United States National Risk Management

Environmental Protection Research Laboratory

Agency Cincinnati, OH 45268

Project Summary

Research and Development EPA/600/SR-97/131 December 1997

Abstract

A series of computer runs has been

completed using the DOE-2.1E building energy model, simulating a small

(4,000 ft2

)* strip mall office cooled by

two packaged single-zone systems, in

a hot, humid climate (Miami). These

simulations assessed the energy penalty, and the impact on indoor relative

humidity (RH), when the outdoor air

(OA) ventilation rate of the office is

increased from 5 to 20 cfm/person in

this challenging climate to improve indoor air quality (IAQ). One objective

was to systematically assess how each

parameter associated with the building

and with the mechanical system impacts the energy penalty resulting from

increased OA. Another objective was

to assess the cost and effectiveness of

off-hour thermostat setup (vs. system

shutdown), and of humidity control (using overcooling with reheat), as means

for reducing the number of hours that

the office space is at an RH above 60%

at the 20 cfm/person ventilation rate.

With the baseline set of variables selected for this analysis, an OA increase

from 5 to 20 cfm/person is predicted to

increase the annual cost of energy consumed by the heating, ventilating, and

air conditioning (HVAC) system by

12.9%. The analysis showed that the

parameters offering the greatest practical potential for energy savings are

conversion to very efficient lighting and

Energy Costs of IAQ Control Through

Increased Ventilation in a Small

Office in a Warm, Humid Climate:

Parametric Analysis Using the DOE-2

Computer Model

D. Bruce Henschel

equipment (1.5 W/ft2) and conversion

to very efficient cooling coils (electric

input ratio = 0.284). If the increase to

20 cfm/person were accompanied by

either of these conversions, the 12.9%

HVAC energy penalty for the increased

OA rate would be eliminated; the modified system at 20 cfm/person would

have a lower annual HVAC energy cost

than the baseline system at 5 cfm/person. Other parameters offering significant practical potential for energy savings are: conversion from packaged

single-zone units to a variable air volume system; conversion to cold-air distribution (minimum supply air temperature = 42°

F); or improvements in the

glazing or in the roof resistance to heat

transfer. If the OA increase were accompanied by any one of these modifications, the 12.9% penalty would be

reduced to between 2 and 7% (the modified system at 20 compared against

the baseline at 5 cfm/person).

According to the DOE-2.1E model,

the increase in ventilation rate could

be achieved with an 85% reduction in

the number of occupied hours above

60% RH, compared to the baseline system at 5 cfm/person — with only a $19/

year increase in energy cost — if the

economizer were eliminated. That is,

most of the elevated-RH hours in the

baseline case were predicted to be the

result of economizer operation. If the

control system were modified so that it

controlled the humidity as well as the

temperature in the office space, all of

the elevated-RH occupied hours would

be eliminated, at an energy cost of $90/

year.

*Readers more familiar with metric units may use the

factors provided at the end of this Summary to convert

to that system.
2

Neither economizer elimination nor

humidity control would address unoccupied periods, when most of the elevated-RH hours occur. Building operators concerned about biological

growth at elevated RH should consider

operation of the cooling system during

unoccupied hours, perhaps with the

thermostat set up, rather than system

shut-down off-hours. Off-hour setup

from 75 to 81°F would add only $10/

year to energy costs, and would provide some modest reduction in unoccupied elevated-RH hours. Setup to

79°F would provide a greater reduction, at an energy cost of $38/year.

DOE-2.1E underestimates the number of elevated-RH hours because it

does not address the moisture capacitance of building materials and furnishings, or re-evaporation off the cooling

coils when they cycle off with the air

handler operating. As a result, the performance of the RH reduction steps

above may be overestimated, or the

costs of the steps underestimated.

This Project Summary was developed

by the National Risk Management Research Laboratory’s Air Pollution Prevention and Control Division, Research

Triangle Park, NC, to announce key

findings of the research project that is

fully documented in a separate report

of the same title (see Project Report

ordering information at back).

Introduction

Among the three basic techniques for

improving IAQ — improved ventilation, air

cleaning, and source management — improved ventilation is perhaps the most

commonly utilized. In ANSI/ASHRAE Standard 62-1989, ASHRAE recommended

that OA ventilation rates in office space

be increased from 5 to 20 cfm/person to

maintain acceptable IAQ. There will be an

energy penalty associated with an OA increase, which will usually be most pronounced in hot, humid climates. Also of

particular concern in humid climates, an

OA increase can result in increased indoor RH levels, which can be of concern

both from the standpoint of occupant comfort, and from the standpoint of fungal

growth.

Objectives and Approach

To assess these energy and RH penalties associated with increased ventilation,

a systematic series of computer simulations have been run using the DOE-2.1E

software to model a small (4,000 ft2) office

in a hot, humid climate (Miami). These

simulation runs comprised a parametric

analysis to systematically quantify how

each of the building and HVAC system

variables impacts energy consumption and

cost, and HVAC performance (in particular, indoor RH levels), at ventilation rates

of both 5 and 20 cfm/person.

By defining the building and HVAC parameters having the greatest impact on

HVAC energy consumption and cost, this

assessment was intended to suggest those

parameters which — if modified in conjunction with the increase in OA — could

at least partially offset the energy and

cost penalties associated with the increased ventilation rate. Likewise, by defining the parameters having the greatest

impact on indoor RH, the assessment was

intended to suggest parametric modifications which could reduce the RH impacts

of the OA increase.

