AllTHU Ifl J

NBS

pUBticATlONS

AlllDS lS7Maa

NBSIR 80-2094

A Laboratory Study of A Gas- Fired

Condensing Boiler

George E. Kelly

Mark E. Kuklewicz

Building Equipment Division

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

U S. Department of Commerce

Washington, DC 20234

November 1980

HIG*-' . -Sponsored by

100 The Department of Energy

.056 A/ashington, DC 20585

80-209^

1980

c. 2

National Bureau of Standards

Library, £-01 Admin, Bldg,

FEB 2 7 1981

A LABORATORY STUDY OF A GAS-FIRED

CONDENSING BOILER

NBSIR 80-2094

O.C. . - {jrc.

Octoo

. Us6>

PC>. ^6-^qcj

f^20

George E. Kelly

Mark E. Kuklewicz

Building Equipment Division

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

U.S. Department of Commerce

Washington, DC 20234

November 1980

Sponsored by

The Department of Energy

Washington, DC 20585

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Jordan J. Baruch. Assistant Secretaty for Producth/itY, Technology, and Innovation

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
.^wa .r«nfcA 10-3

feet T s:

' '

.'" <'* i:i-aAD A iip Ycnn-?. a.

y»j4' 1 *7tr’‘-''“9r;

,'i '''..V

1} |V,>:^.viC ,I'4H

;;i|^

J rj i,pft 9 i ' • v-

>“

ftriittfrtr .oO Hj VU,

'''Od

'OSPf ibvUnt»ii«>|!l';,'

‘C’^ /j' .

'»'•> m^*'Mne^TiO «flt,

Qi!' DO . , 'j

\

JB V

" fwrvv !W3 aAmm J’ASS
ABSTRACT

As a part of the Department of Energy's energy conservation program for

consumer products, the National Bureau of Standards (NBS) developed test

procedures for conventional gas and oil-fired furnaces and boilers. The

Department of Energy (DoE) published their finalized version of these

procedures in the Federal Register on May 10, 1978. In an effort to

update and refine these test procedures, DoE directed NBS to develop a method of testing condensing furnaces and boilers which could be used to

compare the annual performance of condensing and non-condensing residential heating systems. This report summarizes the laboratory tests of a gas-fired pulse-combustion condensing boiler that were carried out as a part of the development effort.

The performance of the pulse-combustion boiler was evaluated under both

steady-state and part-load operating conditions. The efficiency of the unit was determined by the input/output method which measured the heat transferred to the circulating water and the energy input during each test period.

Steady-state laboratory tests of the unit were conducted at constant

return water temperatures of 100, 110, 120, 130 and 140°F (37.8, 43.3,

48.9, 54.4 and 60.0®C). Part-load performance tests were carried out

at a number of these return water temperatures at on-times of

approximately 5, 15, 22.5 and 42 percent.

A modified version of the heat loss procedure for estimating the seasonal

perfoirmance of a residential central furnace or boiler was also used to

evaluate the boiler's steady-state efficiency and part-load efficiency

at a 22.5 percent on-time. A cool-down test and heat-up test were performed to obtain dynamic information which was used to calculate the

unit's cyclic performance. The predicted steady-state and part-load

efficiencies from the heat loss method were found to be within three

percent of the performance determined using the Input/output method.

Key Words: Boilers; central heating; condensing boilers; efficiency,

part load; fossil-fuel heating systems; gas-fired boilers;

hydronic heating; pulse combustion.

iii
ACKNOWLEDGEMENTS

The authors are Indebted to the Department of Energy for funding this study.

They also wish to acknowledge the valuable contributions of Mr. John McAuley,

who assisted in setting up the test apparatus and gathering the data, and

of Mrs. Mary Reppert for editing this report, and of Mr. David Brace who

assisted with installation of the test boiler.

iv
TABLE OF CONTENTS

Page

ABSTRACT iii

ACKNOWLEDGMENTS iv

LIST OF TABLES vi

LIST OF FIGURES . vi

NOMENCLATURE vii

1. INTRODUCTION 1

2. DESCRIPTION OF TEST BOILER 2

3. TEST INSTRUMENTATION DESCRIPTION 3

4. TEST AND CALCULATION PROCEDURES 5

4.1 Steady-state Test Procedure 6

4.2 Cool-down and Heat-up Test Procedures 6

4.3 Cyclic Test Procedure 7 4.4 Jacket Loss Test 7 4.5 Calculation Procedures 8

5. DISCUSSION OF RESULTS 11

6. SUMMARY 13

7. REFERENCES 14

APPENDIX Error Analysis 29

V
LIST OF TABLES

Page

Table 1. General Information on Tests Performed 24

Table 2, Steady-state Test Results Obtained Using Input/

Output Method 25

Table 3. Steady-state Test Results Using Heat-loss Method. • 26

Table 4. Condensing Boiler Cyclic Test Results 27

Table 5. Cool-down and Heat-up Test Results and Predicted

Efficiency at a Percent On-time of 22.5 Percent . . 28

LIST OF FIGURES

Page

Figure 1. Condensing boiler test arrangement ... 15

Figure 2. Pulse-combustion condensing boiler schematic ... 16

Figure 3. Test apparatus schematic ..... 17

Figure 4. Exhaust/alr Intake installation 18

Figure 5. Flue gas temperature dependence on return water

temperature from steady-state tests 19

Figure 6. Condensate vs. return water temperature during

steady-state tests 20

Figure 7. Steady-state efficiency vs. return water

temperature 21

Figure 8. Cyclic efficiency at percent on-tlmes of 4.9,

14.7, 22.5, and 42.0 percent as determined using

input/ output method 22

Figure 9. Cyclic efficiency at 22.5 percent on-time vs. return water temperature ..... 23'

vi
NOMENCLATURE

As

2 area of equal jacket surface temperatures, in ft

^L» latent heat loss coefficients

s

constant-pressure specific heat for liquid and water vapor

in the flue gases

coefficient of convection for each area of equal surface

temperature, in Btu/ (h ft °F)

^rs coefficient of radiation heat loss for each surface of equal

surface temperature, in Btu/(h ft ®F)

HHV measured higher heating value of test gas, in Btu/lb

Hs jacket heat loss for each surface in Btu/h

1 L , L

c * c cyclic and steady-state loss which corrects for hot condensate

going down the drain instead of up the flue as heated vapor, in

percent

^L,A latent heat loss, in percent

^,OFF off-cycle sensible heat loss, in percent

^,ON on-cycle sensible heat loss, in percent

^,SS,A sensible heat loss at steady-state operation, in percent

•m average water mass flow rate in Ibm/h

Pv partial pressure of water vapor in the flue gases at atmospheric

pressure of p.

