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NBS

PUBLICATIONS

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NAT’L INST. OF STAND & TECH

AlllDb OSlbSS

NBSIR 85-3142

Laboratory Tests of A Gas Fueled

Modulati ng Type Hot Water Boiler

E. R. Kweller

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Building Technology

Building Equipment Division

Gaithersburg, MD 20899

December 1984

Issued April 1985

Prepared for: -QC

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85-3142

1935

3. Department of Energy

Ice of Conservation and Renewable Energy

shington, DC 20585
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NATIONAL BUREAU OF STAND.^S

LIBRARY

NBSIR 85-3142

LABORATORY TESTS OF A GAS FUELED

MODULATING TYPE HOT WATER BOILER

E. R. Kweller

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Building Technology

Building Equipment Division

Gaithersburg, MD 20899

December 1984

Issued April 1985

Prepared for: U.S. Department of Energy

Office of Conservation and Renewable Energy

Washington, DC 20585

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

NATIONAL BUREAU OF STANDARDS. Ernest Ambler. Director

Disclaimer

Any commercial equipment, instruments, or controls identified in this paper

does not imply recommendation or endorsement by the National Bureau of

Standards nor does it imply that the materials or equipment identified are

necessarily the best available for the purpose.

Table of Contents

Page

1.0 Introduction 1 1.1 Background 1

1.2 Purpose of Fuel Modulation 2

1.3 Objectives 3

2.0 Overview of DOE Test Procedures 4

2.1 Effect of Heating Load on Burner Modulation 4

2.2 Effects of Cycling Rate and Oversizing on Part Load

Efficiency 6

3.0 Test Equipment and Procedures 10

3.1 Scope , 10

3.2 Test Boiler 10

3.3 Boiler Load Simulator 12

3.4 Fuel Modulating Control Valve 16

3.5 Instrumentation 18

3.6 Test Procedures 19

3.6.1 Heating Load 19

3.6.2 Cycling Rate 20

3.6.3 Boiler Water Temperature Control Setting 20

3.6.4 Piping System Volume 20

3.6.5 Zone Control 21

4.0 Test Results 21

4.1 Introduction 21

4.2 Variable Heating Loads and Boiler Water Control Settings 22

4.3 Zone Control 25

4.4 Burner Cycling Rate 33

4.5 System Volume 35

5.0 Discussion 36

5.1 Operation in the Cyclic Mode 36

5.2 Operation in the Non-cycling Mode 40

6.0 Recommendations 42

References 44

Appendix 49

Figure A-1 50

Figure A-2 51

Figure A-3 52

ABSTRACT

The objective of this study was to set up a modulating controlled hot water

boiler in the laboratory and to simulate a variety of conditions that were

cited by manufacturers of boilers as influencing and being distinct

operating parameters for boilers, A further objective of these tests was

to compare these responses to the fuel input rate with the mode of

operation which was previously described for modulating controlled space

heaters and furnaces. The variation of controlled fuel rate to the burner

via the fuel modulating valve was measured under several controlled

conditions. Effects of heating load, burner cycling rate and zone control,

were investigated. The response of gas pressure modulation to the burner

of a hot water boiler heating system was studied in several series of tests

in the labortory. A boiler load simulator was set up and used for these

tests to control the heating load (heat transfer rate at the radiators) and

to simulate a variety of operating conditions that would be expected to

exist with a boiler installed in the home. The effects of heat transfer

rate and boiler water operating temperature on the modulated gas pressure

are presented as a series of data in charts showing controlled gas pressure

versus time. Cycling rate (burner on-off time) was also shown to have an

effect on the operating gas pressure when operating in a cycling mode.

1.0 INTRODUCTION

1 ,1 Background

On June 16, 1982, NBS presented a summary of a test procedure developed for

modulating controlled household heaters to the Gas Appliance Manufacturers

Association Test Procedure Committee. At that meeting, a manufacturer of

boilers using modulating controls contended that the NBS recommended test

procedures addressing fuel modulating controls on warm air furnaces or

household heaters [1]* did not adequately address the testing of modulatingcontrolled hot water boilers. The manufacturer contended that many

variables specific to the installed system, control the firing rate of the

burner other than the heating load as determined from the indoor to outdoor

difference. For example, the amount of water in the system as well as

oversize would be a determining factor on the percent of operating time at

reduced input. In order to support the Department of Energy (DOE) proposed

changes to test procedures to include modulating boilers, and to better

quantify the effects of several variables, a modulating controlled boiler

was set up and tested at NBS.

Proposed test procedures were subsequently published by DOE in June of 1983

[2]

, Comments received responding to the proposed procedures were

generally favorable with the exception of one commentor representing two

manufacturers of boilers, stating their concern that the test procedure did

not address zone control. In an analysis of comments by NBS, it was

concluded that a separate test procedure for zone control applications was

not practical "because of the many control schemes that can be employed, it

would be impractical to devise a test procedure that would cover each one"

[3]

.

* Numbers in brackets refer to references in Section 6.0.

1
1 ,2 Purpose of Fuel Modulation

The purpose of this control is to modulate (or throttle) the gas flow so

that boiler output is matched to the heating load. The boiler is in effect

resized to match the number of zones in operation.

This control is specifically recommended with systems having variable

water-flow rates, such as zoned systems. One boiler manufacturer described

the advantages of this type of controlled boiler [4] as follows:

"We have found that it is fairly common for a heating boiler to have

more output capacity than the heating radiation system installed in

the home. When this occurs, the boiler may overdrive the heating

system (that is add heat to the water faster than it is being removed

via the radiation system) which will result in an ever increasing

boiler water temperature until eventually the boiler will shut off on

its high limit, even though the room thermostat is still calling for

heat. This may result in several on/off cycles of the boiler during

one sustained call for heat from the room thermostat. This phenomenon

is especially true on a multi-zone system where only one zone may be

calling for heat. On a field test of our boiler by one of the control

manufacturers with the modulating system blocked open to act as an

on/off boiler, there were as many as three complete on/off cycles per

minute while delivering heat to a single zone. Modulation effectively

eliminates these on/off cycles that would otherwise occur during a

sustained call for heat."

Another advantage was described as "especially important when a hydronic

boiler is used with an air handling fan and coil type of device. With a

fan and coil unit, fluctuation in water temperature will result in

2
fluctuations in delivered air temperature. Modulation achieves a more

constant delivery of water temperature and results in a more comfortable

stable delivery of heat to a room."

The advantages of reduced number of on-off cycles, as results with fuel

modulation, is reduced cycling loss. If the burner cycles less, the off

period cyclic loss is less. This has been recognized and has been applied

in the current DOE test procedures [5] in terms of the adjusted burner onoff periods assigned for two stage and step-modulating controlled boilers

(i.e. 15 minutes on and off versus 9.7 minutes on and 33.8 minutes off for

single state thermostat). For those boilers which modulate between their

maximum and reduced input condition, there is no cyclic loss assigned for

the modulating mode of operation. (See 1.5.2 for further details.)