As part of this analysis, the DOE-2.1E

model was used to further assess the

energy penalty and the effectiveness of

two specific approaches for reducing the

number of hours at RH levels greater than

60%. These approaches are: 1) turning

the thermostat up (rather than shutting

the HVAC system down) during unoccupied cooling hours; and 2) use of a humidity controller on the HVAC system,

employing overcooling and reheat as necessary to maintain the RH below 60%

during occupied hours.

This analysis did not address the equipment/installation costs associated with the

parametric variations, or any impact of the

variables on maintenance costs.

The Baseline Building and

HVAC System

The building type selected for this analysis was a small, one-story office in a strip

mall, with adjoining space (occupied by

other tenants) on either side. The office

had a frontage of 40 ft and a depth of 100

ft for a total floor area of 4,000 ft2, and

was subdivided into two 2,000 ft2

 zones

(of 40 by 50 ft). A small office was selected because the U. S. population

spends a substantial number of hours inside offices, and Government statistics indicate that approximately half of the office

buildings in the U. S. are 5,000 ft2 and

smaller.

The floor plan for this office is illustrated

in Figure 1.

Full occupancy is 27 persons (150 ft2/

person). The occupancy varies throughout the day on weekdays, between 6 am

to 7 pm. The building is unoccupied overnight (7 pm to 6 am), and all day on

weekends and holidays.

The baseline HVAC system consists of

two rooftop, constant-volume, packaged

single-zone (PSZ) units, one dedicated to

each of the 2,000 ft2 zones. The units

included electric resistance heating; annual heating requirements are minimal in

the Miami climate. Ventilation rates of both

5 and 20 cfm/person were considered.

The cooling setpoint was 75°F during occupied hours; the cooling was shut down

overnight and on weekends. The heating

setpoint was 70°F, set back to 55°F during off-hours. The cooling electric input

ratio (EIR) was 0.341 Btu/hr of electric

input per Btu/hr of cooling output, considered to be representative of modern PSZ

units.

The Impact of Building and

HVAC Parameters on the

Penalties Associated with

Increased Ventilation

Table 1 summarizes how each of the

building and HVAC system parameters

impacts the computed cooling coil capacity, the annual HVAC energy cost, and the

percentage of occupied hours having an

RH above 60%.

For ease in comparison, the impact of

each parameter is presented in Table 1

as the percentage change from the

baseline building and baseline system operating at a ventilation rate of 5 cfm/person. Under baseline conditions at 5 cfm/

person, the cooling capacity computed by

the software is 103.6 kBtu/hr (8.6 tons of

refrigeration), the annual HVAC energy

cost is $2,510, and the number of occupied hours above 60% RH is 40 hours per

year (1.2% of 3,276 occupied hours), as

shown by the first entry in the table.

All of the other entries are for operation

at 20 cfm/person.

The second entry in the table shows

the predicted impacts when the baseline

building and system are simply operated

at 20 cfm/person, without any other variations in the building and HVAC variables.

For example, this entry shows that operation at the increased ventilation rate increases HVAC energy cost by 12.9% (an

increase of $325, to $2,835 per year).

Use of the HVAC energy costs in this

table is intended to emphasize the impact

on the HVAC system. If one instead used

the total building energy costs — which

are $4,273 per year, including lighting and

equipment, at 5 cfm/person — the $325

increment caused by the OA increase

would correspond to only a 5.4% increase.

The second entry in the table also shows

that the OA increase in the baseline system is computed to decrease the percentage of elevated-RH occupied hours by

25%, from 40 to 29 hours per year.

The remainder of the entries show the

predicted impacts (at 20 cfm/person) as

each of the building and HVAC param-
3

Figure 1. Floor plan for the baseline 4,000 ft2 office in a Miami strip mall.

Adjacent

Office

Adjacent

Rear Wall Office

Front Wall

Front Overhang

Rear

Zone

Front

Zone

50 ft.

50 ft.

40 ft.

10 ft.

Interior Wall

Table 1. Effects of Building and HVAC Variables on HVAC Capacity and Energy Cost, and on

Occupied Hours Above 60% RH

Annual Occupied

Cooling HVAC Hours

Coil Energy with RH

Capacity Costa >60%

OA Rate = 5 cfm/person

Baseline system with OA rate of 5

cfm/person 103.6 kBtu/hr $2,510 40 hr/yr

OA Rate = 20 cfm/person Results below are expressed as the percentage change

from the baseline numbers at 5 cfm/person, above

Baseline system with OA rate of 20 15.1 12.9 -25

cfm/person (increase to (increase to (decrease

119.2 kBtu/hr) $2,835) to 29 hr/yr)

Effect of (baseline):

Building (LOADS) Variables

Building orientation (building faces north)

- Building faces south 15.5 10.2 -25

Building shading (door, window overhangs)

- Delete all overhangs 21.0 16. -25

Occupant density (150 ft2/person

- Reduce density to 300 ft2/person -0.2 -1.5 -31

- Increase 60 4.0 W/ft2 29.6 26.9 -25

Lighting/equipment power use (2.55 W/ft2)

- Reduce to 1.5 W/ft2 0.4 -5.1 +7

Infiltration rate (0.1 ACHb)

- Decrease to 0 ACH 13.1 11.4 -25

(continued)

eters are systematically varied from its

baseline values. The percentage changes

with each parameter should be compared

with the percentage changes with the

baseline system at 20 cfm/person, discussed in the preceding two paragraphs.