P atmospheric pressure of 14.7 psia

p?» p5’ saturated vapor pressure of water at average on-period flue gas

temperature and at steady-state flue gas temperatures respectively

in psia

Po partial pressure of water vapor at 14.736 psia

Psw partial pressure of water vapor at 60°F of 0.2563 psia

^IN fuel energy input rate during steady-state and cyclic operation

test periods, in Btu/h

^OUT heat transfered to circulating boiler water during steady-state

and cyclic operation test periods, in Btu/h

vii
At length of test period

"^LAB laboratory air tempeaturing during test periods, in *F

Ts jacket surface temperature during jacket heat loss rate

test, in ®F

AT average temperature rise across the boiler during steady

state and cycle tests, in AT®F

^F,ON on-cycle flue gas temperature while the system is in cyclic

operation, in *F

^F,SS flue gas temperature at steady-state, in ®F

'’ss steady-state efficiency

^22.5% part-load efficiency at 22.5 percent on-time

viii
1 . INTRODUCTION

Condensing furnaces and boilers achieve high efficiencies through low

flue gas temperatures and by condensing part of the water vapor generated as a result of burning hydrogen in the fuel, to recover its heat

of vaporization. Since the formation of condensate requires cooling the

flue gas to temperatures below the water vapor's saturation temperature,

the heat exchanger's operating temperature must be kept in the condensing

region if condensation is to be achieved. In addition, the unit must

be equipped with a drain so that the condensed water can be continuously

removed.

Manufacturers are in the process of developing condensing furnaces and

boilers for the residential home heating market and there is a need

for a standard testing and rating procedure to estimate the seasonal

performance of these units and to allow their performances to be com- pared with each other and with non-condensing units. The purpose of

the study described in this report was to provide technical data which

would assist in the development of such procedures. Test methods for

evaluating the performance of non-condensing furnaces and boilers were previously developed in references [1] and [2] , but no credit was given

in those documents for the recovery of the heat of vaporization of

condensed water.

Laboratory tests were conducted on a condensing boiler having a counter

flow, fire-tube heat exchanger and a natural gas fired pulse combustor.

The unit employed a direct vent system which was designed to bring outdoor air in for combustion and to directly vent the flue products to

the outside. A condensate drain was incorporated in the base of this

unit to allow condensed water to exit the unit separately from the

flue gases.

Steady-state laboratory tests of the unit were conducted at constant

return water temperatures of 100, 110, 120, 130, 140®F (37.8, 43.3,

48.9, 54.4 and 60.0“C). At a number of these return water temperatures

part-load tests were also performed corresponding to percent on-times

of 4.9, 14.7, 22.5 and 42.0 percent.

Each steady-state test was also accompanied by a cool-down test and a heat-up test. The procedure employed required operating the boiler

until steady-state conditions were achieved and all necessary steadystate measurements obtained. The unit was then shut down for a 50-

minute cool-down period, during which the flue gas temperature was measured at several different times after the burner was shut off. It was then turned on and flue temperatures read after 1.0 and 5.5 minutes

of operation.

The efficiency of the unit during the steady-state and part-load operation was determined by measuring the total heat transferred to the water

during each test and dividing it by the measured energy input to the

boiler during the test period. In addition to using this Input/output

1
method, the steady-state efficiency and part-load efficiency at a percent

on-time of 22.5% were also calculated using a slightly modified version

of the heat-loss procedure given in reference [3].

The heat-loss procedure subs tracts from the complete input energy identified losses, such as latent and sensible percent flue gas and condensate

losses, attributable to operation of a heating systems.

Descriptions of the condensing boilers tested and the Instrumentation

employed are presented in sections 2 and 3 of this report, respectively.

Section 4 discusses the test and calculation procedures, while sections

5 and 6 contain, respectively, a discussion of the experimental results

and brief summary. A brief error analysis is presented in Appendix A.

2. DESCRIPTION OF TEST BOILER

The condensing boiler used in this study was a gas-fired 100 000 Btu/h

(29307 W) input unit. It used a pulse combustion principle in conjunction with a counter flow, fire-tube heat exchanger and direct venting

of flue gases. The unit was designed to take air for combustion directly

from outside the residence. The boiler, which is shown in Figures 1 and

2, had a cylindrical shape and was 44 inches (112 cm) high with a 14-inch

(35 cm) diameter. An external control box, mounted on the side, housed

a starting timer circuit, a coil used to fire the spark plug, and

pressure and temperature safety switches.

The pulse combustor operated in a cyclic manner. A thermostat "needfor-heat*' signal to the controls began operation of the blower, which

purged the combustion chamber and forced air into it at the same time

the gas valves opened to allow gas to enter. The spark plug ignited

the first few air/ gas charges to cause combustion. As the charges

exploded, the expanding gases closed the air/ gas inlet valves and the

rapid build up in pressure forced the flue gases to exit through the

heat exchanger tubes and venting system. The drop in pressure in the

combustion chamber, as a result of the flue products discharging through

the heat exchanger, helped to draw in a new charge of air/ gas which was ignited by the residual heat from the preceding charge. After a few

combustion cycles, the process became self sustaining, and the spark

plug and blower automatically shut off. The boiler continued to operate

until the thermostat was satisfied and the gas valve was de-energized.

A counter flow heat exchanger allowed a low return water temperature to

reduce the temperatures of flue gases exiting the unit and thus maximize

the heat transfer to the circulating water. With moderate return water

temperatures (below approximately 130°F (54.4°C)), the counter flow

heat exchanger was capable of cooling the flue gases below the saturation

temperature of the water vapor present in the combustion products. This

resulted in part of the water vapor condensing and giving up its heat of

vaporization. The unit was equipped with a condensate drain line (see

Figure 2) for removal of this condensed water.

2
The high temperature safety shut-down control was supplied pre-wired

with the unit. The settings of operating control devices were not

changed from their manufacturer's settings during the tests described

herein.

A flue pipe and a combustion air-inlet pipe were installed using 1.5

inch (3.8 cm) l.D. polyethylene pipe as specified in the installation notes supplied by the maufacturer.

Electrical power required by the boiler was supplied from a 120-volt

service panel in the laboratory. Automatic switch contacts for the

circulating pump, included as a part of the controls to cycle the

pump on and off with the burner, were not used. Instead, the boiler

and the water circulating pump were manually switched on-and-off.

3. TEST INSTRUMENTATION DESCRIPTION

Testing was conducted using the boiler test apparatus schematically

shown in Figure 3. An insulated 125 gallon (0.47 m^) water tank,

circulating pump, hot water dump, auxiliary boiler, and city water

makeup comprised the major support equipment. In order to achieve

repeatable water flow between tests, pairs of manually operated valves

were employed - one valve for flow control and the other for open/

shut (flow/no flow) operation.

The auxiliary boiler, a gas-fired 85 kBtu/h input unit, was used to

raise the temperature of the water in the storage tank between tests

and to maintain uniform return water temperatures to the test boiler

during cycling tests involving long off-periods. Pump and boiler operation was controlled by manual operation.

The direct vent exhaust/air intake system shown in Figure 4 was used

during all tests. It was designed and installed in accordance with the

recommendations of the manufacturer. Vent lengths were selected to be

typical of what would be expected if the boiler had been installed in a single-story dwelling with the exhaust vent passing through the roof.

