developed for the system. The software includes routines for supervisory

control as well as direct control (DDC). The information in this report is a product of the development effort. The supervisory control software has been

tested using a "building system emulator* (BSE) rather than an actual building

system. The emulator is an independent pair of microcomputers. One computer

uses a variant of a MUX to send analog and digital values representing the

state of an HVAC system to the actual BMCS MUX. The second microcomputer

supervises the first and runs a real time simulation of the HVAC system to

represent the response of the system to control actions. Eventually, the

control software will be tested in an actual building.

Since it is sometimes difficult to construct actual working programs from

general algorithms, a description of working software from the NBS FID is

included in this chapter. It should be understood that the software described

is one possible implementation of the control strategies, and is not

necessarily the best implementation. Other system architectures are certainly

possible and would require different software for the control algorithms.

The algorithms presented as examples are intended for operation in the FID

level of the BMCS. As background information, section 7.1 describes the basic

architecture of the FID. Section 7.2 describes a DDC controller for an air

handling unit which is used by the FID. The sections 7.3 through 7.5 describe

the example software in detail.

7.1 Example Field Interface Device Software Architecture

The software to operate the FID is a complex computer program which is mostly

written in the high-level computer language, FORTRAN. A number of utility

routines are written in microprocessor assembly language, specifically for the

microprocessor used in the FID.

The activities of the FID can be divided into two major categories:

communication with the CCU, and execution of tasks. A task is defined as a software procedure that has a specific purpose such as turning on a fan or controlling a valve position and begins at a certain starting point in the FID

computer memory. In the NBS FID, a task is in the form of a FORTRAN

subroutine without arguments. Information is passed between tasks through

common data areas. When there is no communication from the central level,

tasks are executed with a multi-tasking’* scheme in which a number of software

tasks are each executed in a periodic fashion in conjunction with the other

tasks so that it appears that tasks are operating simultaneously. The two

most essential types of tasks of the FID are collection and processing of data

and execution of control software (DDC, duty cycling, etc.) through the MUXs.

43
An example of a task is the supply air temperature reset software which

controls setpoints used by other control tasks. The day/night setback control

software is a task which is scheduled to execute when the building becomes

occupied or unoccupied at a specific absolute time-of -day.

Any "multi-tasking” software system must have some sort of "task manager

software" to control when tasks will execute and resolve timing conflicts

between tasks. The task manager used for the NBS FID is coded in assembly

language and makes use of a hardware real-time clock issuing interrupts to

divide time into 0.1 second intervals. The manager uses this division of time

to coordinate all tasks in the FID. The FID task manager has three basic

characteristics which are desirable for task control. First, the task manager

allows the periodic execution of multiple tasks and has a priority scheme for

use if two tasks are set to execute at the same time. The period of any task

may be changed during execution of the FID software. Second, any periodic

task may be stopped on demand. Third, any periodic task which is stopped may

be started after a variable delay which is independent of the period of the

task, or tasks may be set to execute only once after a variable delay (such a task may be called a "one-shot" task).

To control tasks, the task manager for the NBS FID uses a table of task

starting points and associated time intervals at which these tasks are to be

executed. This table will be referred to as the task table. Table 4 is a facsimile of the basic task table in the FID. There is a row in the task

table for each task in the FID. There are two groups of columns (four columns

per group) used for timing purposes. One group of four columns is designated

the counter, and the other group is called the interval. The four columns in

a group hold values for hours, minutes, seconds, and tenth-seconds. Every one tenth second, the manager software executes. The counter for each task is

decremented each time the task manager is executed. If any counter becomes

all zeros, this indicates that the associated task i6 to be executed. After

the task is executed, the counter columns are reset using values from the

interval columns, and the countdown resumes. The interval is not changed by

normal operation of the task manager.

table 4. NBS FID task table

task no. status code hr

COUNTER

min sec ts hr

INTERVAL

min sec ts TASK NAME

01 1 00 00 60 00 00 00 60 00 CONTROL

02 1 00 00 01 07 00 00 02 00 DATA

03 0 00 05 25 09 00 10 00 00 INACTIVE

04 -1 00 01 30 00 00 00 00 00 ONESHOT

05 1 00 59 59 09 01 00 00 00 PERIODIC

06 1 00 00 00 00 00 00 20 00 ACTIVE

07 1 00 00 14 02 00 15 00 00 DUTCYC

08 1 00 48 08 00 01 00 00 00 SCHEDULR

44
The first column in the task table contains the task number. This number is

also a priority ( a lower number represents a higher priority). All tasks are given priorities and lower priority tasks will wait if it is time for a higher

priority task to execute. The second column of the task table contains a code

to indicate whether the task is stopped (0)» set to execute once ( _ 1)» or set

to execute in a continuous, periodic mode(+l). The last column contains a symbolic name which represents the starting address of the task.

A utility routine, separate from the task manager routine, is used to change

the entries in the task table used by the task manager. This task table

editor routine can be called as a subroutine from a FORTRAN program. There

are three arguments which must be passed to the routine. The first argument

is the number of the task in the table whose table entries are to be edited.

The second argument is the value of the code to be loaded into the second

column for that row of the table. The third argument is equal to 1 if the

task intervals and the task counters for this row in the table are to be set

to the same new values. The third argument is set to 2 if only the task

counters are to be set to new values. The values to be loaded into the

counters and intervals are passed to the subroutine using data in a common

data area.

Another utility routine is used to determine the current values of the entries

in the task table. The routine can be called as a subroutine from a FORTRAN

program. There are two arguments. The first argument is the number of the

task in the table whose table entries are to be checked; the second entry will

contain the value of the code in the second column of the table. The values

of the interval columns in the table will be stored in a common data area

(same area used by the task table editor) as four consecutive values

representing the hours, minutes, seconds, and tenths of seconds. The routine

is very similar to the editor routine, but data are ’read from the table

rather than being written" to the table.

7.2 Basic Air Handling Unit Control Task

The NBS FID software contains a task which is designed to maintain the supply

air temperature from an air handling unit. The air handler control software

is oriented towards a constant volume reheat air handler, since this is the

type of air handler in the majority of lab buildings at NBS. The air handler

control task gets the supply air setpoint from a table in the FID and

modulates valves to maintain the air being discharged from the air handler at

the setpoint. The supply air setpoint table can be reset by other tasks or changed from the central level. Supply air reset software determines the

proper setpoint and changes the value in the table periodically.

In the NBS configuration, the air handler control task has control over three

actuators in the air handling unit. These are a chilled water valve actuator

to control flow through a chilled water coil, a steam valve actuator to

control steam flow through a preheat coil, and a damper actuator to control

the quantity of outside air. The task must actuate the proper control led

element to the position which most economically produces discharge air at tin' setpoint. If only one valve were available, the task would use a simple

velocity form proportional-integral (PI) control algorithm to position the

valve [63. With three controlled elements, there is a need for sequencing of

45
the actuators so that no two valves are open at the same time* to avoid

reheating or recooling of air (unless humidity control is required).