If, for example, the percentage change in

annual HVAC energy cost becomes less

than 12.9% when a given parameter is

varied, this parametric variation is predicted to consume less HVAC energy at

20 cfm/person than would the baseline at

20 cfm/person. In concept, the HVAC energy penalty associated with increasing

the baseline from 5 to 20 cfm/person could

be correspondingly reduced if the OA increase could practically be accompanied

by this variation in this parameter.

In some cases, the percentages become negative. This means that a building or HVAC system incorporating that

parametric variation could operate at 20

cfm/person at a savings compared to the

baseline at 5 cfm/person.

Table 2 — presented in the same format as Table 1 — lists those entries from

Table 1 that are predicted to offer the

greatest potential reductions in HVAC energy cost at 20 cfm/person, compared to

the baseline at 5 cfm/person. These entries are listed in descending order, with

the parametric variation offering the greatest reduction listed first.

Similarly, Table 3 lists those entries from

Table 1 that are predicted to offer the

greatest potential reductions in hours at

elevated RH at 20 cfm/person, compared

to the baseline at 5 cfm/person. Again,

the entries are listed in descending order.

Parameters Creating the

Greatest Reductions in HVAC

Energy

Six of the ten parameters listed in Table

2 are associated with the building: elimination of all exterior surfaces (a hypothetical consideration); reduced lighting/equipment wattage; reduced occupant density;

decreased glazing; improved glass type;

and increased roof insulation.

That each of these parameters would

significantly reduce annual HVAC energy

cost, of course, is not surprising. However, it is instructive to explore why these

parameters fall in the order they do in

Table 2.

As shown, lighting and equipment are

the largest individual contributors to the

HVAC load, contributing about half of the

total load from all sources. Thus, it is not

surprising that a 40% reduction in lighting

plus equipment wattage (from 2.55 to

1.5 W/ft2) would provide the greatest reduction in HVAC energy costs among the

practical alternatives in Table 2. (Only the
4

Exterior wall resistance (Uo = 0.16 Btu/hr ft2 F0)

- Decrease to 0 Btu/hr ft2 F0 14.2 11.4 -25

- Decrease to 0.6 Btu/hr ft2 F0 14.8 12.3 -25

Amount of glazing (33% of exterior walls)

- Decrease to 0% 10.4 3.5 -43

OA Rate = 20 cfm/person

Glass type (Uo = 0.94 Btu/hr ft2 F0, S-C = 0.55c)

- Improve to Uo= 0.32, S-C = 0.16 13.3 6.1 -40

Roof resistance (Uo = 0.066 Btu/hr ft2 F0)

- Reduce to Uo = 0 15.1 6.7 -34

Total office insulation (exterior walls, roof)

- Eliminate all exterior surfaces

 (hypothetical) 7.6 -8.4 -60

HVAC (SYSTEMS) Variables

Thermostat setup off-hours (shuts down

 off-hours)

- Cooling setpoint 810F off-hours 15.1 13.3 -25

Alternative HVAC systems (2 PSZ units/2 zones)

- 1 PSZ unit/1 zone 12.1 11.4 -30

- 1 PSZ unit/1 zone + 1 subzone 15.1 10.1 -33

- 1 PVAVSd unit/2 zones 16.8 4.9 +135

- 2 PTACe units/2 zones 7.9 9.1 -100

Ducted return air (plenum return)

- Air return via ducts 15.1 12.4 -33

Cold-air distribution (PSZ/550F min. supply T)

- PSZ/420F minimum supply T 22.2 6.2 -72

- PVAVS/420F minimum supply T 23.3 1.6 -52

Economizer modiciations (T-controlled econo.)

- No economizer 15.1 13.7 -85

- Enthalpy-controlled economizer 15.1 13.0 -55

Cooling electric input ratio (EIR = 0.341)

- Cooling EIR = 0.284 15.1 -1.8 -25

- Cooling EIR = 0.427 15.1 105.5 -25

Cooling capacity and SHRf (8.6 tons/SHR = 0.75)

- 10 tons/SHR = 0.78 15.8 13.8 -25

- 10 tons/SHR = 0.73 15.8 13.7 -25

- 11 tons/SHR = 0.78 27.4 15.4 -25

- 11 tons/SHR = 0.73 27.3 15.6 -25

Weather File Variables

Alternative weather files (typical meteorological

 year)

- Weather year for energy calcs. 30.8 12.2 -8

aEnergy costs include electricity for: the air-conditioning compressor and condenser fan; the electric

resistance heating coils; the motor for the central air handling fan; and auxiliaries (compressor

crankcase heaters). Cost of electricity is $0.0473/kWh plus a demand charge of $9.96/kW above 10

kW.

bACH = Air changes/h.

cS-C = Shading coefficient.

dPVAVS = Packaged variable-air-volume system.

ePTAC = Packaged terminal air conditioner.

f

SHR = Sensible heat ratio.

Table 1. (continued)

Percentage Increase Over Baseline at 5 cfm/person

Annual Occupied

Cooling HVAC Hours

Coil Energy with RH

Capacity Costa >60%

hypothetical scenario of eliminating all exterior surfaces provided a greater reduction.) This reduction in lighting plus equipment wattage could be achieved by converting from the prescriptive or average

wattages in ASHRAE 90.1-1989 to very

efficient lighting (e.g., including daylighting)

and more efficient (or more limited) equipment usage.