Condensate was collected in a stainless steel beaker placed under the

drain line attached to the boiler and weighed immediately after each

collection period on a scale having an error of less than + 10 milligrams. A glass flask was also connected to the bottom of the flue pipe

outlet (see detail A of Figure 4) to catch any condensate which formed

within the flue pipe. The latter was not included in the reported con- densate quantities since the heat of vaporization given up when it

condensed would be transferred within the flue pipe and not within the

boiler.

A turbine meter in the return pipe to the boiler was used to measure the

water flow through the boiler. A manually read totalizer which counted

pulses from the turbine meter was used to determine the total water flow

through the boiler during each test. The accuracy of the turbine meter

3
was verified using a weigh tank and a timer at return water temperatures

from 100 to 140“F (37.8 to 60.0“C). Occasional calibration checks were made throughout the period of time that tests were being conducted.

A 40-junction thermopile made from 30 AWG type T (coppper/constantan)

thermocouple wire was used in stainless steel wells inserted in the supply

and return water lines to measure the temperature differential across

the boiler. The stainless steel wells had an outside diameter of 3/8

inches (0.95 cm) and a 6-inch (15.2 cm) immersion length, as recommended

in ASHRAE Std. 41.1-74 (4). All other temperature measurements were made using type T thermocouples.

A multipoint strip-chart recorder was used, in addition to the digital

data logger, to provide a continuous recording of the temperature differential across the boiler. A planimeter with a minimum scale division of

0.01 square inches (0.065 cm^) was used to find the area under this curve

in order to determine the average temperature differential across the

boiler during the cyclic tests. The actual temperatures of the supply

and return water and the flue gases were also continuously recorded for

the purpose of monitoring the test results. All other temperatures were recorded on the digital data logger at ten-minute intervals.

The wet-bulb and dry-bulb temperatures of the entering combustion air were measured using a pair of thermocouples and a blower as described in

reference 4. The wet bulb thermocouple was equipped with a wick, which

was supplied with distilled water from a reservoir. This psychrometer

was located at the same elevation as the combustion air intake, as illustrated in Figure 4.

The flue gas temperature was measured using bead-type thermocouples

manufactured in an inert gas thermocouple welder. They were installed

as a 5-ln-l averaging thermocouple placed in the flue pipe 15 inches

(38 cm) from the boiler exit. Radiation shields were not necessary

because the design of the test boiler prevented the thermocouple from

"seeing" the flame. The first 18 inches (46 cm) of the flue pipe

downstream from boiler were insulated with foil-covered insulation.

Flue gas analyses were performed to determine the concentration of

carbon dioxide, oxygen and carbon monoxide present in the flue gases.

Samples were drawn from the center of the flue pipe at the temperature

measurement plane. The carbon dioxide concentration was determined

using an infrared absorption analyzer. The sample train included a vapor

trap, particle filters, and a flow meter. The instrument was checked

each test day using a zero (0.0%), a midrange (5.13%), and a full-scale

(15.2%) span gas consisting of CO2 in nitrogen. The percentage by volume

of oxygen present in the flue gases was measured using an electrochemical

sensor with oxygen-sensitive electrolyte. The concentration of oxygen

provided a check on the CO2 measurement and was not used in the calculation procedure. The carbon monoxide concentration in the flue gases was determined using a length of stain detector sample tube. The sample

was passed through a vapor trap and the sample tube by a manually operated

4
pump. The concentration of CO2 in parts-per-million (ppm) was based on

the length of color change from yellow to brown of potassium palladosulfite impregnated silica gel in a glass tube. This gave a reading within

+ 10 ppm in the 0 to 175 ppm range. The carbon monoxide concentration

readings were taken as a check of the boiler's combustion performance

and did not enter into the efficiency calculations.

The higher heating value of the natural gas used was continuously recorded

on a gas calorimeter located at a nearby NBS building. The amount of gas

consumed during a test was measured with a dry-type positive displacement

meter with a one cubic foot per revolution register. The gas temperature

at the meter was measured using a type T thermocouple and the gas pressure

at the meter was determined using an open tube manometer and a barometer.

Electrical energy consumption of the boiler was measured by a 2 wire,

115-VAC, watt-hour meter having a resolution of .01 kWh. The line

voltage was monitored with a digital voltmeter.

4. TEST AND CALCULATION PROCEDURES

Three types of tests were performed on the boiler: steady state, cooldown/heat-up, and cyclic. During each steady state test a constant

return water temperature was maintained and the water flow rate was adjusted to give a constant temperature rise across the test boiler.

A steady-state test always preceded a cool-down/heat-up test. The

latter consisted of a 50-minute off-period, during which there was no water flow through the boiler, followed by a brief on-period at the same return water flow rate and return water temperature as the preceding

steady-state test.

The cyclic tests consisted of several on/off cycles at a constant return

water temperature. During both the cool-down and heat-up portion of the

tests, the water flow rate was maintained at a value which resulted in a 20“F (11.1®C) temperature rise across the boiler under steady-state

operation at the same return water temperature. Maintaining water flow

through the boiler during the off-period of the cyclic tests was necessary in order to accurately determine the heat transferred to the circu- . lating water using the input /output method of testing.*

* This is because the time constant of the thermocouples and wells tended

to yield results which underestimated the amount of heat transferred

to the water during the first few minutes of boiler operation. Operation with continuous water flow minimized this problem, since the measurement errors immediately after start-up and after shut-down tended

to cancel each other.

5
4.1 STEADY-STATE TEST PROCEDURE

Prior to beginning the actual steady-state tests, the boiler was operated

at the desired return water temperature until changes in the temperature

differential across the boiler were less than + 0.5**? (0.28"C) per hour.

At least two hours of boiler operation were necessary to achieve this

condition, since the firing-rate of the test boiler tended to slowly

decrease with time after the initial start-up. In order to assure a con- stant return water temperature to the test boiler, the temperature of the

water in the storage tank was maintained at a constant value by adjusting

the rate at which heated supply water was dumped and replaced by 60“F

(15.6®C) city water. The flow rate through the boiler was adjusted to

give a constant 20°F (11.1°C) temperature rise through the boiler under

steady-state operation.

The actual steady-state test consisted of one hour of continuous operation at the desired return water temperature, during which the gas input,

gas pressure, barometric pressure, water flow rate, electric consumption,

and voltage were monitored and recorded at ten-minute intervals. The

concentrations of CO2 , CO, and O2 in the flue gases were measured once during each steady-state test. The condensate was collected over a period of time which was long enough to give an accurate estimate of the

rate of condensate formation. The temperature differential across the

boiler, the return water temperature, the supply water temperature, and

the flue gas tempertures were recorded on a multipoint strip-chart

recorder and, at ten minute intervals, on a digital data logger.

4.2 COOL-DOWN AND HEAT-UP TEST PROCEDURES

The cool-down and heat-up test was always preceded by a steady-state

test. They were conducted as described in Sections 3.2 and 3.3 of

Reference 1.

To assure that no water flow occurred during the cool-down period (as

required in [1]) a valve was closed on the supply side of the boiler

immediately after the burner was turned off. During the off-period,

flow was maintained through a by-pass loop in order to keep the "return

water" at a uniform temperature prior to the start of the heat-up test.