1 .3 Obiective

The objective of this study was to set up a modulating controlled hot water

boiler in the laboratory and to simulate a variety of conditions that were

cited by manufacturers of boilers as influencing and being distinct

operating parameters for boilers. A further objective was to study the

effects of these operating parameters under varying heating load

conditions. In order to accomplish this task at minimal cost and time, it

was necessary to run these tests under laboratory controlled conditions

rather than in field tests. In order to simulate the varying load

conditions that would be imposed on a boiler throughout the heating season

(i.e. outdoor temperature variations), it was necessary to design a

variable load simulator for these laboratory tests, A further objective of

the study was to observe the response of the modulated fuel input (actually

controlled gas pressure was monitored and converted to fuel input rate)

3
with variation of iirposed load and other parameters that would be expected

to influence the controlled input of fuel.

A further objective of these tests was to compare these responses of the

input energy with varying load to the mode of operation which was

previously described for modulating controlled space heaters and furnaces

in [1], An overview of the previous stu(^ is included here in /^pendix A,

2.0 Overview of, DOE Test_ Procedures for, Modulating. Boilers

2.1 Effect of Heating.Load, QD_ Burner Modulation

The effect of heating load on burner modulation was previously reported [1]

(see figure 1) as a figure showing burner output versus outdoor

temperature. In [1] and in this report, outdoor temperature below 65^ is

considered as the only determinant of the heating load. And heating load

is synonymous with the output energy delivered by the boiler. Burner

response was shown (see figure 1) as a series of on-off burner cycles at

the minimum input rate (cross sectioned blocks represent burner "on"

condition in figure 1). At temperatures below the balance point

temperature (Tc), burner cycling at the reduced input rate was no longer

considered sufficient to satisfy heating required and the burner firing

rate gradually increases in order to maintain the desired setting. For

household heaters or furnaces, this setting is the room temperature

thermostat setting, and for boilers, it is the boiler water temperature

control setting as determined by the modulating control set point. Figure

1 is intoided as a pictorial representation of the entire heating season,

where the total heating load in the year is divided up into temperature

bins and totaled for each 5®F bin. The objective being to obtain a

weighting factor that ^plies to each mode of operation (i.e. cyclic mode

and the modulating or nonr<Yclic mode). In actual operation during any day

4
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of the heating season either of these inodes will be in effect. As the sun

rises and sets, the solar heat gain affects the heating load; as windows or

doors are opened cool drafts will affect the burner response, and as

thermostat is set up or down the burner will respond independent of outdoor

temperature. Internal heat gains or intermittent use of supplemental

heaters, or a fireplace are other effects that cannot be quantified in this

analysis. In reality, it is likely that burner on-off cycling occurs

throughout the heating season irrespective of outdoor temperature because

of these real-life effects and because a wall thermostat in the living area

can override the modulation by cycling the burner off at any time. Other

effects such as piping system volume, zone control, oversizing are

considerations that have been identified. These were investigative tests

and were run in an attempt to quantify their efforts, as reported in the

following sections.

2 .2 Effects of Cycling Rate or Oversizing on Part Load Efficiency

The test procedure prescribed by DOE [5] does not explicitly address the

effects of oversize. However, there is iiq> licit within the test procedure

for modulating controlled furnaces or boilers an accounting of the oversize

effects on part load efficiency. That test procedure assigns a zero

oversize to the on-off cycling mode, which applies to the portion of the

heating season between 65°F and the "balance point" temperature (i.e. the

outdoor temperature where the minimum input rate setting is no longer

sufficient to satisfy the heating requirements). A background report [6]

that describes the effects of on/ off times was reported in January of 1983.

As a result of that study [6], the typical burners on and off periods (ton,

t off) for modulating controlled furnaces were assigned the values of 10

minutes on and ten minutes off, and boilers were assigned 15 minutes on and

6
15 minutes off. The conventional single stage thermostat on/off periods

remains at 3.9 minutes and 13.3 minutes for furnaces; and 9.7 minutes and

33.3 minutes for boilers. The significance of this is an increase in the

typical part load efficiency of approximately 6% for a modulating

controlled boiler (see Ref. 6). The reason for this increase in efficiency

is due to the increased proportion of burner on time to off time (i.e.

reduced cyclic loss) and a total discounting of cyclic loss for a fraction

of the heating season.

The non-cycling mode applies between the balance point and the coldest

expected outdoor temperature (i.e. the design temperature which is 5°F as a

national average). There are no cyclic losses assigned for that portion of

the annual heating load because the modulating control is considered able

to modulate the fuel and size the boiler output to just meet the load. In

reality 100% modulation is unlikely to occur. Since there is a wall

thermostat which is the primary control of burner on off operation, it is

likely there will be some on/ off cycling of the burner at even the coldest

times of the year (i.e. due to night set back, internal heat gains, etc.).

These effects have not been quantified and therefore the test procedure

assumes no cyclic operation for that fraction of the total annual heating

load when theoretically the burner should not be cycling off. That

fraction is referred to as (X

2

) in the DOE procedure [5] (see figure A-2 of

Appendix). The fraction (X2

) is obtained from a relationship which

identifies the balance point temperature (i.e. that outdoor temperature

which divides the total heating season degree days between modulating mode

and cycling mode. This balance point (Tc) is determined by a proportional

relationship between the minimum (B) and maximum (A) output rates and the

degree of oversize («0.

7
Te. = 45 -ATi (1)

Where ATd is 60°F? (the difference between 65°F and the design tenperature

of 5°F). In previous work published for vented space heaters [1] and [5],

the outdoor design temperature was taken as 15®F and there was no oversize

factor considered. Therefore, the equation used previously to determine Tc

(as shown in figure 1) was based on T = (65 - 15) = 50.

In the analysis used to develop this procedure, the range of outdoor

temperatures were divided into temperature bins of 5°F width. The numbers

of hours within each bin was averaged from data from weather stations

across the United States, The number of heating degree hours was therdDy

obtained for each site. The ratio of degree hours between 65®F and einy

outdoor temperature (Toa) below 65°F to the total annual heating degree

hours is taken as the fraction of total annual heating load at Toa. This

fraction between 65°F and the balance point temperature (Tc) is ^plied to

the minimum input rate, and is referred to as (X^^) in the DOE procedure.

The corresponding fraction for the modulating mode is (1 - Xj) which is

(X2

)o See reference [1] and [6] for a further description of this

procedure, and for examples of the test site weather data used to develop a

national average for the values used for (X^) and (X2

) as related to

balance points.

From the above equation (1), it can be seen that as the oversize (oC) of a

boiler increases, the balance point (Tc) shifts to a lower outdoor

temperature. Consequently, the fraction of the year in which the noncycling mode applies (X2

) is reduced and the fraction applicable to the

reduced input cycling mode (X-j^) increases.

8
Another effect causing a shifting of the balance point is the ratio (B/A),

This effect can be seen by returning to figure 1.

Given two boilers with different oversize conditions and thermal

efficiencies and that otherwise are identical, the boiler that is more

oversize would operate more of the season at the reduced input on-off

cycling mode. This is because the oversized boilers reduced input rate is

greater, therefore, it can satisfy a greater fraction of the heating season

at its reduced input. Since the part load efficiency is generally less for

the reduced input cycling mode, and that load fraction (Xj^) is greater,

this would reflect a lower weighted average annual part load efficiency for

that boiler with the greater oversize condition.