Sequencing is performed in the digital control system by having feedback from

the actuators as to whether or not they are at the ends of their strokes. The

three controlled elements are sequenced to produce increasingly warm air as each element is opened in turn. For this configuration* four end-of -travel

sensors are required. These are chilled water valve closed, preheat valve

open, outside air damper open, and outside air damper closed. The air handler

controller only controls one element at a time. If modulation of one element

will maintain the supply temperature, no other element needs to be controlled.

If, for example, the chilled water valve were under control,and the supply air

was too cold, the control algorithm would call for a reduction in the opening

of the chilled water valve. If, however, the control action closed the

chilled water valve, then the controller would leave the chilled water valve

and attempt to modulate the outside air damper to achieve the setpoint.

There is a sequencing problem to be avoided at the transition between

controlled elements. The solution used in the NBS FID is to accumulate

control actions after the current element is either fully open or closed as indicated by the limit switches. The accumulated control actions must build

to a certain level before transition from one element to another takes place.

If control actions in the opposite direction occur before the switchover, the

accumulated control actions are then reduced. This way, if there is slight

oscillation about a control setting, there will be no continous switching

between the two controlled elements.

The air handler control task uses a table of values to allow general control

of any types of control elements in any sequence. Since PI velocity algorithm

control is used, each of the controlled elements requires different control

parameters, since their gains are different. These parameters are entered in

the table. Other parameters are actuator travel time, maximum control action,

size of ’dead band’* for sequencing between elements, and whether the device is

for heating or cooling.

The outside air damper is treated as a cooling device. A simple dry bulb

economizer algorithm is used to lock the damper shut if the outside air is too

warm or too cold [7]. When locked out, the outside air damper is passed over

in the sequencing.

7 .3 Example Outside Air SuppIv Ait Easel Algorithm

A supply air reset control algorithm using outside air temperature, based on

the algorithm in figure 6, was developed to run in the NBS FID. The algorithm

was implemented in FORTRAN as a software task (section 7.1). Appendix A

contains the FORTRAN listing of the outside air reset task software.

In this example of outside air reset, the setpoints to be reset are contained

in a data area ( "common block’*) common to several tasks, in particular the

outside air reset task and the air handling unit control task. Therefore when

the outside air reset task changes the setpoint in the common data area, the

next time that the air handler control task executes, it will use the changed

setpoint

.

46
The outside air reset task uses a reset schedule which has the shape of the

curve in figure 5. For every value of outside air temperature there is a scheduled value of setpoint temperature. To implement the schedule, the

outside air reset task uses an equation with variable parameters. The

parameters may be changed from the central level of the system and may be

different for each setpoint to be reset. The parameters are stored in a table

where there is a row for each setpoint and columns for the parameters

associated with that setpoint. In the example software there is one air

handler control task with a maximum of five controlled elements, and each

element can be used to maintain a different setpoint. Therefore, the reset

table has five rows. Table 5 is a representation of a possible outside air

reset table.

Table 5. NBS FID outside air supply air reset parameter table

setpoint

index no. Toa-max

Toa-mid Toa-min Tr-max

Tr-mid Tr-min arstat

1 35.0 22.0 0.0 16.5 15.0 12.2 1 2 37.0 20.0 -5.0 16.0 14.5 13.0 1 3 37.0 20.0 -5.0 16.0 14.5 13.0 1 4 37.0 20.0 -5.0 16.0 14.5 13.0 0

5 37.0 20.0 -5.0 16.0 14.5 13.0 0

Note: All temperatures are in degrees C,i

The first column of Table 5 designates the setpoint index number. In the NBS

FID this points to a particular device used to maintain the setpoint of the

supply air temperature and indicates the place that a controlled element takes

in the sequencing of control. Values in the last column, marked arstat, are

used to indicate what setpoint reset algorithm is to be used. The 1 indicates

outside air reset. The 0 for setpoints 4 and 5 indicates no reset. These are under manual control only. The other columns are used to define the

relationship between setpoint and outside air temperature. This is done by

specifying three points on a plot of setpoint versus outside temperature.

Figure 19 shows three points on such a plot.

When the outside air temperature is below Toa-m in

» the setpoint is fixed at

T

r

_ max» If the outside air temperature is above T oa - max » the setpoint is

fixed as T

r

_ nj £n

. Between outside air temperatures of Toa - m i n and T

oa - m ^d» the

setpoint varies linearly with outside air temperature between T

r _^ and T

Between Toa-m i<i and TQa_max , the setpoint varies linearly with outside

air temperature between T

r

_ and T

r

- m in

*

47
figure 19. Example of outside air supply air reset schedule specified by

three points.

The operation of the outside air reset task follows figure 6 closely. After

the current outside air temperature is determined by examining the correct

analog inputs a software loop is used to reset all five setpoints. Before the

setpoint is reset the arstat variable for the setpoint is checked. If not

equal to one, the setpoint is not reset. To determine the reset value for the

setpoint, the region of figure 19 bounding the outside air temperature is

determined. If the outside temperature is bounded by one of the regions where

the setpoint is changing with outside temperature, the following equations are used

Tr-lo

" Tr-hi

slope = T • - T oa-hi oa-lo

(3)

setpoint = slope * (TQa ^oa-hi^ + Tr-mm ( 4 )

where the subscript r indicates setpoint, oa is outside air temperature, lo is

the lower of the two values which bound the outside temperature or setpoint in

the correct region of figure 19 and hi refers to the higher of the two values.

7 .4 Example Zgng UsaaM Su-PpJLy Ain Re.s&t Al&gxi-thm

An algorithm to perform supply air reset based on measurement of zone demand

was developed to run in the NBS FID. The algorithm is a specific

implementation of the algorithm in figure 10, written in FORTRAN as a software

task (see section 7.1). A FORTRAN listing of the the actual software may be

found in appendix B.

In this example of a zone demand reset algorithm, the setpoints to be changed

are assumed to be located in a data area common to several tasks. The most

significant task which can access the setpoints is the air handler control

task which uses the setpoints to control the temperature of the supply air

from an air handling unit.

48
The implementation of the supply air reset algorithm of figure 10 is dependent

on what type of HVAC system is being controlled, unlike the implementation of

the outside air reset algorithm of figure 5. The zone demand reset algorithm

was designed to work with a specific HVAC system emulated on the Building

System Emulator (see introduction to chapter 7.). The HVAC system for this

example is a constant volume, terminal reheat system, supplying conditioned

air to two zones. The zones each have a reheat coil controlled by a local

thermostat which senses room temperature. The reheat coil is a hot water coil

and has a control valve which is either open or closed in response to an

on/off signal from the thermostat. It is assumed that there is a switch type sensor attached to the reheat coils in each zone which changes state when the

reheat coil is turned on or off. This sensor is connected to the BMCS system

as a digital input and the state of the sensor can be determined by the supply

air reset algorithm.

There are two time interval values that must be assigned before the algorithm

can be run. One of these is the setpoint reset sampling interval, given the

variable name DRSAMP. This is the time between adjustments of the setpoint.

As an example, this might be five minutes. The other important time value is

the task execution interval, given the name DRTSKI. This is how often the

setpoint reset task runs, and an example of this value might be 15 seconds.