Tlie water flow rate at which the preceding steady-state test was conducted was maintained during the heat-up test.

Five flue gas temperature measurements were made during the cool-down and

heat-up tests. Flue temperatures were recorded at 3.75, 22.5, and 45.0

minutes after the unit was turned off, and 1.0 and 5.5 minutes after

burner start-up. These flue temperatures were used to estimate the partload efficiency of the condensing boiler at a cycling rate having an on-period of 22.5%, as described in section 4.5. No attempt was made to

measure the heat transferred to the circulating water during the heat-up

tests.

6
4.3 CYCLIC TEST PROCEDURES

The boiler was manually cycled on-and-off based on observed clock test

times. The lengths of the on and off-periods for the four different

percent on-times studied are given in Table 1. Cyclic tests actually

performed are indicated by the "X's" in this table.

Prior to each cyclic test, either a previous cycle test was performed or

a brief period of continuous operation was carried out in order to establish the return water temperature and other test conditions. The boiler

was operated at the desired cycle rate and return water temperature for

at least one cycle preceding any test cycle used for calculating the

efficiency

.

The flow rate was set to maintain a return water temperature which would

result in a 20®F (11.1®C) temperature differential across the boiler if

the unit were operated in a steady-state manner at the same return water

temperature. This flow rate through the boiler was maintained during

both the on and off-periods.

The temperature differential across the boiler was measured using both a multipoint strip-chart recorder and a digital data-logger. A smooth

curve was drawn through the individual temperature differential points

on the strip chart and a planimeter was used to find the area under the curve. From this information, the average AT across the boiler during

the entire on/off test cycle could be determined and the heat transferred

to the circulating water per cycle could be calculated as described in

section 4.5. The amount of gas consumed (corrected to standard conditions) by the boiler during a test cycle and the higher heating value of

the fuel were measured with the instrumentation discussed in section 3.

Other temperature and pressure measurements were also performed and

recorded throughout the test to assure that the unit was operating properly. Most of these measurements were identical to those made during

steady state operation and are discussed in sections 3. and 4.1.

4.4 JACKET LOSS TEST

Jacket loss tests were conducted during steady-state operation at approximately 100“ and 130“F (37.8“C and 54.4*C) return water temperatures.

The boiler jacket was subdivided into areas of 5-inch squares (32.3

cm"^) and the surface temperature measured at the center of each with

a thermocouple. This thermocouple was moved from point to point on

the jacket surface. The temperatures were read on a digital temperature meter and were manually recorded. The radiation and convection

losses were calculated by the procedures given in Appendix F of ANSI

Z21. 47-1973.

7
4.5 CALCULATION PROCEDURE FOR DETERMINING STEADY STATE AND PART-LOAD

EFFICIENCIES

Using the input/output method, the efficiency of the test boiler was defined for both steady state and cyclic operation as;

n = ^Qput^ . (Qin ) where is the heat transferred to the circulating boiler water during

the test period and is the energy input to the unit during the same period of time. For a steady state, this test period consisted of one hour of steady state operation, while for a cyclic test it consisted of one or more complete on/off cycles.

For all tests, the energy input, was calculated by correcting the

measured volume of gas consumption during the test period to standard

conditions* and multiplying the result by the higher heating value (in

Btu/ft^) of the fuel. To account for the fact that the gas consumed

by the test boiler was very dry (i.e., contained very little water vapor),

the latter was adjusted by using the equation:

HHVl = HHV| (. dry saturated P - P o ^sw

) ,

where P^ is atmospheric pressure of 14.736 psi and Pg^ is the saturated

water vapor pressure at 60®F of 0.2563 psia. The heat output of the

boiler, Qqu^> calculated using the formula;

«out ' “ Hp AT At ,

where the average water mass flow rate, m, was determined by dividing

the measured value of the total mass of water passing through the boiler

during the test period by the length of the test period. At. The average

specific heat of the water, Cp, was assumed to be equal to the specific

heat of water at a temperature equal to one-half the sum of the return

water temperature and the average supply water temperature. For all

steady state tests, the average temperature rise across the boiler.

At, was determined by calculating the difference between the supply and

return water temperature at 10 minute Intervals and then averaging these

temperature differences for the test period. For all cyclic tests, the

instantaneous temperature differential across the boiler, AT, was directly measured on a multi-point strip chart recorder and the average

water temperature rise. At, was calculated by finding the area under

this AT-time curve (for the test period) and then dividing it by the length

of the test period.

* Standard conditions are defined as 60®F (15.6°C) and 14.736 psia

(101.6 kPa).

8
Using the heat-loss method, the steady-state efficiency and part-load

efficiency at a percentage on-time of 22.5 percent were determined by the

calculation procedure specified in reference 3. The only exceptions to

this were: (1) the steady-state efficiency was found for the actual

return water temperature measured (not 180°F (82°C)), (2) credit was given in the steady-state efficiency calculation for the heat gain resulting from condensate formation, and (3) since the primary interest was

a comparison of laboratory results, the measured room air temperature

was used in place of the recommended 42°F average outdoor winter

temperature in calculating the part-load efficiency.

The basic equations employed in the heat loss method of calculating

performance were:

^SS ^,A “ ^S,SS,A

“

^c»

^22.5% “ ^,A “ ^S,ON

“ ^c*

where the last equation follows from the fact that the test boiler was assumed (for the purpose of this analysis) to use indoor air for combustion (i.e. Cg = 1), it did not employ a pilot light (i.e. PF = 0) , and

its design prevented air flow through the heat exchanger during the

off-period (i.e. Dj, and, therefore, Lg equal 0).

The quantities ^

and accounted for the latent heat loss due ’l h.,A

to that part of the water vapor (generated by the burning of hydrogen in

the fuel) going up the flue during steady-state and cyclic operation,

respectively. For a natural gas, the assigned value of 9.55% was used

for A [1»2]. The heat loss coefficients and were calculated

using £he equations:

t and

^

"

where ^v is the partial pressure of water vapor in the flue gases if there

were no condensation and the atmospheric pressure, P, was assumed to be

14.7 psia. The equation for calculating p^ is given in [3]. The quantities

p

s' s V and P^ correspond, respectively, to the saturated vapor pressure

of water at the steady-state flue temperature, Tp gg, and the average

on-period flue gas temperature, Tp qj^, defined in* [3].

t The quantities Lq and account for the loss due to hot condensate

going down the drain and for the fact that this condensate did not go

up the flue as heated vapor [3]. They were calculated using:

9
L'c ~ ••^ ~^io53,3

^ ^liquid Pvapor) (Tp^sg "

'^LAB^^ ,and

" ^^*^1053,3^^^ ((^Pliquid " ^Pvapor) (Tp^gS "

'^LAB^^»

where Tp^g was the laboratory air temperature during the test period.