Applying the test procedure to the test boiler used here with its minimum

and maximum output rates of 15,880 Btu/h and 56,375 Btu/h, the balance

point would be calculated as follows using the average assigned 70%

oversize condition for ( ),

TC = (65) - 60 • [15,880 J 56,375] * (1.7) = 36°F.

From the test procedure (6), the fraction of total annual heating load

applied to the modulating (or non-cycling mode) would be (X

2

) = 48%, See

Appendix fig, A-2, The weighted part load efficiency would be based on 52%

of the part load cycling efficiency calculated at reduced input rate plus

48% of the non-cycling (modulating) mode efficiency. Efficiency in the

modulating mode is determined based on the steady state efficiency at

maximum input and at minimum input, and is proportioned using a calculation

that takes oversize into consideration (see Appendix A Figure A-3 for a

9
further description of that calculation procedure).

3.0 TEST EQUIPMENT AND PROCEDURES USED

3 .1 Scope

The response of gas pressure modulation to the burner of a hot water boiler

heating system was studied in several series of tests in the laboratory. A

boiler load simulator was set up and used for these tests to control the

heating load (heat transfer rate at the radiators) and to simulate a

variety of operating conditions that would be expected to exist with a

boiler installed in the home. The effects of heat transfer rate and boiler

water operating temperature on the modulated gas pressure are presented as

a series of data in charts showing controlled gas pressure versus time.

Cycling rate (burner on-off time) was also shown to have an effect on the

operating gas pressure when operating in a cycling mode.

The variation of controlled fuel rate to the burner via the modulating

controller was measured under several controlled conditions. Effects of

heating load, burner oversizing, zone control, and system volume, were

invest igated

,

As a result of these tests, recommended modification of the DOE test

procedure is being made for fuel modulating boilers equipped with manually

adjusted boiler water control valves. (See Section 6.0 for

Recommendat ions .)

3

.2

Test Boiler

The prepackaged hydronic boiler used in these tests is shown installed in

Figure 2. Figure 2 shows a cross section through this boiler. This is a

low mass type of design using copper coils and a finned tube to absorb

radiant and convective heat from the gas fueled burner. The copper tubing

10
11
is coiled around the burner to form a "water wall combustion chamber." Hot

combustion products rise upward and heat water flowing through the finned

tube tube heat exchanger. Rated input of this boiler was 80,000 Btu/h, and

maximum rated output was 64,000 Btu/h. Maximum fuel input as received was

measured at 74,205 Btu/h and minimum input measured 21,456 Btu/h. Maximum

output including jacket loss based on a measured flue loss of 24% with the

as received maximum input was 56,395 Btu/h. Minimum output based on flue

loss was 15,880 Btu/h.

3 .3 Boiler Load Simulator

Figure 3 shows a flow schematic of the boiler system, meters, controllers,

and the data recording instrumentation used. Figure 4 is a photograph of

this equipment. In Figure 4, the modulating boiler is shown in the center,

data recording equipment to the right and the boiler load simulator (100

gallon water holding tank at the left).

As shown in figure 3, there is a closed flow of water from the boiler to

four groups of five ft long fin tube radiators (four per group) immersed

(vertically) in the holding tank. The top of these fin tube radiators can

be seen in the open top of the holding tank in figure 4. Energy picked up

by the boiler water is circulated from the boiler through the fin tube

radiators. Some of this energy is removed from the hot boiler water

through heat transfer to the cooler water in the holding tank. Boiler

water is then returned to the boiler through a pump, and the cycle is

repeated. Metered water flow through this closed loop along with the

temperature rise across the boiler allows for measuring the output energy

(Q out) of the boiler water. If the amount of energy removed from the

circulating boiler water is less than energy added by the burner, the fuel

gas flow to the burner will eventually be reduced (or modulated) by actions

12
MODULATING

BOILER

TEST

SETUP

13

dln^ran'

of

laboratory

test

equipment

and

Instrumentation

used

in

boiler

testing.
14

Figure

4.

Laboratory

test

equipment

showing:

(1)

Boiler

load

simulator-holding

tank,

(2)

Finned

tube

radiators

(3)

Test

Boiler

(4)

Zone

control

valves,

(5)

Data

logger

and

recorder

(6)

electronic
of the boiler water temperature controller, (i.e. the fuel modulating

control). Conversely, if the amount of energy being removed is greater

than energy being supplied by the burner, the boiler water temperature will

drop and the modulating control will respond by increasing the flow of gas

to the burner. This process continues in order to maintain boiler water

temperature to a predetermined temperature. In operation, the modulating

control is set to any of nine settings from "1" to "9" on the control which

corresponds to a water temperature from 120®F at No. 1 to 240°F at No. 9

(i.e. 15®F increase per each setting). See section 3.3 for further details

of modulating controller. By controlling the flow of cooling water through

the holding tank, it was possible to control the temperature of the holding

tank water and the degree of heat transfer from the boiler water flowing

through the fin tube heat exchangers. Rate of heat transfer is synonymous

with the heating load and is measured by the rate of heat picked up and

removed from the holding tank. Heating load was there-by adjusted by flow

of cold water through the holding tank. Metered water flow and water

temperature rise from (T in) to (T out) of the tank was calculated as (Q

out) or the load imposed on the radiators (i.e. Q out is the product of

mass flow of water (M), specific heat (Cp) of water, and the temperature

rise (T out - T in)).

The tank water level was maintained constant throughout the test by the use

of a liquid level control of the water supplied to the tank. Drain rate

was controlled using a manual value at the base of the tank.

Flow rate of water through the boiler was initially adjusted to obtain

approximately a 20°F temperature rise at maxinum imput rate of the burner.

15
3 .4 Modulating Control Valve

The DOE test procedure [5l applies to either of two types of modulating

controls defined as either two stage or step modulating. This test boiler

uses the step modulating type of control. The step modulating type is

believed to be the most prevalent type used for boilers.

The mode of operation of a step modulating control was previously described

in reference [1] for space heaters. In most cases, thermostatic space

heater controls use a closed hydraulic sensing and actuating devices

consisting of a bulb, capillary and bellows or diastat. One manufacturer

[7] describes the operation of the hydraulic control as follows:

Figure 5. Hydraulic and Actuating Mechanism

The interior of the system is filled with a liquid which has a high

coefficient of expansion. A rise in temperature sensed by the bulb causes

an expansion or increase in volume of the liquid which is transferred

16
through the capillary to expand the bellows. The amount of movement of the

bellows depends on the temperature being sensed which provides for

temperature calibration. The bellows is normally spring loaded in

compression and the resulting movement under variable temperature

conditions utilized for operating snap mechanisms for valve opening and

closing. The movement is also used in some cases for securing varying

pressure regulator settings for modulating valves, A schematic of a

hydraulic sensor and actuating system used for snap action control is shown

in f igure 5

.