Each time the task executes, it samples the digital inputs connected to the

reheat coil sensors. The value of the input will be 1 if the coil is on and 0

if it is off. For each zone, the integer equivalent of this digital input is

added to a summation of previous digital input values. When a time equal to

DRSAMP has passed, the sums are used to calculate a correction for the supply

air setpoints. The number of samples in the sum is given by DRSAMP/DRTSKI.

For the example given, the number of samples would be (5*60)/ 1 5 or 20 samples.

The values of DRSAMP and DRTSKI must be adjusted for each system being

controlled to yield the best results. DRSAMP is stored in FID memory and can be changed from the central control level. DRTSKI is equal to the task

interval found in the main task table (see table 1). In the setpoint reset

software a special routine is used to read this value from the task table.

With each execution of the task, a counter variable is used to hold the

current number of samples taken of the reheat coil sensors. If this counter

is not equal to DRSAMP/DRTSKI, the task is exited. If the counter is equal to

the maximum number of samples, the setpoint reset section of the task is

executed and the sample counter is reset to zero.

In the setpoint reset section of the algorithm, the zone with minimum use of

the reheat coil is determined by examining the summation of digital inputs

from all the zone reheat coils. The minimum value is converted to a fraction of time that the reheat was on by dividing the summation with the

total number of samples taken (DRSAMP/DRTSKI).

Once a value for the minimum reheat on-time fraction is obtained, a software

loop is entered. Each transition of the loop is used to set one of several

possible supply air setpoints that the algorithm is capable of modifying. To

determine the setpoints, a simple method is used. Six parameters must be

specified and stored in a software table. Sample data for such a table is

given in table 6. The parameter ARSTAT, in the last column, indicates wh it task should be used for supply air reset. A 2 indicates zone demand supply

49
air reset, and a 0 is for no reset (manual adjustment). This value is checked

for each setpoint and if the value is not 2, the rest of the loop is skipped.

If the current value of the setpoint is above the value T

r

- max or below the

value T

r

_m £n

, the setpoint is not changed.

If the setpoint is within bounds, the parameters in the first two columns of

the supply air reset parameter table are used to determine what action to

take. If the minimum reheat on-time fraction is greater than Rmax » this

indicates that there is too much reheat being used and that the setpoint is

too low. If the setpoint is too low, an error value is calculated as the

minimum reheat on-time fraction in percent minus Rmax» This error value is

multiplied by the parameter R ste p» which is really a proportional gain, to

produce a correction to the setpoint. The setpoint is increased by this

correction, checked to ensure that the setpoint is not greater than the

maximum specified, and the task is exited.

If the minimum reheat on-time fraction is less than than R •„ or equal to

zero, this indicates that the setpoint is too high and that the zone with

minimum reheat is probably too warm. If the zone is too warm it cannot be

cooled unless the supply air setpoint is lowered. If an error were calculated

between the minimum reheat on-time fraction and Rman» it would be very close

to R - n

» since the minimum reheat on-time fraction is probably zero. Therefore in this case, the setpoint correction is determined by multiplying

R step

^ Rmin“ The set P°int is decreased by this correction, checked to ensure that the setpoint is not less than the minimum specified, and the task

is exited.

If the minimum reheat on-time fraction is between Rm ^n and Rmax» no correction

to the setpoint is made. Under normal conditions, this condition would be

expected to occur most of the time, unless the sample interval is very large.

In tests of the FID connected to the building system emulator, the supply air

setpoint successfully adapted to changes in conditions such as outside air

temperature and building occupancy. Eventually the algorithm will be tested

on an actual building.

Table 6. NBS FID zone demand supply air reset parameter table

setpoint

index no. ^ax ^in Rstep Tr-mm Tr-max arstat

1 10.0 5.0 0.033 12.2 16.5 2

2 8.0 2.0 0.025 13.0 16.0 2

3 9.0 6.0 0.050 13.0 16.0 2

4 6.0 4.0 0.050 13.0 16.0 0

5 10.0 1.0 0.050 13.0 16.0 0

Note; ^ax and ^in are in Rstep is in de S rees c/ % » and Vmin and Vmax

are in degrees C.

50
7.5 Example oJL Dav/Night jgflthftfik

An implementation of a day/night setback algorithm was installed in the NBS

FID software. The algorithm follows closely the algorithms presented in

figures 11 and 12 for shutdown and startup of a zoned air handling system.

The startup and shutdown algorithms were written in FORTRAN and organized as two separate software tasks (see section 7.1). A FORTRAN listing of the the

actual software may be found in appendix C. The startup and shutdown tasks

were executed under control of a time-of-day control task* allowing them to be

scheduled for any time of day on any day of the week. A description of the

FID time-of-day control software may be found in reference [4].

For this example of day/night setback algorithms, there is a special air

handling unit control task which maintains the supply air temperature from the

unit at a setpoint by manipulating outside air dampers and heating and cooling

coils. The air handler control task is described in section 7.2. The air

handler control task was designed to always operate in one of five modes:

normal, stop, off, vent* and purge. The air handler controller is in normal

mode when the building is occupied and space thermostats are not in a setback

condition. If the air handler controller is placed in the stop mode, fans are

stopped, outside air dampers and all cooling and heating valves are shut, and

the controller then enters the off mode. In the off mode, the air handling

unit is inactive, and provides no space conditioning. The vent and purge

modes are used for ventilation purge, as described in section 7.6.

The detailed implementation of the setback shutdown and startup algorithms of

figures 11 and 12 is dependent on what type of HVAC system is being

controlled. The setback algorithm was designed to work with a specific HVAC

system emulated on the Building System Emulator (see introduction to chapter

7.). The HVAC system for this example is a constant volume, terminal reheat

system, supplying conditioned air to two zones. The zones each have a reheat

coil which is controlled by a local on-off thermostat. The thermostats, which

must have a normal and setback setpoint manually programmed at the thermostat

location, have a digital input which causes the thermostat to use the normal

setpoint when the input is in a logical low state, and to use the setback

thermostat when the input is in a high state. The FID controls the input to

the thermostat by using one of the FID hardware digital outputs. All

thermostats in the zones serviced by the air handler are assumed to be

connected to a single output.

The shutdown task is a short routine. When the task is executed, it begins by

shutting off four motors driving pumps and fans which are considered to be

unnecessary when the building is unoccupied or setback. These devices are controlled using a routine, DIGOUT, which is designed to turn off a specified

digital output. As a next step, the shutdown task places the air handling

control task in the stop mode. This is done by changing a mode variable,

AHSTAT, stored in a memory area common to the setback tasks and the air

handler control task. The next time that the air handler control task

executes, it will go through the actions to stop the air handler, which

consist of stopping the supply air fan 0 and closing, in sequence, al , controlled devices such as dampers and coils. The space thermostat, art- commanded to use the setback setpoint by using the digital output control

51
routine, DIGOUT, to turn on an output which is connected to ail of the space

thermostat setback/normal control inputs.

The startup task is slightly more complicated than the shutdown task.