The terms Lg gg ^

and Lg qjj represent the sensible heat loss during the

steady state an^ cyclic ^ests, respectively. Tliey were calculated using

the procedures prescribed in [1,2,3], except that Cg was set equal to

1 . 0 , which meant that the temperature of the combustion air was not

corrected to 42°F (5.6“C). This was done in order to allow the efficiencies calculated by the heat loss method to be properly compared with

those obtained using the input/output method.

Since the input/output method accounts for the boiler jacket losses

while the heat loss method does not, it was necessary to directly measure the jacket losses in order to fairly compare the results obtained

with the two methods. Using the surface and air temperatures as measured in section 4.4, steady state percent jacket losses were calculated for the test unit operating at approximately 100 and 130“F (37.8

and 54.4“C) return water temperatures.

The heat loss rate from the jacket was calculated using the sum of the

radiant and convective losses given for each surface area. The equation

employed was:

»S - + he) (T3 - I3

) where Hg is jacket heat loss rate for each surface,

Ag is area of equal surface temperatures,

hj.g and h^, are coefficients of radiation and convection for each area

of equal surface temperatures, and

T and T„ are the surface and room temperatures respectively. 0 cL Specific values of the hj.g and h^ coefficients were determined for each

area of similar surface temperatures. The loss rate for all surface areas was totaled and divided by the measured input fuel rate, to

give the percent jacket losses.

10
5. DISCUSSION OF RESULTS

The close relationship between the flue gas temperatures and the return

water temperature for this condensing boiler is clearly illustrated in

Figure 5 for a number of steady state tests conducted at different return

water temperatures. The difference between these two temperatures varied

from about 14“F (7.8°C) to approximately 4®F (2.2®C) as the return water

temperature was increased from 100®F (37.8®C) to 140°F (60.0®C). This

resulted in a rapid drop off in the rate at which condensate was generated as the return water temperature was increased, as shown in

Figure 6. As a consequence, the steady-state efficiency measured using

the input/output method dropped from approximately 97.8% at a return

water temperature of 100“F (37.8®C) to approximately 90% at a return

water temperature of 140°F (60.0®C).

The exact steady-state efficiencies, calculated using both the

input/output and the heat-loss method, are presented, respectively, in

Tables 2 and 3, along with information on the various test conditions.

Table 2 also contains the results of two steady-state jacket loss tests

conducted at return water temperatures of 100°F (37.8°C) and 130®F

(54.4®C). This Information is plotted in Figure 7, where the dash-dotdash line through the heat loss data was "estimated" in order to give an inflection point at 125*F (51.7®C)* and the straight solid line

through the input/output data represents a linear least-squares fit.

Since the heat loss efficiency data do not include jacket losses while

the input/output data do, the latter were adjusted by adding the

jacket losses measured at return water temperatures of 100 and 130°

(37.8 and 54.4°C) and plotting the dashed line shown in Figure 7 through

the results. Comparing this dashed line with the heat-loss results

(the dash-dot-dash line) indicated that the input/output method yielded

steady state efficiencies that were slightly greater (between 1 to 3

percentage points) than the corresponding heat loss efficiencies. The

above differences tended, however, to be either within or nearly within

the range of experimental error at most of the return water temperatures

studied.

The results of the cyclic tests conducted at different return water temperatures and different percent on-times are presented in Table 4. The

cyclic efficiencies contained in this table, which were calculated using

the input/output method, are plotted in Figure 8 along with linear lines

representing a least-squares-f it to each set of data. It can be seen from this figure that the efficiency of the boiler in the range of conA return water temperature of 125°F (51.7°C) corresponds to a flue

gas temperature (see Fig. 6) which approximately equals the flue gas

vapor’s calculated saturation temperature. Flue temperatures below

this value should result in condensation, while those above it should

not

11
ditions studied, is much more strongly dependent on the return water

temperature than upon the percent on-time. For a percent on-time of

22.5% (the rating point used in both the non-condensing and condensing

furnace/boiler test procedures [1, 2, 3]), the efficiency of the test

boiler was found to drop 11 percentage points as the return water

temperature was increased from 100°F to 140®F (37.8®C to 60.0®C).

Table 5 contains the results of the cool-down and heat-up tests and the

part-load efficiencies predicted for a percent on-time of 22.5% using

the procedure described in section 4.5. This calculation procedure is

the same as the method proposed for condensing furnaces and boilers in

[3], except that a slight adjustment was made for the use of indoor

laboratory air for combustion (instead of outdoor air). The predicted

part-load efficiency decreased from approximately 94% to 89% with

increasing water temperatures from 100 to 140°F (37.8 to 60.0“C).

The efficiencies in Table 4, corresponding to values measured using the

input/output method at a percent on-time of 22.5%, are plotted in

Figure 9 along with the predicted efficiencies (for the same percent

on-time) from Table 5. Lines were "estimated" to both sets of data,

and made to have inflection points between return water temperatures

of 120 to 125°F (48.9 to 51.7®C). If the line through the measured

cyclic test data is adjusted for the effect of jacket loss , the dashed

line in Figure 9 is obtained. Comparing this dashed line with the line

through the predicted efficiencies in this figure (the dash-dot-dash

line) shows that the input /output method tended to give efficiencies

which were two to five percentage points higher than the values obtained

using the heat-loss method over the range of return water temperatures

studied. However, a large part of this discrepancy can be attributed

to the expected experimental error calculated for such tests (see

Appendix A)

.

Another feasible explanation for the input/output method yielding both

steady state and part load efficiency results which were slightly higher

than those obtained with the heat loss method is the fact that the heat

loss method recommended in [3] uses the existing flue gas temperature

to estimate the amount of water vapor which was not condensed. If a substantial part of the heat exchanger was actually at temperatures

below the measured flue gas temperature, this procedure would tend to

overestimate the amount of uncondensed water vapor and thus the latent

To adjust for the effect of jacket losses under cyclic operation, it was assumed that the cyclic jacket losses at a given return water

temperature could be approximated by the steady state jacket loss

(as shown in figure 7) at the same return water temperature divided

by 0.225. This approximation, which assumes that the rate of heat

loss from the jacket during the off-period is the same as during

steady state operation, is likely to overestimate the cyclic jacket

losses

.

12
heat losses. An examination of the amount of condensate actually collected tended to indicate that this was indeed the case, but that the

overestimated latent heat loss resulted in efficiencies that were on

the average only about one percentage point low. Consequently, the

use of the existing flue gas temperature to estimate the latent heat

losses contributes to, but does not fully explain, the discrepancy

between the two methods.

6. SUMMARY

The performance of a gas fired condensing boiler was evaluated in the

laboratory for the purpose of providing technical data and background

information that would assist NBS in the development of a testing procedure for condensing boilers and furnaces [3].

Steady state tests were performed on the unit at several return water

temperatures and the efficiency of the boiler was calculated using both

the input/output and heat-loss methods. The steady state efficiency

was found by the input /output method to vary between 98% and approximately 90% as the return water temperature was Increased from 100 to

140®F (37.8 to 60.0‘*C). These efficiencies tended to be between one

and three percentage points higher than the corresponding steady state

efficiencies estimated using the heat-loss method.