Operation of the modulating control used with this boiler is described by

the control manufacturer [8] as follows: "The valve snaps open to a

minimum flame condition and modulates between minimum and maximum flame in

proportion to the demand for heat". As the temperature approaches the set

point of the control, the flame diminishes and the valve either snaps off

or optionally (depending on the model) will operate at the minimum flame

condition, A second valve installed upstream in the burner manifold is

used for burner on-off operation under control of the room thermostat. See

Figure 2 which shows this burner manifold where the upstream valve is

referred to as the combination control valve.

The same type of hydraulic operated control valve used with room heaters is

used with the modulating burner on boilers with the exception that the

sensing bulb, instead of sensing air temperature, is installed to sense the

boiler water temperature and control it to a preselected temperature. The

control used for this boiler had a control knob with settings of one "1" to

nine y .

17
The valve is factory set at the number six "6" position but can be reset to

produce the approximate boiler operating temperature desired. A table of

boiler vater temperature versus control knob setting as listed in the

installation and operation manual for this test boiler is as follows:

KNOB SETTING “ TEMPERATURE EQUIVALENTS (°F)

Knob Setting 123456789

Operating 120 135 150 165 180 195 210 225 240

Temperature

Temperature adjustment is described in the operating instructions as

follows: "The control knob should be set to the desired operating

temperature at the time of installation," An interview with a local

plumbing company familiar with the installation of boilers of this type

revealed that the control knob is unlikely to be changed to any other

setting after being set by the installer,

3,5 Instrumentation

In addition to water flow and water temperatures described above, the gas

fuel input rate was metered through a wet test meter, and fuel input flow

rate in terms of Btu/h was determined using an on-site continuously

recording calorimeter to measure the heating value of fuel. Gas pressure

out of the modulating control was monitored using an electronic manometer.

All temperatures and the modulated gas pressure were scanned at a fixed

interval rate by a data logger and these data were recorded on magnetic

tape. Data tapes were read into a data file at the NBS central computer.

Stack flue loss readings were also measured using a non-dispersive infrared

analyzer to measure carbon dioxide. Flue gas temperatures were measured

using nine parallel connected thermocouples located in the flue pipe cross

section. Six thermocouples were also located on the center line of the

18
holding tank and positioned vertically at equal distances from top to

bottan of the tank.

3.6 Test Procedures

3.6.1 Heating Load

Two methods were used to adjust the heating load. The first method of

using cold water flow into the holding tank and out to the drain line was

described in section 33. In addition to energy flow to drain, the change

in energy content of the holding tank was determined. The difference

between initial and final average tank temperatures was used together with

the holding tank water volume and density to calculate energy stored in the

tank during the test. Six thermocouples positioned in the holding tank

water are used to determine the average tank temperature changes. Water

temperatures during these tests were held as close as possible to room

tarperature. Therefore, it was not necessary to determine the jacket loss

of the tank for this test method. This method of test was used in

obtaining the data reported in sections 43 and 4.4.

A second method used to measure heating load involved using the holding

tank without the flow of water to drain. The tenperature rise of holding

tank water was used to determine the average heating load during the burner

on period. This is the same procedure as in the first method when

correcting for the difference between energy content of the tank over the

test period. In addition, the standby loss of this uninsulated tank was

predetermined in order to correct for jacket loss of the tank. An

adjustment for standby loss was needed during this second test method

because the tank water temperature was above room temperature for all

tests. This second method of test was used in obtaining the data reported

in Section 4.2.

19
f

f The second method was found to be more useful fbr adjusting the load. In

order to adjust to a very low load with a normal controller setting of the

modulating control, it became necessary to reduce the temperature

difference between water in the radiators and the tank water in order to

operate at a reduced input rate. Operation at minimum input rates using

the tank drain method of load adjustment was only possible at the lowest

controller settings,

3.6.2 gyglto_Rate

An oversized installed heating system in the home will result in reduced

burner on-time. Since the boiler heating system used in this study was not

controlling a room temperature, the stu(fy of cycling rate was limited to

controlling burner on/off operation by either manual switching of the

electric power to the gas valve or by on/off timer as shown in figure 1.

The effects of cycling rate also overlap with the study of zone control and

heating load. For further detailed description of the effect of cycling

rate on part-load efficiency see 4,4,

3.6.3 Boiler Water Tenperature Contol Setting

Several series of tests were run with various control settings of the

modulating control. In order to operate at the minimum input rate as

prescribed in the test procedure, it was necessary to set the control to

the No, 3 or lower position. Also, the study of heating load effects was

limited at both extremes by the control setting. In order to operate at

the simulated higher heating loads, the control had to be set up to a

higher water temperature setting. Test results are reported in Section

4.2,

3.6.4 Piping, Svstgn Volume

Initial testing was run with a minimum amount of insulated piping. The

20
insulated piping runs between the boiler and water holding tank can be seen

in figure 2, In order to study the effect of additional water volume as

would be found in the field, two 10 ft lengths of 2 inch diameter steel

pipe were installed in the piping system. These lengths can be seen at the

lower right background of figure 2 (beneath the instrument table).

Internal volume of these two pipes (3.4 gallons) is approximately

equivalent to 75 ft of the one inch pipe size or 140 ft of 3/4 inch copper

tubing (type L), Internal volume of this pipe contains 3.4 gallos of

water

.

3,6.5 Zone Control

In order to simulate zone control up to three of the four sets of radiators

were by-passed using shut-off control valves. These four valves are shown

schematically in figure 2, and can be seen in figure 3 at the top of the

manifold headers in the piping system which is installed above the holding

tank. These test results are reported in Section 4.3.

4. TEST RESULTS AND DISCUSSION

4.1 Introduction

The overall objectives of the study (previously described in Section 1.3)

were carried out by showing how the fuel gas modulating control of a boiler

would respond to the various test parameters. In order to show this, all

test results in this report are presented in graphical form as curves of

the modulated gas pressure measured at the outlet of the modulating control

valve (or inlet to the burner) versus elapsed time after burner start-up.

An example of one set of data obtained is shown in Figure 6. The

equivalent fuel gas input rate corresponding to the fuel gas pressure is

also plotted in Figure 6, Since fuel input rate Btu/h varies as the square

root of gas pressure drop the effect of a change in gas pressure is not as

great expressed in terms of fuel input. Boiler output rate with this

21
boiler varies practically directly as boiler input rate. Boiler output

(including jacket loss) was 76% of the boiler input at steady state

operation at maximum input rate, and 74% of the input at the minimum input

rate.

In most tests with this boiler, the response curve for on-off cycling was

similar to the curve shown in Figure 6. Tests were run with on-off times

of 15 minutes for the test runs shown in Section 4,2 because the DOE

procedure [5] uses 15 minutes as the assigned burner on and off periods in

the calculation of the part-load cyclic efficiency. Other burner on-off

times were investigated and those results are reported in Section 4,3.