Basically, all actions in the startup task are the opposite of the shutdown

actions. The four miscellaneous motors which were turned off by the shutdown

task are turned back on, but to avoid a large electrical demand peak, they

must not be turned on at the same time. Therefore a special routine, DELAYD,

is used to turn on the outputs. This routine is called in the same manner as DIGOUT, used to turn off the outputs, but DELAYD allows the specification of a delay time before the output is turned on. The use of staggered delays

prevents the demand peaks. Since there is no ’start'* mode for the air handler

controller, the air handler fan is turned on directly from the startup task,

using the DELAYD routine. Also the air handler mode variable is set to the

’normal” state. After the air handler has been started, the space thermostats

are ordered to cease using the setback space setpoint by turning off the

digital output connected to the zone thermostats. Since a task to reset the

supply air setpoint is active in the FID (section 7.4) and the task may have

based its current setpoint value on data taken from an inactive air handler,

it is desirable to pick an initial value for the supply air setpoint. This is

accomplished by calling an outside air supply air setpoint reset task (section

7.3) to select a setpoint based on the current outside air temperature.

In tests with the FID connected to the building system emulator, the building was successfully setback and returned to normal operation on a programmable

schedule. No difficulties were encountered in the tests. Eventually the

algorithms will be tested on an actual building.

7 .6 £x.ampl£ Ventilation Purging Al&flUktfcffi

An implementation of a ventilation purging algorithm was installed in the NBS

FID software. The algorithm follows closely the algorithms presented in

figures 14, 15, and 16. The ventilation purge algorithm was written in

FORTRAN and organized as an independent software task (see section 7.1). A

FORTRAN listing of the the actual software may be found in appendix D. The

purge algorithm was intended to be executed under control of a time-of-day

control task, allowing it to be scheduled for a time of day a short time prior

to the scheduling of the startup task described in section 7.5. A description

of the FID time-of-day control software may be found in reference [4].

For this example of a ventilation purging algorithm, there is a special air

handling unit control task to maintain the supply air temperature from the

unit at a setpoint by manipulating outside air dampers and heating and cooling

coils. The air handler control task is described in section 7.2. The air

handler control task was designed to always operate in one of five modes:

normal, stop, off, vent, and purge. The normal and stop modes were described

in section 7.5. In the off mode, the air handling unit is inactive, and

provides no space conditioning. To start a purge, the purge algorithm causes the air handling unit controller to enter the vent mode. In the vent mode,

the air handler fan is turned on, and the outside air damper is moved to the

fully open position. The controller then automatically enters the purge mode,

where the fan is operating and the damper is open. To stop purging the

52
algorithm causes the controller to enter the stop mode which shuts down the

fan and closes the dampers.

The ventilation purging task is organized as a main subroutine and three

subordinate function subroutines. The main routine is a direct implementation

of figure 14. The decision points in the figure are implemented as two

subroutines, one for determining if the building space requires cooling and

one for determining if cooling by outside air is available.

The main ventilation purging task begins by determining the current values of

the outdoor air temperature and the space temperature. Humidity

considerations were not included in this example, so outdoor air and space dry

bulb temperature were used. The outdoor air temperature is contained in one element of an array of analog input variables, whose index is stored in a table of parameters used in conjunction with the outdoor air economizer

algorithm for the air handler (see section 7.2).

The space temperature is approximated by using a value for the return air

temperature pointed to in the same manner as the outdoor temperature. When

the fan is running, this is a valid method of obtaining an average space

temperature. In an actual building, when the fan is stopped, the return air

temperature no longer represents the space temperature. In this example,

however the FID is connected to the building system emulator device (see

chapter 7 introduction). In the emulator the return air temperature can be

defined to be equal to the space temperature when the fan is off. This step

was taken to reduce the number of sensors and the algorithm development and

testing time. In a normal building the space temperature would be taken from

sensors actually located in the space.

Once the room and space temperatures are known, the remainder of the main

routine is implemented using logical ”if-then” structures. The coolingavailable and space-needs-cooling decisions are implemented as FORTRAN in-line

functions. These two functions make use of a number of parameters which must

be selected by the building engineer or operator to customize the algorithm to

a particular building air handling unit. Table 7 lists the parameters, brief

descriptions and values used in the NBS FID software.

The space-needs-cooling function closely follows figure 15. The tests for

whether the current room temperature is within certain bounds are implemented

by comparing the room temperature to the values for ROOMAX and ROOMIN as defined in table 7.

A more difficult problem is determining the trend of the room temperature.

This test requires more than a simple comparison with a constant. A separate

routine is used to perform this test (RTDROP). The output of the routine is a logical value for whether or not the room temperature is steadily falling.

Each time the routine is used, the current room temperature is subtracted from

the former room temperature. This change is compared to a constant, PDT.W IN. defined in table 7. If the change is smaller than PDTMIN (including negativ

values), the change is then identified as an upward or steady movement. I! the change is larger than PDTMIN, the change is identified as a downward

movement. A counter keeps track of how many times an increase has occurred

and how many times a decrease has occurred. Basically as long a3 the up tr-u :

53
counter is less than the value NPURSA# defined in table 7, the room

temperature is considered to be falling steadily. If the down trend counter

exceeds NPURSA, the up trend counter is reset to one. If the movement is

upward the down counter is reset to one. This means that a steadily falling

temperature will keep the logical output at true, but an oscillating or rising

output will cause the logical output to be false, after a delay which is

determined by the value of NPURSA. If the room temperature is determined to

be falling steadily using the room temperature trend test from the coolingneeded routine, then this means that cooling is considered unnecessary.

The cooling-available routine is constructed very much like the cooling-needed

routine, except that the logic is based on figure 16. If a purge is not

occurring, then ventilation purging is considered to be capable of providing

cooling if the outside temperature is above OATMIN, defined in table 7, and

the difference between the outside and indoor temperatures is greater than

STPDEL# also defined in table 7. If purging is currently occurring, then the

room temperature trend test routine is used to determine if the room

temperature is falling steadily. If it is, then cooling is still considered

to be available. If room temperature is not falling steadily, this means that

it has not been falling steadily for a reasonably long time, and then cooling

is considered to be unavailable.

name

Table 7 Parameters used by the ventilation purging algorithm

definition

e.g.

value

ROOMAX -

ROOMIN -

PDTMIN -

NPURSA -

STPDEL -

OATMIN -

maximum room temperature, above which the building space is

considered to require cooling by ventilation purging, when

no purging is currently occurring.

minimum room temperature, below which building space is

considered to not require further cooling# when purging is

currently taking place.

the smallest decrease in the value of the space temperature

that can occur between successive executions of the purge

algorithm and be recognized as indicating a definite

downward trend.

maximum value of a counter used to determine whether the

building space temperature has a downward, upward, or constant trend. The count is of the number of samples of

room temperature taken at algorithm executions which exibit

a definite upward or downward trend, based on PDTMIN.

minimum differential between outside air temperature and

indoor temperature which the current differential must

exceed to start purging.

minimum outside temperature, below which there is a danger

of freezing heat exchange coils if ventilation purging is

performed

.