As shown in Table 2, the HVAC energy

cost savings from more efficient lighting/

equipment would more than offset the increase in HVAC energy costs resulting

from an increase in OA from 5 to 20 cfm/

person. The building with efficient lighting/

equipment could operate at 20 cfm/person with a HVAC cost savings of 5.1%

compared to the baseline 5 cfm/person

case. Of course, efficient lighting/equipment would provide even greater savings

in total building energy costs, by reducing

the energy costs for lighting and equipment as well as for the HVAC system.

As shown in Table 4, occupants are

tied with glazing and (at 20 cfm/person)

with OA as the second largest contributor

to HVAC energy consumption. Accordingly,

it is not surprising that cutting occupancy

in half (from 150 to 300 ft2/person) would

provide the next greatest reduction among

the 6 building parameters in Table 2. Of

course, reducing occupant density will not

generally be a viable option for reducing

energy costs.

Table 4 shows that — among the exterior surfaces — conduction and radiation

through the glazing are the most important contributors to HVAC energy consumption in this office. As a result, it is not

surprising that adjustments to the glazing

— eliminating it altogether, or increasing

its resistance to conduction and radiation

— should show up on Table 2 as the next

most effective building parameters for reducing HVAC energy costs. Eliminating

(or substantially reducing) the glazing might

not often be a viable option. However,

improving the glazing resistance is a viable option, if the building owner is prepared to accept the increased construction costs.

Finally, Table 4 shows that — among

the exterior surfaces — roof conduction is

the second most important contributor,

about half as important as glass conduction and radiation. The roof is important

because it represents such a large exterior surface area for this building (4,000

ft2, compared to only 700 ft2 for the unglazed portion of the exterior walls), and it

has the most consistent direct exposure

to solar radiation. Consequently, it is not

surprising that hypothetically increasing the

roof resistance to infinity (i.e., reducing

the roof Uo — heat transfer coefficient —
5

Table 2. Building and HVAC Variables Creating the Greatest Reductions in Annual HVAC Energy Cost

at 20 cfm/person (from Table 1)

Annual Occupied

Cooling HVAC Hours

Coil Energy with RH

Capacity Cost > 60%

OA Rate = 5 cfm/person

Baseline system with OA rate of 5

 cfm/person 103.6 $2,510 40 hr/yr

OA Rate = 20 cfm/person Results below are expressed as the percentage change from

the baseline numbers at 5 cfm/person, above.

Baseline system with OA rate of 20

 cfm/person 15.1 12.9 -25

Variables giving the greatest reduction in HVAC energy cost at 20 cfm/person, in descending order.

Eliminate all exterior surfaces (ideal) 7.6 -8.4 -60

Reduce lighting/equipment to 1.5 W/ft2 0.4 -5.1 7

Reduce cooling electric input ratio to 0.284 15.1 -1.8 -25

Reduce occupant density to 300 ft2/person -0.2 -1.5 -31

Convert to PVAVS with cold-air distribution

 (minimum supply air T = 420F) 23.3 1.6 -52

Decrease glazing to 0% of wall area 10.4 3.5 -43

Convert from 2 PSZ units to 1 PVAVS unit -

 standard minimum supply air T (550F) 16.8 4.9 135

Improve glass type to Uo = 0.32, S-C = 0.16 13.3 6.1 -40

Convert PSZ to cold-air distribution (420F) 22.2 6.2 -72

Increase roof resistance to Uo = 0 15.1 6.7 -34

Table 3. Building and HVAC Variables Creating the Greatest Reductions in Hours above 60% RH at

20 cfm/person (from Table 1.)

Annual Occupied

Cooling HVAC Hours

Coil Energy with RH

Capacity Costa > 60%

OA Rate = 5 cfm/person

Baseline system with OA rate of 5

 cfm/person 103.6 $2,510 40 hr/yr

OA Rate = 20 cfm/person Results below are expressed as the percentage change from

the baseline numbers at 5 cfm/person, above.

Baseline system with OA rate of 20

 cfm/person 15.1 12.9 -25

Variables giving the greatest reduction in hours at elevated RH at 20 cfm/person, in descending order.

Eliminate economizer 15.1 13.7 -85

Convert PSZ unit to cold-air distribution

 (minimum supply air T = 420F) 22.2 6.2 -72

Eliminate all exterior surfaces (ideal) 7.6 -8.4 -60

Convert to enthalpy-controlled economizer 15.1 13.0 -55

Convert to PVAVS with cold-air distribution 23.3 1.6 -52

Decrease glazing to 0% of wall area 10.4 3.5 -43

Improve glass type to Uo = 0.32, S-C = 0.6 13.3 6.1 -40

Increase roof resistance to Uo = 0 15.1 6.7 -34

aThe DOE-2.1E model used here does not account for moisture capacitance of the building materials/

furnishings, or for re-evaporation of moisture from the cooling coils when the coils cycle off with the air

handler operating. As a result, the number of hours computed to have RH>60% at any given set of

conditions will usually be low.

from 0.066 Btu/hr ft2 Fo to zero) is the

exterior surface parameter that provides

the next greatest reduction in HVAC energy cost in Table 2 (cutting the cost penalty from the OA increase about in half,

from 12.9% to 6.7%).