4,2 Variable Heating Loads and Control Settings

Table 1 summarizes the results obtained in seven tests of varying heating

loads varying from 18,500 Btu/h to 33,400 Btu/h. Figure 7 shows the data

obtained in four of these tests (tests 1, 2, 4 and 7 from Tab le 1),

This series of tests was run using the second method of test described in

Section 3,3 (i,e, the temperature rise of water in the holding tank over

the test period to calculate the load). Boiler water temperature in the

piping system leaving the boiler and entering radiators in the holding tank

is labeled "Water In" in Figure 7, Boiler water out of the holding tank at

the entrance side to the boiler is labeled "Water Out."

The effect of the cold water in the line on start-up is apparent in Figure

7, At the start of the first test (A), water temperature in the piping

system is shown to be near 80°F. As this water heats up to approximately

157®F leaving the boiler (water into holding tank radiators), the gas

pressure drops to approximately 2 inches W,C, after 8 minutes. Temperature

22
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24

0.00

4.00

8.00

12.00

16.00

20.00

e.00

4.00

8.00

12.00

16.00

20.00

TIME.

MINUTES

MINUTES

(C)

li-aliiii;

Li

dcl

-

!'

:,

749

15tii/h

(D)

He.itii’c

l.u.i.l

-

18,4

V)

litii/h

Figure

7.

Modulated

gas

|iiessure

and

boiler

water

temperature

in

and

out

of

holding

tank

vs.

elapsed

time

af

lei

bui

ner

si

irt-up

uniler

four

heating

load

conditions,

using

control

setting

No.

6
rise across the boiler was then approximately 40®F (i.e. 157®F less 117°F),

The average gas pressure, equivalent fuel input rate, and holding tank

water starting and ending temperatures data are shown in Table 1. The

change in energy content of the tank plus the tank surface heat loss is

taken as the tank imposed load in this series of tests. Boiler output rate

was also determined by continuous measurement of boiler water temperature

rise and boiler water flow rate. Boiler output rate is, therefore,

practically the same as the heating load measured at the holding tank.

Part load efficiency from the data of the boiler input and output rates may

also be determined. For the data shown in Table 1, the part load

efficiency based on output to input ratio varies from 60% for Run No, 1 to

56% for No, 7. In this test, the boiler loop water flow rate was 1.8

gallons per minute. Results of three additional series of tests at control

settings No. 8, No, 2, and No, 4, are shown in Figure 8, 9, and 10, Figure

11 summarizes these test results for control settings No. 6 and 8, with gas

pressure versus time for the various heating loads tested.

4,3 Zone Control

The procedure used for simulation of zone control for these laboratory

tests was described in Section 3.6.5. During these tests, the pump was

cycled off with the burner. The method of the load control for these tests

was by a measure of the flow of water through the holding tank and

temperature rise of the water from the water supply line to the drain line

(See 3,3 for details).

Results of these tests do not impact on the existing DOE test procedure

[5] because zone control is not considered in the calculation of part-load

or annual efficiency. The objective of these tests as with previous tests

with load control was to observe the effect of modulated fuel gas pressurt

25
0.00

4.00

8.00

12.00

16.00

20.00

8

-.®®

12.00

16.00

20.00

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26
Figure

0.00 4.00 8.00 12.00 1G.00 20.00

(A) Heating Load -15390 Btu/h

0.00 4.00 8.00 12.00 16.00 20.00

(B) Heating Load -12109 Btu/h

9. Moduldfed Gas Pres-ure ind boiler water u^mperaLure in and out f tank vs elapsed time atter burner start-up usinii c..nir. 1 .elfin - No

27
0.00

4.00

8.00

12.00

16.00

20.00

12.00

16.00

20.00

3

•MCO

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fOo

OZH'Ht *3»nsS3dd SirO

I

28

0.00

4.00

8.00

12.00

16.00

20.00

0.00

8.00

12.00

16.00

20.00

TIME.

MINUTES

TIME.

MINUTES

(C)

Heating

Load

-

15,888

Btu/h

(0)

Heating

Load

- 1

1,724

Btu/h

Figure

10.

Modluated

Gas

pressure

and

water

temperature

entering

boiler

(out

of

holding

tank)

vs.

elapsed

time

after

start-up

using

control

setting

No.

4
GAS

PRESSURE,

IN.H20

GAS

PRESSURE,

IN.H20

BOILER - INPUT GAS PRESSURE VS. TIME

re 11 . Modulated gas pressures for various heating loads vs. elapsed time after start-up

29
during the burner operation period. In these tests, results r^xjrted are

from the third cycle of a three cycles test and represent a repeatable

condition. Tests were run with one, two and four zones in operation.

Results with one zone in operation was almost identical to the two zone

results and are therefore not r^x^rted.

Figure 12 shows the results obtained with two zones in operation (two

sections of 4 radiators per section in use) versus four zones (sixteen

total radiators) using control setting no. 6 . These test results are also

reported in Table 2. These tests are ccxrpared using the same burner on-off

periods because the length of burner on-off periods (cycling rate) also

affects the modulated gas pressure.

Figure 13 shows the effect of zone control using the modulating control

setting No. 3. Figure 13 also shows the secondary effect of burner on-off

times. The comparisons in Figure 13 are between (A) versus (B) and (C)

versus (D) for the zone control effect, v^ile (A) versus (C) and (B) versus

(D) shows the effect of cycling rate. Test results are also shown in Table

3 for the test runs of Figure 13. (Section 4.4 discusses the effect of

burner cycle rate results in more detail.)

The effect of these zone control tests could be considered as simply a

change in the heating load due to the increase in heat transfer surface

area of the radiators in operation. An increase from two zone to four zone

operation was in effect an increase in heating load by 50% at control

setting No. 6 (see Figure 12 and Table 3), and 25% increases for control

setting No. 3 (Figure 13).

30
TIME. MINUTES

(A) Two Zones

TIME. MINUTES

(B) Four Zones

Figure 12. Modulated gas pressure vs. elapsed time after start-up at contro

setting No. 6 showing effect of two zone vs. four zone operdtion

31
-

,1111 J-l-J-i 1111 1 1 X-4-

. . w

2J 5

z

OSH'Nl *3ttnSS3ttd SVO

s

ci

COoCsJc

o

c

0>co

rvi

OSH NI *!3«nss3bd Svo Ii

32

Figure

13.

Modulated

gas

pressure

vs.

time

after

start-up

at

control

setting

No.

3

showing

effects

of

two

zone

vs

four

zone

operation

for

two

cycling

rates
4.4 Burner Cycling tUite

Effects of burner on-off times on the modulated gas pressure was shown in

Figure 13 in combination with zone control. Another set of test data is

shown in Figure 14 for three on-off periods. Test results for Figure 14

are also summarized in Table 4, The results of these tests all show that

the shorter off-period decreases the peak gas pressure on start-up. This

is due to the fact that water temperature drop in the piping system is less

on start-up and the subsequent response of the modulating control gas

valve

.