22 C

19 C

0.01 C

20

3 C

5 C

54
In tests of the ventilation purge algorithm in the FID connected to the

building system emulator* the building was setback using the routines

described in section 7.5. When conditions were correct the ventilation purge

correctly started the purge. The purge was terminated when the room

temperature ceased to decrease. Figure 13 represents data from an actual test

using the BSE. Eventually the algorithms will be tested on an actual

building.

55
8. CONCLUSIONS

This report has described algorithms developed for use in the NBS Building

Management and Controls Laboratory for several supervisory building control

strategies. The strategies implemented were supply air setpoint reset*

day/night setback, ventilation purging, and supply water reset. The

algorithms were tested using an actual microprocessor-based FID running

software incorporating the algorithms. The algorithms developed performed

successfully on a test system using a microprocessor driven building system

emulator (BSE) rather than an actual building system. Further research will

be required to determine how the algorithms perform when connected to a real

HVAC system and to measure the performance of the algorithms in terms of

energy savings, accuracy of control, and reliability.

56
9. REFERENCES

[1] U.S. Army Corps of Engineers. Energy Monitoring and Control Systems.

Technical Manual TM5-815-2/AFM 88-36/NAVFAC DM-4.9. Dept, of the Army,

Air Force, and Navy, Washington, D.C. 1981 September.

[2] American Society of Heating, Refrigerating, and Air-Conditioning

Engineers, Inc. ASHRAE Handbook and Product Directory, 1980 Systems.

Chapter 3. 1980.

[3] Paoluccio, J.P. Dead Band Controls Guide. ASHRAE Transactions. Vol 86,

part 1, 1980.

[4] May, W. B. Time of Day Control and Duty Cycling Algorithms for Building

Management and Control Systems. Nat. Bur. Stand. (U.S.) NBSIR 83-2713;

1983 June, 60 p.

[5] Park, C. An Optimum Start/Stop Control Algorithm for Heating and Cooling

Systems in Buildings. Nat. Bur. Stand. (U.S.) NBSIR 83-2720; 1983 May, 69

P*

[6] May, W.B.; Borresen, B.A.; Hurley, C.W. Direct Digital Control of a Pneumatically Actuated Air Handling Unit. ASHRAE Transactions. Vol 88,

part 2, 1982.

[7]

Park, C. Economizer Algorithms for Energy Management and Control Systems.

Nat. Bur. Stand. (U.S.) NBSIR 84-2832, 1984 February, 81 p.

57
APPENDIX A - Outside Air Supply Air Reset Control Routines

This appendix contains the FORTRAN IV source code for the outside air supply

air reset control routines used in the NBS FID. The routines are taken out of

the complete FID program that they are designed to work with, but still

provide an example of actual source code for outside air supply air reset.

• •••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••a

SUBROUTINE ARESET

C

C This routine is used to reset the supply air setpoint of an air

C handling unit. This version uses the outside air reset method, where

C the setpoint is a linear function of outside air temperature within

C a range of outside air temperatures.

C

C VARIABLE DEFINITIONS:

C

C COMMON BLOCK SETPOINT:

C TREF - array of setpoints used by the air handler task, each resettable.

C

C COMMON BLOCK AIRSET:

C TOAMAX - array of outside air temperatures where the setpoints reach TRMAX.

C TOAMIN - array of outside air temperatures where the setpoints reach TRMIN.

C TRMIN - array of minimum possible values for setpoints in TREF.

C TRMAX - array of maximum possible values for setpoints in TREF.

C TOAMID - array of outside air temperatures where setpoints reach TRMID.

C TRMID - array of values for setpoint in TREF between two linear regions.

C ARSTAT - array of values to indicate what type of reset strategy to use.

C 0 = no reset; 1 = outside air reset; 2 - zone demand reset

C

C COMMON BLOCK ANALOG:

C ANAI - array of analog input values obtained by data input task.

C

C COMMON BLOCK OADAMP: from economizer task; TRETPT, TCODIF, TOAMN not used.

C TOAPT - two part integer pointing to the value in ANAI for outside temp.

C 1st 8 bits = MUX number; 2nd 8 bits = point number

C

C FULL - copy of TOAPT used to split TOAPT into two parts: HALF (1),HALF(2)

C MUX - mux ID number where outside air temperature is taken.

C IPNT - point ID number within MUX for outside air temperature sensor.

C TOA - current outside air temperature.

C TRLO - low setpoint limit for linear interpolation.

C TRHI - high setpoint limit for linear interpolation.

C TOAHI - high outside air temperature for linear interpolation.

C TOALO ~ low outside air temperature for linear interpolation.

C SLOPE - slope of line joining (TOALO, TRLO) and (TOAHI, TRHI)

.

C

INTEGER FULL, TRETPT, TOAPT

REAL TCODIF, TOAMN

INTEGER* 1 HALF( 2)

REAL*8 ANAI

REAL TREF

58
REAL TOAMAX , TOAMIN , TRMIN , TRMAX, TOAMID * TRMID

INTEGER ARSTAT

C

COMMON/AIRSET/TQAMAX( 5) , TOAMIN ( 5) ,TRMIN(5) ,TRMAX(5)

& . TOAMID ( 5 ) , TRMID( 5 ) . ARSTAT ( 5

)

COMMON/ SETPNT/TREF( 5)

COMMON/ANALOG/ANAI ( 1 ,32)

COMMON/ OADAMP/TRETPT , TOAPT , TCODIF , TOAMN

EQUIVALENCE ( FULL, HALF ( 1 )

)

C

C determine outside air temperature

FULL=TOAPT

MUX=HALF(2)

IPNT=HALF( 1

)

TOA=ANAI ( MUX, IPNT)

C change setpoint value as a function of outside air temp.

DO 3000 1=1,5

IF(ARSTAT(I) .NE.l)GO TO 3000

IF(TOA.LT.TOAMIN(l))GO TO 1000

IF(TOA.GT.TOAMAX(l))GO TO 2000

IF(TOA.GT.TOAMID(l))GO TO 500

C outside air temperature between TOAMIN and TOAMID

TRLO=TRMID( I)

TRHI=TRMAX(l)

TOAHI=TOAMID(l)

TOALO=TOAMIN(l)

GO TO 600

C outside air temperature between TOAMID and TOAMAX

500 TRLO=TRMIN(l)

TRHI=TRMID(I)

TOAHI=TOAMAX(l)

TOALO=TOAMID(l)

C calculate new setpoint by linear interpolation

600 slope=(trlo-trhi)/ (toahi-toalo)

TREF(l)=SLOPE*(TOA-TOAHI)+TRLO

GO TO 3000

C if air temperature is outside of limits, fix setpoint1- 1000 TREF ( I

) =TRMAX( I

)

GO TO 3000

2000 TREF(I)=TRMIN(I)

3000 CONTINUE

RETURN

END

59
APPENDIX B - Zone Demand Supply Air Reset Control Routines for

Constant Volume Reheat System

This appendix contains the FORTRAN IV source code for the zone demand supply

air reset control routines used in the NBS FID. The routines are taken out of

the complete FID program that they are designed to work with, but still

provide an example of actual source code for zone demand supply air reset.