Of course, reducing the roof Uo all the

way to zero is not practical. However,

these results show that — if additional

resources are going to be expended to

better insulate the shell of this particular

office configuration — one is best served

directing those resources towards improved glazing and increased roof resistance, rather than towards increased wall

or slab resistance.

The other 4 of the 10 parameters listed

in Table 2 are associated with the HVAC

system: improving the cooling system efficiency; converting from a constant-volume

PSZ system to a packaged variable-volume system (PVAVS); and conversion to

cold-air distribution (i.e., a minimum supply air temperature of 42°F rather than

55°F), with either the PSZ or the PVAVS.

Of these four, the parameter providing

the greatest reduction in HVAC energy

cost is improved efficiency of the PSZ

cooling coils. In this calculation, the EIR

was decreased from the baseline value of

0.341 — corresponding to an energy efficiency ratio (EER) of 10 Btu/hr per W),

representing a typical efficiency — to an

EIR of 0.284 (EER = 12 Btu/hr per W),

representing a high-efficiency unit. If the

building owner were prepared to invest in

high-efficiency cooling units, this office

could operate at 20 cfm/person while simultaneously saving 1.8% of the HVAC

energy cost compared to operation at 5

cfm/person with the baseline, moderateefficiency system. This 1.8% savings corresponds to a modest $46/year.

As shown in Table 2, conversion of the

pair of PSZ units to a single two-zone

PVAVS (operating at the standard minimum supply air temperature of 55°F) would

reduce by 60% the HVAC energy cost

penalty associated with the OA increase.

That is, the penalty would drop from 12.9%

to 4.9%. PVAVS can be slightly more complicated and more expensive than the PSZ

units, and hence do not appear to be as

widely used in strip mall space of the type

being modeled here. However, PVAVS of

this capacity are commercially available,

and can reasonably be considered as a

means to reduce the energy penalty in

this application.

PVAVS reduce energy consumption and

cost by reducing the volume of supply air

being delivered. Most of the savings result from reduced power consumption by

the central air handling fan, since power
6

consumption varies with the cube of the

volumetric flow rate. A small portion of the

savings results from reduced cooling coil

consumption, since reduced central fan

operation results in less heat being added

to the circulating air stream by the fan

motor.

Finally, Table 2 shows that conversion

to cold-air distribution (with either the

PVAVS or the PSZ system) will provide a

significant reduction in the HVAC energy

penalty associated with the OA increase.

Operation at a minimum supply air

temperature of 42°F instead of 55°F reduces volumetric flow rates, thus reducing

fan power consumption as well as the

amount of heat added to the air stream by

the fan motor. Superimposing cold-air distribution and a PVAVS — for which volumetric flows are already significantly reduced — provides the greater reduction in

HVAC energy costs, among the two HVAC

types.

The use of cold-air distribution creates

a number of design and operating complications that could make such an approach

impractical for small offices such as the

one modeled here, where simplicity in

maintenance is important. Among these

complications is the need for: a) increased

care to reduce the risk of moisture condensation on the ductwork and the diffusers; and b) possible powered terminals to

provide adequate throw of the reduced

volume of air out through the diffusers (a

step which would offset part of the energy

savings achieved through the reduction in

volumetric flow).

Parameters Creating the

Greatest Reductions in Hours

at Elevated RH

According to the DOE-2 model, occupied hours having RH levels greater than

60% occur on cool mornings in Miami.

During the first hours after system startup

on cool mornings, the outdoor RH can be

high (over 90%), but the indoor and outdoor temperatures can be sufficiently low

that the cooling coils operate at greatly

reduced capacity (or remain off altogether).

As soon as the cooling coils begin operating at a significant fraction of their capacity — usually within 2 or 3 hours after

startup — the indoor RH drops below 60%.

(On warm summer mornings, the coils

begin operating near full capacity immediately upon startup; thus, elevated-RH indoor hours never occur during warm

weather, despite the high outdoor RH levels that exist.)

On some cool morning hours, when the

economizer is able to provide all of the

sensible cooling required by the space,

the economizer will activate in lieu of coil

operation. (The economizer and the cooling coils cannot operate simultaneously in

the packaged PSZ units.) During economizer operation — when a large amount

of untreated, potentially high-moisture-content outdoor air can be introduced into the

building — there is an increased potential

for indoor RH levels to exceed 60%. In

practice, the economizer on the HVAC

systems being modeled here does not

operate often.

As shown in Table 3, simply increasing

the OA rate from 5 to 20 cfm/person using

the baseline system (with no changes in

any other variables) is predicted by DOE2 to reduce the number of elevated-RH

occupied hours by 25%. Although this percentage may seem significant, the actual

number of hours involved is small, corresponding to a reduction from 40 hours per

year at 5 cfm/person (1.2% of all occupied hours) to 29 hours at 20 cfm/person

(0.9%).

The number of elevated-RH hours occurs because, on average, the increased

OA rate increases the sensible load. In

attempting to address this increased load,

the PSZ coils operate at a lower temperature during the cool morning periods when

elevated-RH hours occur. This increases

the latent cooling provided by the system.

This increase in latent cooling at 20 cfm/

person is predicted by DOE-2 to more

than offset the increase in latent load

caused by the increased OA rate.

Other researchers have made similar

calculations using a model that includes

factors not addressed by DOE-2, namely,

moisture capacitance and re-evaporation

off the cooling coils. These researchers

predict that — in contrast to the DOE-2

predictions — an increase in OA rate in

Miami would significantly increase, not decrease, hours at elevated RH. Also, when

capacitance and re-evaporation are considered, it is predicted that some of the

elevated-RH hours will occur during warm

weather, not just on cool mornings.