Cyclic tests at several different return water temperatures and percent

on-times were also conducted. For a percent on-time of 22.5%, the

input /output method resulted in efficiencies which varied between 98%

and approximately 88% with increasing return water temperatures from

100 to 140“F (37.8 to 60.0°C). Cool-down and heat-up tests were also

conducted and the results were used to predict the part load efficiency

at different return water temperatures and a fixed percent on-time of

22.5%. The calculation procedure employed was the same as the one proposed for rating condensing furnaces and boilers in [3]

,

except that

a correction was made for using indoor laboratory combustion air. A

comparison of the efficiencies obtained for the cyclic tests employing

a 22.5% on-time with the predicted efficiencies when credit was given

for jacket losses, showed that the efficiencies measured using the input/

output method were between two and five percentage points greater than

the corresponding heat loss method results.

A part of the discrepancy between the results obtained using the input/

output method and those obtained using the heat loss method can be

explained by the fact that the heat loss calculation procedure [3] uses

the exiting flue gas temperature to estimate the amount of uncondensed

water vapor. This tends to overestimate the latent heat loss and results

in efficiencies which were on the average about one percentage point

low. The remaining portion of the difference observed between the two

methods is probably due to the large experimental error associated with

the input/output method (see Appendix A).

13
7 REFERENCES

1. Kelly, G.E., Chi, J., Kuklewicz, M.E., •'Recommended Testing and

Calculation Procedures for Determining the Seasonal Performance of

Residential Central Furnaces and Boilers," NBSIR 78-1543

(Washington, DC: NBS, 1978) Available from NTIS,PB289484.

2. "Test Procedures for Furnaces and Vented Home Heating Equipment",

Federal Register, Volume 43, No. 91, Wednesday, May 10, 1978,

pp. 20147-20205.

3. Kelly, G. E., Kuklewicz, M. E., "Recommended Testing and Calculation

Procedures for Estimating the Seasonal Performance of Residential

Condensing Furnaces and Boilers," Draft NBSIR 80-2110 (Washington, D.C.

National Bureau of Standards).

4. ASHRAE Standard Measurement Guide: Section on Temperature Measurements, 41.1-74, American Society of Heating, Refrigerating and AirConditioning Engineers, New York, N.Y., 1974.

5. Chi, J., Kelly, G.E., "A Method for Estimating the Seasonal Performance of Residential Gas and Oil-Fired Heating Systems," ASHRAE

Transactions, Vol. 84, Part 1, AF78-1 No. 3, 1978.

6. Kelly, G.E., Didion, D.A., "Energy Conservation Potential of Modular

Gas-Fired Boiler Systems;" NBS BSS 79, (Washington, D.C.: NBS

1978). Available from Sup. of Doc., SD Cat. No. C13: 29/2:79.

7. ASHRAE Standard Measurement Guide: Engineering Analysis of Experimental Data, 41.5-75, pp. 4-5, American Society of Heating, Refrigerating and Air-Conditioning Engineers, New York, N.Y., 1976.

14
I 15
\i
' COMBUSTION

! AIR INTAKE

>C

NATU RAL GAS —cm

115V AC

FLUE GAS EXHAUST

PRESSURE

RELIEF

INLET AIR PLENUM

C BLOWER > /

GAS and AIR

INJECTION VALVE

THERMOSTAT LINE

COMBUSTION

CHAMBER

(3

^

SPARK

PLUG

5

SUPPLY

HIGH TEMPERATURE

LIMIT SENSOR

HEAT EXCHANGER

CONDENSATE &

FLUE GAS

SEPARATION

5

RETURN

1

CONDENSATE

DRAIN

Figure 2* Puls e“Combust ion condensing boiler schematic)

16
»f,5

m ^'1

Ilf" li’^i,

‘.m

i'P' m.

m ’-; € . -V: HOifeuafi

' S3<>yTHi «l

>81!

#

js3‘ Yjs^ue w m

’h?TP>

r1^

SUUfAllSWsJI not#

i»o?w3e JW'i

-j

wmi'

•30' ^?!3?> *A£- lA»lf^

W.JN

i*Ai .‘irfnAuj

• " " ' *^ J*

' :-m

''k'fW ''Ai .Lis

M»UT?J5 %

>“' '"#' Ot m:

:js^ m

%A yen:

1 'l«l#UTi.'^i ' •

#' ‘

itv

rmttt.-.-

' -i"" IT

AllAlse

. %

N»rfi

• " J’V

'i''*^83iMK13IBBi^ ^ ~

'.4T , -jjyiM

j^:i, M

.‘i;. * •'ju*:^:,1lli:
60°F

CW

PRESSURE

REDUCING

VALVE

MAKE

UP

XI

x-

.25'

°i< z«> o

•H4J

(Uu

CD

CDP

4-1 CD

>-l CDO.a

CD

4-1 CD

Oi H

0)

S-i D

c»c •H

P-i

17
Figure 4. Exhaust/air intake installation.

18
TEMPERATURE

OF

FLUE

GAS

-’F

RETURN WATER TEMPERATURE - °C

37.8 43.3 48.9 54.4 60.0

65.6

60.0

54.4

48.9

43.3

37.8

Uo

I

Figure 5. Flue gas temperature dependence on return

water temperature from steady-state tests.

19

TEMPERATURE

OF

FLUE

GAS
RETURN WATER TEMPERATURE - °F

Figure 6. Condensate vs. return water temperature

during steady-state tests

20

CONDENSATE

Kg/h
o MEASURED BY INPUT/oUTPUT METHOD

LEAST SQUARES FIT TO INPUT/OUTPUT DATA

LEAST SQUARES FIT TO INPUT/OUTPUT DATA ADJUSTED TO GIVE

CREDIT FOR HEAT LOSS THROUGH JACKET.

O MEASURED BY HEAT LOSS METHOD WITH CREDIT FOR

CONDENSATE FORMATION

"estimated" fit TO HEAT LOSS DATA

RETURN WATER TEMPERATURE - °C

RETURN WATER TEMPERATURE - °F

Figure 7. Steady-state efficiency vs. return water temperature.

21
RETURN WATER TEMPERATURE -®C

37.8 43.3 48.9 54.4 60.0

Figure 8. Cyclic efficiency at percent on-times of 4.9, 14.7, 22.5,

and 42.0% as determined using input/output method.

22
^ __ MEASURED BY INPUT/OUTPUT METHOD — "estimated" fit to input/output data — "estimated" fit to input/output data adjusted to give

CREDIT FOR HEAT LOSS THROUGH JACKET

EFFICIENCIES BASED UPON C00L“D0WN/HEAT“UP TESTS AND

CALCULATION PROCEDURE DESCRIBED IN SECTION ^,5 •— "estimated" fit to efficiencies derived from cool-down/

HEAT-UP DATA

RETURN WATER TEMPERATURE - °C

RETURN WATER TEMPERATURE . °F

Figure 9. Cyclic efficiency at 22.5% on-time

vs. return water temperature.

23
TABLE

1.