C :::::::::: s :::::: : s :::::::::::::::::::::::::::::::::::::::::::::::::::

SUBROUTINE DRESET

C ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

C This routine is used to reset the supply air setpoint of an air

C handling unit. This version uses the demand determination method, where

C the demand is determined from the measured usage of reheat coils in the

C zones served by the air handler. The setpoint is moved up or down by a

C step change when the reheat usage is outside of specified limits.

C

C VARIABLE DEFINITIONS;

C

C COMMON BLOCK TSKINT:

C H,M,S,TS - task execution interval times from task table (hrs, mins, etc.).

C

C COMMON BLOCK DIGITA: from digital data base. DOPRI, DIGO not used here.

C DIGIN - array of current digital input values.

C

C COMMON BLOCK RHDEMA

C RHMAX - array of maximum desirable values for % of maximum reheat used.

C RHMIN - array of minimum desirable values for % of maximum reheat used.

C RHSTEP - proportional constants to correct setpoint, applied to ERROR.

C DRSAMP - sampling period (s) for adjusting setpoint and summing reheat.

C

C COMMON BLOCK AIRSET:

C TOAMAX - array of outside air temperatures where the setpoints reach TRMAX.

C TOAMIN - array of outside air temperatures where the setpoints reach TRMIN.

C TRMIN - array of minimum possible values for setpoints in TREF.

C TRMAX - array of maximum possible values for setpoints in TREF.

C TOAMID - array of outside air temperatures where setpoints reach TRMID.

C TRMID - array of values for setpoint in TREF between two linear regions.

C ARSTAT - array of values to indicate what type of reset strategy to use.

C

C COMMON BLOCK SETPNT;

C TREF - array of setpoints used by the air handler task, each resettable.

C

C DRTSKI - task interval(s) at which DRESET executes. From FID task table.

C ERROR - difference between measured and maximum % of maximum reheat.

C MAXCNT - number of samples required before reset calculation can be made.

C MCOUNT - counter holding current number of samples that have been taken.

C MINRHV - current minimum value of reheat used for one zone.

C NRHZON - number of zones with reheat status sensors.

C RHPCT - minimum measured percentage of maximum reheat usage.

C RHZONE - array of counters for summing times reheat is on when sampled.

INTEGER ARSTAT

60
INTEGER RHZONE( 2)

BYTE DIGO ,DOPRI , DIGIN

REAL TREF

BYTE H,M,S,TS

COMMON/TSKINT/R.M,S,TS

COMMON/DIGITA/DIGO( 1 , 24) ,DOPRl( 1 . 24) ,DXGIN( 1®X6)

COMMON/ RHDEMA/RHMAX( 5) » RHMIN ( 5 ) ,RHSTEP( 5) .DRSAMP

COMMON/AIRSET/TOAMAX( 5) ,TOAMIN( 5) ,TRMIN( 5) ,TRMAX( 5)

& , TOAMID ( 5 ) , TRMID ( 5 ) , ARSTAT ( 5

)

COMMON/ SETPNT/TREF ( 5

)

DATA RHZONE/2*0/ .MCOUNT/O/ .NRHZON/2/

C determine execution interval --- CALL TSKCHK ( 7 » IEDOS

)

DRTSKI=M*60+S

MAXCNT=DRSAMP/DRTSKI

C determine reheat coil usage

DO 100 1=1 »NRHZON

J=I+6

RHZONE(I) =RHZONE ( I ) +DIGIN ( 1 , J

)

100 CONTINUE

MCOUNT=MCOUNT+

1

IF ( MCOUNT . LT . MAXCNT ) GO TO 9000

MCOUNT=0

C Determine minimum reheat usage for all zones---

MINRHV=MAXCNT

DO 200 1=1 .NRHZON

IF(RHZONE(l) .GT. MINRHV) GO TO 190

MINRHV=RHZONE ( I

)

190 RHZ0NE(I)=0

200 CONTINUE

RHPCT=FLOAT(MINRHV) /FLOAT(MAXCNT )*1 00

.

C change setpoint value as a function of reheat usage--

DO 5000 1=1,5

IF (ARSTAT ( I ) . NE . 2 ) GO TO 5000

C if air temperature is outside of limits, fix setpoint

IF ( TREF (I) .GT.TRMAX(l) )TREF(l)=TRMAX(l)

IF ( TREF ( I ) . LT . TRMIN ( I ) )TREF ( I

) =TRMIN ( I

)

IF (RHPCT .GT .RHMAX(l) )GO TO 3000

IF ( RHPCT . LE . RHMIN ( I ) ) GO TO 4000

C no change in setpoint desired

GO TO 5000

C too much reheat - raise setpoint

3000 ERROR=RHPCT-RHMAX( I

)

TREF ( I

) =TREF ( I ) +RHSTEF ( I

) *ERROR

IF (TREF ( I) .GT . TRMAX( I) )TREF ( I

) =TRMAX( I

)

GO TO 5000

C not enough reheat - lower setpoint - 4000 TREF ( I

) =TREF ( I

) -RHSTEP ( I

) *RHMIN ( I

)

if(tref(i) .lt.trmin(i) )tref(i)=trmin(i)

5000 CONTINUE

9000 CONTINUE

RETURN

END

61
APPENDIX C - Day/Night Setback Control Routines

This appendix contains the FORTRAN IV source code for the day/night setback

routines used in the NBS FID. The routines are taken out of the complete FID

program that they are designed to work with, but still provide an example of

actual source code for day/night setback.

C: ::::::::::::::::::::::::::::: :::::::: :::::::: ::::::::::::::::: SUBROUTINE STARUP

C This routine is used as a task for morning startup of a building

C including space thermostat setup, air handling unit controller

C activation and starting of fans and pumps which have been off.

C

C VARIABLE DEFINITIONS:

C

C COMMON BLOCK AIRSET: from supply air reset task, only one variable used:

C ARSTAT - array of values to indicate what type of reset strategy to use.

C

C COMMON BLOCK ACTUA: from air handler control task, only one variable used:

C AHS TAT - variable indicating current status of air handling unit.