Table 3 lists the eight parametric variations predicted by DOE-2 to provide the

greatest reductions in the number of elevated-RH occupied hours.

Two of the most effective of these eight

variations involve adjustments to the

economizer. This is not surprising, since

— in the Miami climate, as discussed

above — the economizer is likely to cause

elevated indoor RH during those hours

when it operates.

When the economizer is eliminated altogether (and the system is operating at

20 cfm/person), as shown in the table,

occupied hours above 60% RH are reduced by 85% compared to the baseline

5 cfm/person case (from 40 to 6 hours/

year). This result confirms that, in this

humid climate, the bulk of the elevatedRH hours are caused by the economizer.

If the economizer were converted to

enthalpy control, rather than standard temTable 4. Approximate Contribution of the Various Heat Sources toothe Annual HVAC Energy

Consumption in the Baseline Building

Percentage Contribution to

Annual HVAC Energy Consumptiona

Heat Source OA = 5 cfm/person OA = 20 cfm/person

Conduction and Radiation Through Exterior Surfaces (sensible)

Exterior walls - conduction 2 2

Glazing - conduction and radiation 14 12

Door - conduction 0.5 0.4

Roof - conduction 8 6

Slab - conduction -0.5 -0.4

Infiltration

(sensible and latent) 2 2

Mechanically Introduced Outdoor Air

(sensible and latent) 2 15

Internal Sources

Occupants (sensible and latent) 15 13

Lighting (sensible) 40 35

Equipment (sensible) 17 15

Domestic hot water heater (sensible) ~0 ~0

TOTAL 100 100

aAnnual HVAC energy consumption for the baseline building is 26,145 kWh/year for a ventilation rate

of 5 cfm/person, and 29,390 kWh/year for 20 cfm/person.
7

perature control, hours above 60% RH

are reduced by 55% (from 40 to 18 hours/

year). Economizer enthalpy control prevents the economizer from operating if

the outdoor enthalpy is greater than the

indoor enthalpy (even if the outdoor temperature is lower). But this controller does

not make any effort to control the indoor

humidity. Thus, if the outdoor enthalpy

were lower, the controller would allow the

economizer to operate — and hence allow the cooling coils to shut down — even

if this meant that indoor RH values would

exceed 60%. Thus, enthalpy control would

eliminate only some of the economizerinduced elevated-RH hours.

Elimination of the economizer altogether

is predicted by DOE-2 to almost eliminate

occupied hours above 60% RH in this

climate, and it does so with only a modest

energy cost penalty. The annual HVAC

energy cost for the no-economizer case

at 20 cfm/person is only $23/year greater

than that for the baseline temperaturecontrolled-economizer case at 20 cfm/person. Thus, this is a viable option to consider for reducing indoor RH. By comparison, the option of economizer enthalpy

control is less attractive, since the cost

and maintenance requirements make such

controllers less desirable for small office

applications, and since enthalpy control is

less effective in reducing elevated-RH

hours.

Two of the other parametric variations

in Table 3, offering significant reductions

in the number of elevated-RH hours, involve conversion to cold-air distribution.

These include conversion of the baseline

PSZ units to cold-air distribution (providing a 72% reduction, from 40 to 11 hours),

and conversion to a PVAVS with cold-air

distribution (providing a 52% reduction,

from 40 to 19 hours). This occurs largely

because — at the very low coil temperatures in cold-air systems — the amount of

latent cooling increases significantly relative to the standard (55 oF supply air temperature) case. Thus — after the coils

activate on cool mornings, when the elevated-RH hours occur — RH levels in

the office space drop more rapidly with

the cold-air system.

However, due to the operating complications and likely increased maintenance

of cold-air systems, it is not likely that this

approach would often be considered for

use in a small strip mall office such as the

one modeled here.

The remaining four parameters listed in

Table 3 involve efforts to make the building shell more heat resistant: hypothetical

total isolation of the space; elimination of

the glazing; improving the glazing; and

increasing the roof resistance. These four

parameters appear in Table 3 in the same

order that they appeared in Table 2.

These parameters have this effect on

the number of elevated-RH hours because

the better insulated the building, the less

it cools off over cool winter nights and

weekends. Consequently, the cooling coils

see a greater cooling load more quickly

after startup on the cool mornings, when

the elevated-RH occupied hours occur.

The temperature-activated coils come on

earlier after startup, and provide greater

total (and hence latent) cooling during

these morning hours, thus reducing the

number of elevated-RH hours. The more

effective the shell insulating step, the better the building retains its heat overnight,

and the greater the resulting latent heat

removal in the morning. For this reason,

the insulation steps that provide the greatest reduction in total HVAC energy cost

(Table 2) also provide the largest reduction in elevated-RH hours (Table 3).

These results show that resources devoted toward improved glazing and increased roof resistance will have the greatest impact, not only on reducing HVAC

energy cost, but also in reducing (modestly) the number of hours at elevated

RH.

It is interesting to note that Table 3

does not include any of the parameters

that involve latent heat entry into, or generation inside, the building. Reducing occupant density to 300 ft2

/person reduces

the number of elevated-RH hours by 31%,

just below the cut-off used in preparing

the table. Reducing outdoor air infiltration

from 0.1 to 0 ACH has essentially no

impact on the number of elevated-RH

hours.