The colder the boiler water is at start-up the more pronounced the peak and

the longer is the duration of elevated pressure on start-up. This can be

seen by comparing the boiler water temperature data on start-up or at 25

seconds after start-up in Table 4 with the curves of Figure 14. (See

Section 5.1 for more discussion of the modulating control response to

varying water temperature in the lines.) Higher control settings

accentuate this effect. The effect is more apparent in the data of Figure

14 at control setting No. 6 then in Figure 13 with a No. 3 setting. This

is due to the faster rate of cooling of water in the piping system at the

higher control settings. Rate of cooling in the lines in this test with

radiators immersed in water was considerably faster than it would be if the

test had been operated with natural convective heat transfer with room air.

However, these data are still considered to reflect field conditions. The

lower heat transfer with natural convectors would be offset by increased

loss through the piping system because the heat transfer surface area of

the total piping system in the home would be very much (perhaps ten to 100

times, depending on the installed condition) greater than in thit.

laboratory test. These tests were run with a very short run of piping

33
BOILER

FUEL

GAS

PRESURE

4-


 «s> o 
 <S> 
 
 o

L/> OJ

OZH'NI *3tjnSS3Ud SVO

34

0.00

4.00

8.00

12.00

16.00

20.00

TIME.

MINUTES

Figure

14.

Modulated

gas

pressure

vs.

time

after

start-up

showing

effect

of

burner

cycling

rate
between the boiler and the tank. It should be expected that a decrease in

this peak effect would result with a large volume of circulating boiler

water.

In order to account for the effect of this peaking of gas pressure on start

up under cycling conditions, the reduced output rate would need to be

adjusted. The effect of adjusting this output would be to reduce the

calculated balance point (TC) with a subsequent increase in effect of

efficiency at the reduced input because of an increase in the value of (X1). The time weighted average gas pressure during the 15 minute on-period

would need to be calculated. The difference between this average pressure

and the steady state pressure as now used in the procedure can be seen in

figure 7 by comparing gas pressures corresponding to the average (see dot

marked on curve) with the steady value (after 15 minutes). This difference

would not be as noticeable if it were shown in terms of fuel input rate

because input varies as the square root of pressure drop. See figure 6 for

that comparison. No changes are being recommended for this effect because

the difference is small in comparison to the greater effect described below

in Section 5.1.

4.5 System Volume

Test results were not conclusive in any of the data comparing results with

and without the added piping volume. Therefore, no data is presented on

this aspect of the testing. An expected effect that may have resulted with

more extensive system volume variation was discussed in Section 4.4.

35
5 .0 Discussion

The purpose of this section is to point out the similarities or differences

between the assumed modes of operation used in development of the DOE

procedures for modulating furnaces and vented heaters and the results found

here with a fuel modulating controlled boiler.

5.1 Operation in the Cyclic Mode

Previously, representations of burner cycling were described schematically

as shown in Figure 1. Figure 1 shows shaded areas of cyclic burner on

periods up to the balance point (Tc). These are shown as distinct step

changes from the off condition of the burner to the minimum input rate.

The burner is then shown to operate at constant rate until the thermostat

caused the gas value to step to the off condition. The representation of

burner response in Figure 1 applied when the sensing element was sensing

room air temperature near the heater. (The sensing bulb with a room heater

is located near floor level behind the heater.) In most tests of this

modulating controlled boiler, the response of the modulating control at

burner start-up was similar to the response shown in Figure 6. This

response differs in appearance from that previously used to describe the

space heaters cyclic operation. The initial peak of the curve (as shown in

Figure 6) is followed by a drop in pressure to a practically continuous and

steady pressure. This response was characteristic of all test results

obtained. The only variation in results was in the accentuation of the

peak condition at start-up with the various tests. The reason for the peak

on start-up is due to a drop in temperature of boiler water in the piping

system to a temperature below the set point temperature of the control.

The first water sensed by the control when the pump starts up is this

relatively cold water in the lines. The controller senses this temperature

and compares it to the control set point. With this water temperature

36
lower than the equivalent control setting, the burner increases fuel input

above the minimum fuel input rate. After all boiler water in the piping

system loop has passed through the boiler, the cold water has been purged

from the system and is replaced by hot water. Immediately after the cold

water is purged, the control senses the water temperature to be above the

set point and responds by reducing gas pressure. This condition results at

approximately one minute after the initial start-up as shown in Figure 6 by

the drop in gas pressure at that time. In all test results, a time

weighted average gas pressure during the burner on-period was calculated.

The average pressure for the curve shown in Figure 6 was 1.04 inches of

water.

The DOE Test Procedure [5] calls for operation at the minimum input rate

for determining a part load cyclic efficiency. In reality, operation with

the minimum boiler water temperature conditions is questionable for this

boiler which would require manually resetting to a lower setting after the

boiler control is adjusted at the time of installation. Discussions with a boiler installer has revealed that it is unlikely the control would be

reset by the homeowner after installation and unlikely he would be told to

reset it by the installer, since that could lead to complaints of

insufficient heat output during coldest weather. In the absence of an automatic reset control that would control to minimum input, it is not

likely that operations at any time would exist with the minimum boiler

water setting. It is, therefore, probable that part load efficiency of

manually adjusted modulating boilers in the field will be somewhat

inconsistent with that predicted by DOE procedures in laboratory testing.

This is because off-period loss would be greater (with conventional

atmospheric burner) since the boiler water temperature would be at a higher
temperature, (i.e. 160 F in Figure 7 at No. 6 setting compared to

approximately 100 F with control setting No, 2). The operating

temperatures specified for these two settings in the manufacturers

instructions shown in Section 3.4 are as follows:

195 F at Control Setting No. 6

135 F at Control Setting No, 2

Tests were not run with this boiler to quantify what effects this change in

water temperature would have on the part load efficiency. However, tests

were run previously at NBS (but not previously published) using a hot water

boiler of the same rated input as this unit (80K Btu/h) at a variety of

supply water temperatures. The following results were obtained for that

boiler:

Boiler Water Part Load Part Load

Temperature Steady State Sensible Loss Efficiency %

Inlet Outlet Efficiency % Ls on (%) Ls off (%) (Nu)

123®F 146°F 80.1 8.4 7.4 71.4

149®F 171°F 80.2 9.5 8.5 69.2

165°F 185°F 79.5 10.2 9.1 67.9

In addition to the lower efficiency that would result at the higher boiler

water temperature, annual efficiency would be less in the field because the

boiler would operate more of the time in a cycling mode. That is, the

fraction of operation in the cycling mode would be greater since the higher

boiler water temperature could satisfy a greater fraction of the total

annual heating load. Therefore, the weighting factors calculated by the

DOE procedure would use less weighting at the reduced input cycling mode

than would result in the field.

38
with this test boiler adjusted to the manufacturers recommended control

setting (No. 6) the data shown in figure 7-D ^jplies. With a supply v«ter

temperature of 140 F and a 20 F rise, the burner output rate after 15

minutes was 22,200 Btu/h. This output rate results in a calculated balance

point temperature of 25 F (vs. 36 F with the current test procedure) and

corresponds to annual heating load fraction for reduced input (X-1) of 81%

(vs. 51% under the curroit method method of test).