C "n" = normal

C

C THREE, FOUR, FIVE, SIX, FAN, SETBAK - pointers to digital outputs for various

C devices. Two parts: MUX and Point no. BYTE ON, AHSTAT, NORMAL

INTEGER SIX, THREE, FOUR, FIVE, T, FAN, SETBAK

INTEGER 0PEN1 , HEAT, DAMPER, DCLOSE.DOPEN, BAND, ARSTAT

REAL TACT , TOAMAX , TOAMIN , TRMIN , TRMAX, TOAMID , TRMID

C

COMMON/AIRSET/TOAMAXC 5) ,T0AMIN(5) ,TRMIN(5) ,TRMAX(5)

&, TOAMID ( 5 ), TRMID ( 5 ), ARSTAT ( 5

)

COMMON/ACTUA/0PEN1 (5) ,HEAT( 5) ,DAMPER( 5) .TACT (5) ,DCL0SE(5) ,D0PEN(5)

&,BAND( 5), AHSTAT

C

DATA SlX/X'OlOe'/ ,THREE/X f 0103V *F0UR/X f 0104*/ ,FIVE/X* 0105 '/

DATA FAN/X’01 18 ’/ .SETBAK/X* 0117'/, ON/1/, NORMAL/'N'/

C activate miscellaneous devices

CALL DIGOUT (THREE, 1.1 27)

CALL DELAYD( 2,FOUR,l 27 ,ON,0 ,1

)

CALL DELAYD(2,FIVE,1 27 ,ON,0, 2)

CALL DELAYD( 2,SIX,1 27 ,ON,0 ,3

)

C activate main air handler fan

CALL DELAYD(2,FAN,127 , ON, 0,5)

AHSTAT=NORMAL

C send signal to space thermostats to set up

CALL DIGOUT (SETBAK.0,1)

C initialize supply air temperature reset value

DO 100 1=1,5

ARSTAT(I)=1

100 CONTINUE

CALL ARESET

DO 200 1=1,5

62
o

o

ARSTAT(l)=2

200 CONTINUE

RETURN

END

• ••••••••••••••••••••a*** • ••••••••••••••••••••••a*

SUBROUTINE SHUTDN

C This task is used to shut down the air handling unit, pumps and fans*

C and set back space thermostats for an unoccupied period.

C

INTEGER SIX, THREE, FOUR, FIVE, SETBAK

INTEGER 0PEN1 .HEAT, DAMPER, DCLOSE .DOPEN, BAND

BYTE AHSTAT, STOP

C

COMMON/ACTUA/ OPEN1 ( 5 ) , HEAT ( 5 ) , DAMPER ( 5 ) , TACT ( 5 ) .DCLOSE ( 5 ) , DOPEN ( 5

)

& ,BAND( 5) .AHSTAT

C

DATA SIX/X' 0106 V.THREE/X' 0103 V,FOUR/X' 0104 V,FIVE/X' 0105 V

DATA SETBAK/X’ 0117 V ,ST0P/ f S'/

C shut off miscellaneous devices---

CALL DIGOUT(SIX.O.l)

CALL DIGOUT (THREE, 0,1)

CALL DIGOUT (FOUR, 0,1)

CALL DIGOUT (FIVE, 0,1)

C set air handler to stop

AHSTAT=STOP

C set back space thermostats

CALL DIGOUT (SETBAK, 1 ,1)

RETURN

END

C=====rr====r=r================r======================rr=r====rr========

SUBROUTINE DELAYD ( MOTSK , OUT , PRIOR , ONOFF ,MIN , SEC

)

C======================r==r==r====r=====rr====r===r====r=rr=============

C this routine is called to turn a load off or on, after a specified time

C interval. An available on-off task is found, where DOTSK is the number

C of the highest on-off task which can be used to control the load. OUT is

C the digital output to control, PRIOR is the priority to control the load

C under, ONOFF is 1 to turn on a load, 0 to turn it off, and MIN and SEC

C are the time interval that should elapse before the output is controlled.

C COMMON BLOCK ONOFF contains a table of on-off task parameters that are

set before the task can be used.

INTEGER PRIOR, OUT, MOTSK

BYTE H,M,S,TS,TSK, ONOFF

BYTE DONOFF, INUSE

INTEGER MIN.SEC.IPR.DNUM

BYTE CNTRL , SCAN0 , SCMAST , SCSLAV .ONOFFT , TSKMAX, DCMAST

COMMON/TSKINT/H.M, S ,TS

COMMON/ ONOFF/DNUM( 1 6 ) .DONOFF (16), INUSE( 1 6 ) , I PR ( 1 6

)

COMMON/TSKLNK/CNTRL( 2) ,SCAN0, SCMAST, SCSLAV(16)

,

&ONOFFT(16) .TSKMAX, DCMAST

DATA N00TSK/16/

CALL LOCK

63
C find unused on/off task pair

ITSK=0

DO 1000 I=MOTSK , NOOTSK

IF(INUSE(l) .NE.O)GO TO 1000

XTSK=I

GO TO 2000

1000 CONTINUE

ITSK=16

2000 INUSE(ITSK)=1

C set parameters in on-off task table

INUSE(ITSK)=1

IPR(ITSK)=PRIOR

DNUM(ITSK)=OUT

DONOFF ( ITSK ) =ONOFF

C cause on-off task to execute after a delay time

TS=0

H=0

M=MIN

S=SEC

CALL TSKEDT(ONOFFT( ITSK). -1,1)

CALL UNLOCK

RETURN

END

64
o

o

APPENDIX D - Ventilation Purging Control Routines

This appendix contains the FORTRAN IV source code for the ventilation purging

routines used in the NBS FID. The routines are taken out of the complete FID

program that they are designed to work with# but still provide an example of

actual source code for ventilation purging.

C ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

SUBROUTINE VEPURG

C: :::::::: ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

C This task is used to determine if ventilation purging would be useful

C and if so to start the purging. If purging is currently occurring# the

task determines if it is useful to continue. If not# purging is stopped.

C VARIABLE DEFINITIONS:

C

C COMMON BLOCK ACTUA: from air handler control task, only one variable used:

C AHSTAT - variable indicating current status of air handling unit.

C

C COMMON BLOCK ANALOG:

C ANAI - array of analog input values obtained by data input task.

C

C COMMON BLOCK 0ADAMP: from economizer task; TRETPT, TCODIF# TOAMN not used.

C TOAPT - two part integer pointing to the value in ANAI for outside temp.

C 1st 8 bits = MUX number; 2nd 8 bits = point number

C

C FULL - copy of TOAPT used to split TOAPT into two parts: HALF (1),HALF(2)

C OFF, PURGE, VENT# STOP - possible air handler states

C MUX - mux ID number where a desired sensor is located.

C IPNT - point ID number within MUX for desired sensor.

C ROOMT - current space temperature.

C OUTSIT - current outside air temperature.

C

LOGICAL NEEDCL , CLAVAL

INTEGER OPENl ,HEAT»DAMPER,DCLOSE,DOPEN, BAND, FULL, TRETPT, TOAPT

BYTE AHSTAT , PURGE , VENT , OFF .STOP , HALF ( 2

)

REAL TACT, TCODIF, TOAMN

REAL*8 ANAI

C

COMMON/ACTUA/0PEN1 ( 5) , HEAT ( 5 ) ,DAMPER(5) .TACT (5) ,DCL0SE(5) ,DOPEN( 5)

&,BAND( 5), AHSTAT

COMMON/ANALOG/ANAI (1,32)

COMMON/OADAMP/TRETPT , TOAPT .TCODIF , TOAMN

C

EQUIVALENCE (FULL, HALF (1 )

)

DATA PURGE/ ’ P

' / .VENT/ * V ’ / , OFF/ ’O'/, STOP/ * S *

/

C --determine outside air temperature

FULL=TOAPT

MUX=HALF( 2)

IPNT=HALF( 1

)

OUTSIT=ANAI(MUX, IPNT

)

65
oooo

o

n

o

oo

o determine space air temperaturein this case, use return air temperature since in emulator return air

temperature equals space temperature also when fan is off.