The Impact of Steps to Reduce

Indoor Humidity

As indicated previously, the DOE-2

model incorporates neither the moisture

capacitance of building materials and furnishings, nor moisture re-evaporation off

the cooling coils when the coils cycle off

with the air handler operating. As a result — unlike a model that does include

these phenomena — DOE-2 does not predict an increase in elevated-RH occupied

hours when the OA rate is increased. On

this basis, DOE-2 might not be expected

to precisely simulate the actual energy

and performance impacts that would result when steps are taken to reduce the

number of hours at elevated RH.

Despite this shortcoming, it is still felt

that a DOE-2 analysis can provide useful

perspective regarding the possible magnitude of the effects of steps to reduce RH.

For example, the conclusion in the preceding section — that elimination of the

economizer would substantially reduce the

number of occupied hours above 60%

RH — is felt to be valid, despite the fact

that the absolute number of computed elevated-RH hours might be low.

A variety of steps can be taken to reduce the number of hours at elevated

indoor RH in warm, humid climates. These

steps fall into two categories:

 a) Utilize an HVAC control system that

relies solely on temperature control, as is typical for office space.

But design and operate the HVAC

system such that — as the system

operates to control temperature in

the space — there will be as few

hours as possible having RH levels

above 60%.

 b) Incorporate humidity control as well

as temperature control into the

HVAC control system, which is not

common for an office of this type.

The humidity control could be

achieved, using overcooling with

reheat or using desiccants.

The RH results presented in Tables 1

and 3 can be viewed as an assessment of

a wide range of building and HVAC parameters that might serve as steps that

would fall into Category a) above. The

most practical conclusion apparent from

Table 3 is that occupied hours at elevated

RH can be substantially reduced at 20

cfm/person if the economizer is deleted in

warm, humid climates.

Two additional RH reduction steps are

considered in further detail here. One —

which falls into Category a) above — involves setting the thermostat temperature

up to 81°F during off-hours (overnight,

weekends, and holidays) rather than turning the system off altogether during cooling periods. The second — which falls

into Category b) — involves using a humidity controller on the system, overcooling

and reheating the supply air as necessary. The humidity control approach was

considered in order to assess the energy

penalty associated with this procedure,

recognizing that humidity control is not

commonly used in small offices, and that

reheat is generally prohibited by Florida

code.

Thermostat Setup vs. System

Shutdown

According to the DOE-2 simulation, setting the thermostat up to 81 o

F during offhours, rather than shutting off the system,

will have no impact on the number of

occupied hours above 60% RH. This result occurs because elevated-RH occupied hours are predicted by DOE-2 to

occur during the first hours after startup

on cool mornings. During such cool
8

weather, the overnight temperatures will

not have been sufficiently high to cause

the overnight office temperature to exceed 81°F. Thus, even if the thermostat is

set up rather than the system being turned

off, the cooling coils will not activate overnight. No latent cooling will be provided

overnight, and, as a result, the latent load

encountered by the system upon startup

in the morning will remain unchanged. Accordingly, the number of elevated-RH occupied hours will remain unchanged.

This predicted result could be different

if the DOE-2 model had addressed moisture capacitance and coil re-evaporation.

In that case, some elevated-RH occupied

hours occur during hot weather, when temperatures sometimes can be sufficient to

activate the coils overnight. This would

reduce the latent load seen by the system

upon startup, by reducing the re-evaporated moisture that remains in the air overnight, and by reducing the amount of

sorbed moisture. It could thus reduce the

number of elevated-RH occupied hours

occurring during warm weather.

With or without consideration of capacitance and re-evaporation, switching to thermostat set-up rather than system shutdown will reduce the number of elevatedRH unoccupied hours during warm

weather. On some hot nights and weekends in Miami, with the system off, the

indoor RH can be above 60% much of the

time, due to infiltration alone (even in the

absence of re-evaporation effects). With

thermostat setup, when the off-hour office

temperature exceeds 81°F and the coils

come on, the RH drops below 60%, at

least for the hours when the coils are

activated. On some warm days, this can

represent 15 to 20% of the unoccupied

hours that would otherwise be at elevated

RH. The same reduction in elevated unoccupied hours will be achieved regardless of the OA rate during occupied hours,

since OA ventilation is not provided during unoccupied hours.

Thermostat setup will not impact the

elevated-RH unoccupied hours that occur

during cool weather in Miami, since the

office temperatures will generally not exceed the 81°F level that would trigger coil

operation.

Of course, reducing the number of elevated-RH unoccupied hours will not improve occupant comfort, since no-one will

be in the building. But it will reduce the

risk of biological growth. Since most of

the elevated-RH hours in this building occur during unoccupied hours — regardless of whether the OA rate during occupied hours is 5 or 20 cfm/person — switching from system shutdown to thermostat

setup would appear to be an important

step for any building operator concerned

about microbiologicals.

The operating cost associated with setup vs. shutdown is low, according to the

DOE-2 predictions. As shown in Table 1,

switching to 81 oF thermostat setup at 20

cfm/person would increase the annual

HVAC energy cost by only 0.4% (amounting to only $10 per year) compared to the

baseline shutdown case at 20 cfm/person. Selecting an even lower set-up temperature of 79°F — which would reduce

the number of unoccupied elevated-RH

hours by an even greater amount — would

increase annual HVAC energy costs by

only $38.