GENERAL

INFORMATION

ON

TESTS

PERFORMED o o O• O O St vO f-i HuHbOWH

<5

m• • !S iri • O

<! • • o

!z

m

><<

X

crt oou

ON o^ 00 o 00 vo ON • • • • 00 ON CM O O CO 00 • • • • CO <H CO m uo CO iH I I X 0 (2

-4- o 0> CO • o cx CJ (U St CO X X 1 X 1 X 3 U 3 m r—(

4) rH

TJ cx (d

(U >

ON CO Cd • o cd cu 00 CM X X X X X X 40 4J 42 ! i cd 4J

(U 42 (U CO S? CX »s • o CJ CU CO f—i X X 1 X X 1 St iH CO CM H H

CO 4-1 CO 00 (U O (U • O 4-1 4J X X X 1 X X

CO I—1 CJ 4-1 CX

•H cd 3

rHM

r

O

H

X 00 cd

W o 3 <u • • •H z < < ON liO o (U rH H S'? • • • • • • 42 O 3 1 z z St CM CM 4J X % Z rH CM 'd- o o o •o cu Is -o

(U rH

O 3 1 4J Z 4-1 rH

(U cd 00 o

rH o CO 3 O

a. •H (U •H

O

cu 0 rH

a >

CO

O 4J (X •H •H CU o CO (X 4-t 00 42

0) 4-* <U <1

U

+J cd 4-1

H 4-1 4J

(0 (U CO 4-1 3 Is

4-1 Z u

O CJ

O (U

O

O

O 4J

O

(/> •H •H •H •H

H z z CO 4s

rt rH rH rH rH c O

X 5 O

O

O

O 1 o a

w o X X X • Is 3

ccJ

Q o o u o X 1 <JJ cu 3 a

(U • 42 42 -H

4-> rH z H +J 4J

n)o o

•H 'S. a, c • X o <—•

4J CO

(U “gc;

24
(0

aoXH0l< H3OH3

&•Z

CA3

tao

’Ji H3

CAH

SdH

blH

CA5

b]H

CA

bl

iJ«<H

<U00

(0IdV>< ^ M -H o ” o 5 ft.

S'?

(Uu S 2.gH s

oa b. • e 3

OQ 5*4 . 0 ao

CM’’ o

0) 4J

(04> a

T3 ja S h3 oO3 3 0*J

O'

M

Z^M 3

O'

u

PO

CO rH

C

u (0 1) U 9 O i 0) 4J Ul o o » C < 0L4 9

0)^49 •

CO C/) CO •

0)00

CO Ci4 m o

0)

H

4"’

b. c • u n , 3 2 § o m

CM 1 1 1 1 i • t O o

00 o nO CM • • • • •

-<r CM ON

m cn

CM CM CO -*4'

ON ON CM CM CO «o •O m >0* m o ON 00 o • • • • • •o *4- CM

sO NO vO

ON 00 cn -a- • • • • •

r*- r>»

r>.

O

O o O o O CM o cn o

in CM 00 • • • • m cn cn CM cn

-0- -o 00 • • • • o o o o o

00 00 1 00 1 »-4 • I • 1 • o o o o o o o o o o o

00 00 o oo m CM o r^. NO

00 00 00 r>«. r>- o o o O o o o o o O ON CM 00 m

00 00 00 00 00

>«• <r cn cn cn • • • • CM CM CM CM CM +I -H +1 -H +1

00 o r*« o • • • • • CM O o ON ON ON ON ON

cn o ON o • • • • • o o ON 00 00

CM CM

cn

O CM CM CM • • • • • o o o o o o CM cn '4'

•9

0)

fH 0) 4J

V o jr •H 4) G 4J d 54 •H 9 00

0) * 4)

0) fH CO •o U .o cd 9

(d cdd • •HH 4J o CO <d

CO •H o iH

0) CO

U 1 O o 9 •H G 5*3 u u

1—1 V

CO

u > 0>

M > G 0) •H C 0 ^4 4> O 00 • .d M O 0> d

CO 4J • cn •o

O 0) CO ON 1 u • • o Cd x:

U CMu cd

G •o On C*4 0) d 0) >> 4J a cd .o •H ON <v 3 0> 41 ki * >N J= ON P 9 u o •H d m 00

CO

G

u

a d) CM 4)

cd Q) •Hu • -o

Q •o

M 3 0) iH Cd

H o •H 9 Ui >% <

iM o a 0 •o o. d> 4) a m (U >>

CO CO cd (4 • 9 u •H 3 00 00 a 4J •H 0) <wf o • Cd G iJ tj fM M 54 D 4J a 5n

•H 4^ IM •o 4J 9 Of

IMO

O s O .d

a

g •

CO 0 CM 0)

M 4J 00

a 0)O •o CO •

0) CO 4J CO iH »H u 9 0) > 00 Cd CO 00 CO pd l-i *H -X •H

•Hd 9 -O

M

CO CM o 9 4J o d

•Hu Cd *o

M 45 45 iH T3 CO PO ^ G M •o s d U d> cd 4= 45 •H d cd 0) B a CM > o > O a CM a CM •H

•H

u

d i-i 3 cn x> •H •o

4J

X o CM

M rH *o

Cd o> o M * c M M u o U M CO p op V4 Wi (0 ^l Vi 9

0) 0)

d 0) 0) 54 o 45 •H

u

X •H > > M > > d 4J d C Xi d d M O o O 9 0 o

CJ

O CO

u

u cS

0) 4J .d •H o O o o o H IM H H H H

<d

25
Table 3. STEADY STATE TEST RESULTS USING HEAT-LOSS METHOD

AVERAGE

RETURN

AVERAGE

TEMP. CALCULATEDSTEADY-STATE

BOILER

condensate'’

02 CO rh of

COMBUSTION

WATER

TEMP.^

rise'’ EFFICIENCY® AIR

*F AT“F % iVh % ppm %

99.8 20.5 94.2 4.19 2.8 160 50

109.7 20.8 92.5 3.62 2.9 140 60

120.4 20.5 89.8 — 3.2 100 78

129.6 20.5 89.1 0.64 3.0 120 60

140.0 20.1 89.0 0.23 2.5 150 60

® Efficiency calculations adjusted for use of laboratory temperature air for

combustion instead of outdoor air.

^ The Internal System of Units of Measurement (SI) values are included within

the text and figures of this document, and within this table the following

conversions should be applied to the U.S. conventional units.

To convert temperatures from degree Fahrenheit to degree Celsius, subtract

32 and divide by 1.8.

To convert Ibm/h to kg/s, multiply by 1.259 979 E-04.

To convert differential temperature AT degree Fahrenheit to AT degree

Celsius, divide by 1.8.

26
TABLE 4. CONDENSING BOILER CYCLIC TEST RESULTS

Percent

On-Time

%

Avg. Cycle^

Return

Water

Temp. ®F

Cyclic^

Effy.