Another inconsistency with the current DOE procedure exists between the

supply water tenperature to the boiler in laboratory testing (i.e. constant

120°F) and the changing supply water temperature found in these simulated

use tests. In all tests run as shown by Figures 7 through 10, the supply

water temperature to the boiler at burner start-up was continuously

increasing after burner start-up. In other words, it can be seen that

supply water tarperature to the boiler is not a straight line. In Figure

7, system return water out of the radiators at start-up compared to four

minutes after start-up was 12°F lower. Che result of this change in supply

water temperature is the peaking of fuel input rate after start-up. No

changes are recommended to correct for this in the DOE procedures for the

following reasons:

The DOE procedures [5] calls for collecting data of the flue terrperature

during a heat-up test to steady state conditions at the minimum input,

followed measurement of flue tenperatures during a cool down test from

the minimum input steady state condition. Flue gas tenperatures on heat-up

are measured at 1.0 minute and 5.5 minutes after start-up. Hie calculation

of an on-period time constant (t on) requires that the steady state flue

gas tenperature be greater than flue gas tenperature measured one minute

39
after start-up. If it is not greater, the on-period time constant (Yon)

would be impossible to calculate and be undefined. It follows from the

curves of modulated gas pressure versus time that fuel input rate may be

greater one minute after start-up than after operating 15 minutes or at

steady state operation, particularly for the higher control settings. No. 6

(see Figure 7) and NO. 8 (Figure 8). If a water supply temperature profile

was to be used in laboratory tests to reflect actual use profile as shown

by these figures, this could result in the temperatures being greater at

one minute after the start-up than at steady state, due to the drop in fuel

input rate. For this reason and the need to calculate the on time constant

as described above, a constant supply water torperature is necessary for

laboratory testing.

5.2 Operation In The Nonrcycling Mode

The response curves of Figure 7 and 8 are at heating loads which would all

be considered to be above the balance point used in the DOE test

procedures. The balance point equivalent load for this boiler is

considered to be 15,000 Btu/h v^ich is the minimum adjusted output rate of

the boiler. Therefore, the cycling mode of this boiler would ^ply to the

heating load conditions shown in Figures 10(C) and (D) with control setting

No. 4, and to the two loads shown in Figure 8 with control setting No. 2.

The DOE procedure specifies on and off times of 15 minutes for boilers.

All the tests were run with burner on periods of 15 minutes followed by a

15 minute off-period. Above the balance point the DOE test procedure does

not consider on/off cycling in the calculation procedures with the step

modulating type control. However, if the control were the two stage type,

on/off cycling would apply.

40
The average gas pressure condition that would exist with burner on/off

cycling can be seen on the pressure curves of Figure 7 where the solid

circle on each curve shows this average pressure. Without cycling and with

continuous modulation (assuming no cycling operation below 36®F), the

average pressures would be similar to those steady pressure conditions for

the heating loads shown at 15 minutes in Figure 7. The design heating load

that would be applicable with this boiler installed assuming it was 70%

oversized would be: (maximum output - 1.7) = 33.173 Btu/h (i.e. maximum

output measured was 56,395 Btu/h). This would apply at a design

temperature of 5®F. Therefore, the heating loads shown for control setting

No. 8 in Figure 8 would apply only when the boiler were less than 70%

oversize. For example referring to Figure 8(D), the oversize fraction at

designs conditions for a 41,961 Btu/h heating load would be: 1 - [56395 -

41,961] = 034, or 34% oversize. This is not to say that burner fuel input

rate would never exceed this rate. Since the boiler water is cold on

start-up, particularly after an extended off-period, such as after night

setback, the maximum fuel input condition as shown by Figure 8-A could well

exist even with 70% oversized boiler. When adjusted at the manufacturers

recommended modulating control setting for this boiler (i.e. No. 6

setting), the operating gas pressure that would be expected to apply in the

modulating modes are those shown in Figure 7. Figure 15-a shows a

representation of the gas pressures versus time for three heating load

conditions simulating Figure 7 data. Figure 15-^d shews these average gas

pressure (solid dots) versus equivalent outdoor temperature. By plotting

these as average loads, versus outdoor temperature as shown in 15-b a curve

results that would be similar to that part of Figure 1 from the balance

point Tc to the design temperature of 5°F. Figure 15-c shows this curve

more specifically by using data from columns (6) of Table 1.

41
6 .0 RECOMMENDATIONS

It is recommended that the DOE test procedures be revised to specify that

when a manufacturers installation instructions include a recommended

setting of the modulating control (i.e. boiler water temperature setting)

that testing at reduced input rate be carried out with a boiler water

temperature consistent with that recommended setting. It is recommended

that a supply water temperature be specified that is 20°F less than the

resulting boiler water temperature found when the boiler is operated with

the manufacturers recommended control setting. If there is no recommended

control setting supplied by the manufacturers installation instructions,

the supply water temperature used should be 20^F less than the resulting

outlet water temperature found when the control is set at the mid-point

setting of the control valve.

It is also recommended that the average output rate measured at the

manufacturers recommended setting (or mid-point setting) be specified as

the minimum output rate to be used in calculation of the balance point

(Tc).

42
LU Light load

QCIDCO

CO

LU

QC

CLCO

<

CD

CD

Figure 15 (A)

I-D

CL

IZ)

o

<

LUI

LOAD —

Figure 15 (B)

Figure 15. Upper left (A) Representation of data obtained. Upper risd'

(B) Representation of data expressed as it relates to out

temperature. Lower figure (C) - Example of actual test data

obtained at No. 6 Setting.

-4d-
Re ferences

(1) Kweller, E.R. and Palla, R.L., "A Test Method and Calculation

Procedure for Determining Annual Efficiency for Vented Household

Heaters and Furnaces Equipped with Modulating Type Controls," NBSIR

82-2497, National Bureau of Standards, Gaithersburg, MD 20899.

(2) Federal Register 48. No. 118, pp. 28014-28046, June 17, 1983.

(3) Kweller, E.R., "Analysis of Public Response to Proposed Department of

Energy Test Procedures for Furnaces, Boilers, and Household Heaters,"

February 7, 1984, Letter Report to Department of Energy, Washington,

DC 20585.

(4) Letter from Mr. L. J. Ashton Raypak Inc. to Esher Kweller,

September 24, 1982.

(5) Federal Register 49. No. 61, pp. 12148-12178, March 28, 1984.

(6) Kweller, E.R., "An Analysis of Burner On and Off Periods and Their

Effect on Part-Load Efficiency for Furnaces and Boilers Equipped With

Modulating Control," Letter Report to Department of Energy, January

1983.

(7) Scharer, Edward B., "Gas Heating Appliances and Controls Training

Manual," Robert Shaw Grayson Controls Company, Form 28-181-0075D,

January 1969, Long Beach, California.

(8) "Thermostat Gas Valves, V5155A,B," Honeywell Form No. 71-92082,

Honeywell, Minneapolis, Minnesota 55408.

44
Test

Data

for

Measured

Heating

Load

for

Control

Setting

No.

vO 3a3C

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33377
Table 2. Zone Control with Modulating Control Setting #6 (See Fig 12)

Zones in Use Boiler

Load * Btu/h

Cycle

On time Off time

Average Gas

Press in. w.c. Input

Equivalent

Btu/h

(A) 2 27,593 11.7 20.0 2.4 58000

(B) 4 41,126 11.7 20.8 3.3 68000

* Using Water flow thru holding tank to control heating load and hold tank water at

approximately room temperatures.