Humidity Control by

Overcooling and Reheat

Overcooling the supply air to condense

moisture, then reheating to achieve the

proper supply air temperature, has historically been a method for controlling humidity. Although humidity control is not commonly utilized in small offices (except in

special cases), and although Florida codes

now generally prohibit reheat due to the

energy penalty involved, it is of interest to

assess the costs and effectiveness of this

approach, in comparison with the other

approaches considered here.

The results of this analysis of a reheatbased humidity control system are summarized in Table 5, presented in the same

format as Table 1.

As shown in the table, the humidity control system operating at 20 cfm/person is

predicted by DOE-2 to increase the HVAC

energy cost by 16.5% compared to the

baseline (temperature-control) system at

5 cfm/person, and by 3.6% (i.e., 16.5 vs.

12.9%) compared to the baseline system

at 20 cfm/person. But the humidity control

system does achieve its objective, of eliminating all occupied hours above 60% RH.

Comparing Tables 3 and 5, it is apparent that — among the parametric variations predicted to provide the greatest reductions in elevated-RH hours — conversion to a humidity controller involves the

largest increase in HVAC energy cost

(16.5%), but provides the greatest reduction in hours above 60% RH (100%). Second to the humidity controller in both these

categories — at least as estimated by

DOE-2 — is elimination of the economizer

(13.7% increase in HVAC energy cost,

85% reduction in elevated-RH hours).

This comparison suggests that elimination of the economizer would eliminate

85% of the elevated-RH occupied hours

at 20 cfm/person, at an energy cost increase of $19/year (compared to the

baseline case at 20 cfm/person). To eliminate the remaining 15% of the elevatedRH occupied hours, one could convert to

a humidity controller, at an energy cost

increase of $90/year (compared to the

baseline at 20 cfm/person). Conversion to

a humidity controller automatically prevents

economizer operation during those hours

when the economizer is responsible for

the elevated RH, and provides the additional cooling/reheat required to address

the remaining elevated-RH hours.

This DOE-2 comparison would change

if one included moisture capacitance and

coil re-evaporation in the model. In that

more rigorous case, the effectiveness of

economizer elimination at 20 cfm/person

would decrease — i.e., the percentage

reduction in elevated-RH occupied hours

would be much less than 85% — since

the new model would show a much greater

number of elevated-RH hours being

caused by factors other than economizer

operation. Conversion to a humidity controller would remain 100% effective, but

the energy cost would increase, since,

again, the new model would show many

more elevated-RH occupied hours.

Most of the energy penalty incurred by

the humidity-controlled system results from

additional sensible and latent cooling on

cool, humid days, when the elevated-RH

occupied hours occur, according to the

DOE-2 model. As would be expected, the

penalty is relatively small during mild and

hot weather. And the contribution of reheat to the total penalty is small, on the

order of 10% of the total; the increased

sensible and latent cooling is responsible

for the remainder.

It is emphasized that humidity control

will maintain the RH in the offices only

during occupied hours, when the HVAC

system is operating. Regardless of which

simulation model is used, a large fraction

of the total elevated-RH hours in the space

occur during unoccupied hours. Thus, if

biological growth is a concern, some offhour operation would be required even if

a humidity control system were used to

eliminate all elevated-RH hours during occupied periods. This is true regardless of

the OA rate during occupied hours.
9

Table 5. Effect of Humidity Control by Overcooling and Reheat

Annual Occupied

Cooling HVAC Hours

Coil Energy with RH

Capacity Costa > 60%

OA Rate = 5 cfm/person

Baseline system with OA rate of 5

 cfm/person 103.6 $2,510 40 hr/yr

OA Rate = 20 cfm/person Results below are expressed as the percentage change from

the baseline numbers at 5 cfm/person, above.

Baseline system (temperature control only)

 with OA rate of 20 cfm/person 15.1 12.9 -25

Humidity control system (temperature plus RH

 control) with OA rate of 20 cfm/person 15.1 16.5 -100

aEnergy costs include electricity for: the air conditioning compressor and condenser fan; the electric

resistance heating coils; the motor for the central air handling fan; and auxiliaries (compressor

crankcase heaters). Cost of electricity is $0.0473/kWh plus a demand charge of $9.96/kW above 10

kW.

Nonmetric multiplied by yields Metric

Btu/hr 0.293 W

cfm 37 L/s

°F 5/9 (°F-32) °C

ft 0.305 m

ft2 0.0929 m2

ton (of 3.520 kW (of

 refrigeration) cooling

 (12,000Btu/hr) capacity

METRIC EQUIVALENTS

Readers more familiar with the metric

system may use the following factors to

convert to that system:
10

• •

United States

Environmental Protection Agency

Center for Environmental Research Information

Cincinnati, OH 45268

EPA/600/SR-97/131

Official Business

Penalty for Private Use $300

BULK RATE

POSTAGE & FEES PAID

EPA

PERMIT No. G-35

D. Bruce Henschel is the EPA Project Officer (see below).

The complete report, entitled “Energy Costs of IAQ Control Through Increased

Ventilation in a Small Office in a Warm, Humid Climate: Parametric Analysis Using

the DOE-2 Computer Model,” (Order No. PB98-113368; Cost: $38.00, subject to

change) will be available only from:

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

Telephone: 703-487-4650

The EPA Project Officer can be contacted at:

National Risk Management Research Laboratory

U.S. Environmental Protection Agency

Cincinnati, OH 45268