FULL=TRETPT

MUX=HALF( 2)

IPNT=HALF( 1

)

ROOMT=ANAI ( MUX , I PNT

)

IF(AHSTAT.EQ.PURGE)GO TO 2000 — a ir handler currently off

IF( AHSTAT .NE .OFF )G0 TO 9000

IF(NEEDCL(AHSTAT,ROOMT).EQ.. FALSE. )G0 TO 9000

IF ( CLAVAL (AHSTAT, OUTS IT, ROOMT) ,EQ. .FALSE. )G0 TO 9000

AHSTAT=VENT

GO TO 9000

---air handler currently in purge mode--

2000 IF (NEEDCL( AHSTAT, ROOMT ).EQ.. FALSE.) GO TO 3000

IF (CLAVAL (AHSTAT, OUTSIT, ROOMT). EQ.. FALSE. )GO TO 3000

GO TO 9000

3000 AHSTAT=STOP

9000 RETURN

END

FUNCTION NEEDCL(AHSTAT, ROOMT)

This function determines whether the space currently needs cooling.

LOGICAL NEEDCL.RTDROP, CLEAR

BYTE AHSTAT, PURGE, OFF

COMMON/ PURGPA/ROOMIN , PDTMIN , NPURSA, STPDEL , OATMIN , ROOMAX

DATA PURGE 9 0FF/»P' ,'OV

IF(AHSTAT.EQ.PURGE)GO TO 500

C ---not currently purging

NEEDCL=. FALSE.

C is space temperature changing by itself, rising or falling?

NEEDCL=( .NOT. RTDROP( NPURSA, ROOMT, PDTMIN)

)

IF( .NOT.NEEDCL)GO TO 900

C does space currently need cooling?

NEEDCL= ( ROOMT .GT . ROOMAX)

IF ( . NOT . NEEDCL ) CLEAR=RTDROP ( 0 , ROOMT , PDTMIN

)

GO TO 900

C currently purging-i6 space becoming too cold?

500 NEEDCL=. FALSE.

IF ( ROOMT . GT . ROOMIN ) NEEDCL= . TRUE

.

IF ( .NOT . NEEDCL ) CLEAR=RTDROP ( 0 , ROOMT , PDTMIN

)

900 CONTINUE

RETURN

END

66
FUNCTION CLAVAL(AHSTAT,OUTSIT*ROOMT

)

C This function is used to determine if cooling is available by the use

C of ventilation purging.

C

LOGICAL CLAVAL , RTDROP

BYTE AHSTAT, PURGE. OFF

COMMON/ PURGPA/ ROOMIN , PDTMIN , NPURSA. STPDEL , OATMIN , ROOMAX

DATA PURGE. OFF/’P’

,

8 OV

C

CLAVAL=. FALSE.

IF(AHSTAT.EQ.PURGE)GO TO 500

C is outside air too cold for purging (coil freeze)

IF ( OUTS I T.LT. OATMIN) GO TO 900

C can outside air provide useful cooling?

IF( (ROOMT- OUTS IT) ,LT.STPDEL)GO TO 900

CLAVAL= .TRUE.

GO TO 900

C is outside air too cold for purging (coil freeze)

500 IF (OUTS I T.LT. OATMIN) GO TO 900

C currently purging-is outside air still cooling ?-

CLAVAL=RTDROP ( NPURSA, ROOMT , PDTMIN

)

900 CONTINUE

RETURN

END

C==============================::======— ===============================

FUNCT ION RTDROP ( NPURSA . ROOMT , PDTMIN

)

C= = = = = = = = = = = = = = = = =: = = = :: = = = = = = = = = = = = = = = = = = = = = :: = = = = = = = = = = = = = = = = = = = = = = = = = = C This function is used to determine if there is a relative drop in

C room temperature since past samples were taken. If so, returns true.

LOGICAL TOP .TCDOWN, RTDROP

REAL OROOMT » OPDT * PDT

INTEGER ICDOWN.ICNTUP

DATA I CDOWN/ 1 / , I CNTUP/ 1 / » OROOMT/ 100./.0PDT/0./

RTDROP=. FALSE.

C determine change in room temperature (PDT)

PDT=OROOMT-ROOMT

C determine if up (TUP = T) or down (TDOWN = T)

TOP=(PDT.LT. PDTMIN)

TCDOWN= ( . NOT . TOP . AND . OPDT . GT . PDTMIN

)

C increment counters for up or down

IF ( TUP ) I CNTUP= I CNTUP+

1

IF ( TCDOWN) ICDOWN=XCDOWN+l

IF( . NOT. TCDOWN) I CDOWN=l

C decide on trend

IF (ICDQWN.GT.NPURSA) ICNTUP=1

IF ( I CNTUP . LT „ NPURSA) RTDROP= . TRUE

.

IF( .NOT. RTDROP )ICNTUP=1

OROOMT= ROOMT

OPDT=PDT

RETURN

END

67
U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date BIBLIOGRAPHIC DATA

REPORT NO.

SHEET (See instructions) NBSIR 84-2846 March 1984

4. TITLE AND SUBTITLE

CONTROL ALGORITHMS FOR BUILDING MANAGEMENT AND CONTROL SYSTEMS —

HOT DECK/COLD DECK/SUPPLY AIR RESET, DAY/NIGHT SETBACK, VENTILATION PURGING,

AND HOT AND CHILLED WATER RESET

5. AUTHOR(S)

William B. May, Jr,

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City. State, ZIP)

Office of Buildings and Community Systems U.S. Navy Civil Engineering Laboratory

U.S. Department of Energy U.S. Department of Defense

1000 Independence Avenue, NW Port Hueneme, CA 93043

Washington, DC 20585

10. SUPPLEMENTARY NOTES

33] Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information,

bibliography or literature survey, mention it here)

If document includes a significant

Software is an important component of building management and control systems (BMCS),

This report describes concepts, algorithms, and software used in BMCS components

developed in the NBS building systems and controls laboratory. The basic concepts,

considerations and general algorithms for hot deck/cold deck supply air setpoint

reset, day/night thermostat and ventilation setback, ventilation purging, and

hot/chilled water supply setpoint reset are presented. Reset is the changing of

a setpoint on a Heating, Ventilating and Air Conditioning (HVAC) system controlled

by a feedback controller to match the system output to the system load. Setback is

the changing of HVAC system operation to reduce energy use during unoccupied periods,

Purging is the use of outdoor air during unoccupied periods to reduce mechanical

conditioning requirements. Specific implementations of the algorithms in software

on an actual BMCS are presented as examples.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) air handling unit control; building management and control systems (EMCS,BMCS); chilled

water reset; computer control; control algorithms; control software; day/night setback;

energy management; hot water reset; heating, ventilating and air conditioning (HVAC);

outside air reset: supply air setpoint res et: ventilation purging.

13. AVAILABILITY

fy~1 Unl imited

[33] F° r Official Distribution. Do Not Release to NTIS

Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

[X] Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

74

15. Price

$ 10.00

USCOMM-DC 6043-P80