Table 3. Zone Control with Modulating Control Setting #3 (See Fig 13)

Run No. of Zones

in use BoilerLoad

Cycle

On

Times

Off

Average Press Average Fu

Input Rate

Btu/h

(A) 2 23,000 11.7 20.8 0.65 31,000

(B) 2 24,000 18.3 12.7 0.59 30,000

(C) 4 29,520 11.7 20.8 1.04 38,500

(D) 4 28,899 18.3 12.7 0.96 37,500

46

Table 5

UNITS CONVERSION TABLE: SI/INCH-POUND/SI

TO CONVERT MULTIPLY BY TO OBTAIN

Lb

( Pounds ) 0.454

Kg

(Kilograms )

Kg

(Kilograms

)

2.2046

Lb

(Pounds )

Cu. Ft.

^Cubic Feet^

28.3

L (Liters

)

L

(Liters

)

0.03531

Cu. Ft.

(Cubic Feet)

in

(Inches)

2.54

cm (Centimeters

)

cm (Centimeters) 0.3937 in

(inches)

ft

(Feet)

0.3048 m

(meter)

m

(Meters)

ft

3.281 (feet)

BTU

(British Thermal Units) 1.055

K J (Kilojoules)

kJ

(kilojoules) 0.9479

Btu

(British Thermal Units)

TEMPERATURES (T) , CONVERSION EQUATIONS:

FOR TEMPERATURE USE:

°F to °C: [(T)°F -32°F] (5/9) = (T)oC

°C to °F: [(T)°C] (9/5) + 32°F = (T) oF

FOR TEMPERATURE DIFFERENTIALS OR TOLERANCE USE:

°F to °C: (T) OF (5/9) = (T)oC

°C to °F: (T) °c ( 9 / 5 ) = (T) °F

48
APPENDIX - OVERVIEW OF TEST PROCEDURE

FOR MODULATING FURNACES

49
Figure AI.

Over

view

of

test

procedure

for

modulating

control

led

vented

heaters

and

furnaces

(Read

down) 1 I o3 O

4> * o<O

-J 5 $ m

IIi o .5 C T3 O 4)

it E I E ^

<0

u « *a 1 I 0 S5 £ 1 & * s u

(A £ S O

m « *o 4) > N (0 ^ H 1 5 S

F> I i i LU S Q. < E - 2?W

(A 3

.E

O

m la

<0 U) , s TM I « 0) •a 5 ^ {A 15 £ « «. o »

•O ^ 0) o ^ <Ac :: 2 V 3 £ c

0> E (A

(UM ^ z Z o

0> c :=

0) O) o

TJ 5 ^

(0 E iV ~ £" ra

0) ^|l O o>

8:

Q <

<0 c |5 £ U

C

’<A ^ JS

0» iJ

TM

S5.U- >*

0> o C 5 in “

ra

®

O 15 = ? c 15

_ > £ _ " n < * I 2 =

(Q 4) (0 Q IL. I-S

JS

O

C c flj 4»

<g Q> 2 ?

(siM) 3Wli o 2 E 3 5 <A ® c -s 4) =

4)

U O) 2 1 - = i 2 «

(4 "2^0- U 4)

0> (4 E

O)

C

O

O c z — a W '<3 W 4> ':t $ J “ « - S -2 o to _e (4 3 O £ •O — «A

flj JO <4 (4

*“ 3 <A tS 6

(4 *0 4) <4 j_ 5 I 2 c ® o ? E ¥ IS CO)?? o ;= E E

50
FRACTION

OF

TOTAL

ANNUAL

HEATING

LOAD

APPLICABLE

TO

REDUCED

OPERATING

MODE

(X.)

12168 Federal Register / Vol. 49, No. 61 / Wednesday, March 28, 1984 / Rules and Regulations

Fraction of Total Annual Heating Load Applicable to Reduced Operating

Mode (XO and to Maximum Operating Mode or Modulating Mode (Xi) vs. Balance Point Temperature for Modulating Furnaces and Boilers.

BILLING CODE 6450-01-C

Figure A-2 Load fractions assigned to reduce;J input

cycling mode and to modulating mode vs. balance point temperature.

51

FRACTION

OF

TOTAL

ANNUAL

HEATING

LOAD

APPLICABLE

TO

MAXIMUM

OPERATING

MODE

OR

MODULATING

MODE

(X2)
AQi AQ2

ATi AT2

Qavg ^DHR • Qradj

At7m

Substituting for Q

for DHR Qmaxi

•vg- T?„,vg-AT]„

1^ AT2

r(PWR)(AT2)-(QrtrfMAT2)l (ATiKAQ)

IM avg AT,

A‘%

J ^ ^aaraS

'

I (^a« WMX " ^aa rad) +

'Stttrd

Fi gure A3

.

cul ation procedure used to derive average output rate (Q^

and average steady state effici ency avg^

lov^er fig

modulating controlled furnaces and boilers

vq)-upp^’'

tor step-

‘i gure

52
NBS-I I4A (REV. 2-80

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 85-3142

2. Performing Organ. Report No. 3. Publication Date

April 1985

4. TITLE AND SUBTITLE

Laboratory Tests c)f a Gas Fueled Modulating Type Hot Water Boiler

5. AUTHOR(S)

Esher R. Kweller

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)

Department of Energy

Office of Conservation and Renewable Energy

1000 Independence Avenue, S.W.

Washington, DC 20585

10.

SUPPLEMENTARY NOTES

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. If document includes a significant

bibliography or literature survey, mention it here)

The objective of this study was to set up a modulating controlled hot water boiler in

the laboratory and to simulate a variety of conditions that were cited by manufacturers

of boilers as influencing and being distinct operating parameters for boilers. A

further objective of these tests was to compare these responses of the fuel input rate

with the mode of operation which was previously described for modulating controlled

space heaters and furnaces. The variation of controlled fuel rate to the burner via

the fuel modulating valve was measured under several controlled conditions. Effects

of heating load, burner cycling rate and zone control, were investigated. The response

of gas pressure modulation to the burner of a hot water boiler heating system was studied in several series of tests in the laboratory. A boiler load simulator was

set up and used for these tests to control the heating load (heat transfer rate at the

radiators) and to simulate a variety of operating conditions that would be expected to

exist with a boiler Installed in the home. The effects of heat transfer rate and

boiler water operating temperature on the modulated gas pressure are presented as a series of data in charts showing controlled gas pressure versus time. Cycling rate

(burner on-off time) was also shown to have an effect on the operating gas pressure

when operating in a cycling mode.

12. KEY WORDS (S/x to twelve entries; alphabetical order; capital! ze only proper names; and separate key word-, by -,em colons Annual efficiency; boiler; gas fueled; heating; modulating; part load efficiency;

hot water.

13. AVAILABILITY

[y~j Unlimited

I I For Official Distribution, Do Not Release to NTIS nn Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

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