%

Qin

Btu/h

%UT

Btu/h

•m

Ibm/h

Air rh

%

Boiler

Condensate

Ibm/h

99.2 95.8 4900 4700 4432 50 0.301

4.9 120.3 89.1 4900 4400 4064 62 0.278

110.0 94.4 14600 13800 4359 56 0.586

14.7 118.6 94.3 14600 13700 4292 52 —

130.2 88.9 14300 12700 4147 56 —

100.9 99.0 23400 23200 5402 65

22.5 108.8 95.6 22300 21300 4359 64 —

118.6 90.8 22200 21200 4279 52 —

138.8 88.0 21400 18800 3922 58 0.190

102.3 97.8 44300 43300 5107 43 2.293

42.0 118.5 93.9 41300 38800 4286 53 0.951

130.6 90.1 40300 36300 4151 55 0.355

a Measured using input/output method.

b The International System of Units of Measurement (SI) values are included

within the text and figures of this document, and within this table the

following conversions should be applied to the U.S. conventional units.

To convert from Btu/h to watt (W), multiply by 2.930 711 E-01.

To convert temperature from degree Fahrenheit to degree Celsius, subtract

32 and divide by 1.8.

To convert Ibm/h to kg/s, multiply by 1.259 979 E-04.

27
TABLE 5. COOL-DOWN AND HEAT-UP TEST RESULTS AND PREDICTED EFFICIENCY AT A PERCENT ON-TIME OF 21.57.

Return

Water

Temp .

** *F

Predicted

Efficiency

at 22.5%

On-Time®

%

Concentration

CO,

%

TFSS

"F TF0FF3“F TF0FF4

“F TF0FF5“F TFONl°F TF0N2

“F TRA“F QinSteady-State

Btu/h

99.8 94.2 10.9 113.1 108.7 100.8 94.4 111.5 112.1 85.8 82940

109.7 92.7 10.9 121.6 116.2 101.4 85.6 117.8 120.6 77.4 85559

120.4 90.2 10.6 130.8 124.0 109.1 85.8 124.0 129.8 76.5 88594

129.6 89.1 10.9 135.9 128.9 110.7 75.5 131.2 134.8 75.5 84683

140.0 89.0 10.9 144.0 134.8 113.3 78.0 135.0 139.8 78.0 84281

® Efficiency calculations adjusted for use of laboratory temperature air for combustion

instead of outdoor air.

^ The International System of Units of measurement (SI) values are included within the text and figures of this document, and within this table the following conversions should be

applied to the U.S. conventional imlts.

To convert from Btu/h to watt (W), multiply by 2.930 711 E-01.

To convert temperatures from degree Fahrenheit to degree Celsius, subtract 32 and divide

by 1.8.

28
APPENDIX

Stiminary of Errors

The uncertainty for the cyclic efficiency tests can be estimated by

reviewing the possible sources of errors in the measured variables used

to determine the input and output energy.

The sources of measurement error in calculating the cyclic heat output

are inaccuracies in water flowrate, the strip chart recorder and the error associated with determining the area under the temperature differential

(across the boiler) vs. time curve. The turbine meter and output totalizers error is estimated at equal to or less than ±1.0%. The strip chart

recorder manufacturer's stated error was ±0.5%. The area under the

temperature differential vs. time curve for each cycle is believed to be

determined within ±0.6%, based upon repeated measurement of a prepared

known sample area. Possible error from variation in specific heat of

water is ignored as the value used in the calculations was based upon

the average temperature of the water within the heat exchanger and should

have little effect on the overall uncertainty.

Other possible sources of measurement inaccuracies are the total gas

input (±0.5%), the gas higher heating value (±1%), the gas pressure

(±2%), and gas temperature (±0.1%). These inaccuracies, when combined

with those of the output measurements, give a total uncertainty of

±2.4% to ± 2.7% in the measured part load efficiencies which were applied to the measured cycle data in Figure 9.

The technique used to calculate the above total uncertainty was the recom- mended ASHRAE Standard Procedure for determining the propagation of

uncertainties in single sample experiments [7]. Briefly, this

technique assumes that the total percentage uncertainty is equal to the

square root of the average of the sum of the squares of the individual

percent uncertainties.

The measurement of the temperature rise across the boiler involved the use of a thermopile. The number of thermocouple junctions in this

thermopile was selected to result in an error of less than ±0.02®F or 0.11% in a 20 degree Fahrenheit (11.1 degree Celsius) temperature differential across the boiler using the automatic data logger. This error combined with the uncertainties cited above to give an uncertainity in

the steady state efficiencies of +2.4 when determined by the input/output

method.

No attempt was made to estimate the accuracy of the heat loss method of

determining boiler efficiency because it involves a fairly complex calculation procedure based on numerous assumptions. Instead, an estimate

of the precision of the method was obtained by running a computerized

version of the procedure to determine the effect of uncertainties in the

input variables (e.g. flue temperature & CO2 reading). The uncertainties

used to adjust the measured flue gas temperature and C02p concentrations

29
were, respectivley , + 0.76% and + 0.1%. The result of this sensitivity

study was that a variation of less than ±0.3 percentage points was observed in the steady-state and part-load efficiencies over the range

of return water temperature studied.

30
Nbb-1I4A (REV. B.78I

U.S. DEPT. OF COMM.

BtBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 80-2094

4. TITLE AND SUBTITLE

A LABORATORY STUDY OF A GAS- FIRED CONDENSING BOILER

5. Publication Date November 1980

7. AUTHOR(S) 8. Performing Organ. Report No. George E, Kelly and Mark E, Kuklewlcz

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS fsrreer, CUy. State. ziP)

Department of Energy

20 Massachusetts Ave. , NW

Washington, D.C. 20585

13. Type of Report & Period Covered Final

15. SUPPLEMENTARY NOTES

16. ABSTRACT - As a part of the Department ^f Energy*^ energy conservation program for'

consumer products, the National Bureau of Standards (NBS) developed test procedures for

conventional gas- and oil-fired furnaces and boilers. The Department of Energy (Do.E)

published their finalized version of these procedures in the Federal Register on May 10,

1978, In an effort to update and refine these test procedures, DoE directed NBS to

develop a method of testing condensing furnaces and boilers which could be used to

compare the annual performance of condensing and non-condensing residential heating

systems. This report summarizes the laboratory tests of a gas-fired pulse-combustion

condensing boiler that were carried out as a part of the development effort.

The performance of the pulse-combustion boiler was evaluated under both steadystate and part-load operating conditions. The efficiency of the unit was determined by

the input/output method which measured the heat transferred to the circulating water and

the energy input during each test period. Steady-state laboratory tests of the unit

were conducted at constant return water temperatures of 100, 110, 120, 130 and 140“F

(37.8, 43,3, 48.9, 54,4 and 60.0®C). Part-load performance tests were carried pu_t at

a number of these return water temperatures at on-times of approximately 5, 15,

22.5 and 42 percent.

A modified version of the heat loss procedure for estimating the seasonal performance

of a residential central furnace or boiler was also used to evaluate the boiler's

steady-state efficiency and part-load efficiency at a 22.5 percent on-time. A cooldown test and heat-up test were performed to obtain dynamic information which was used to calculate the unit's cyclic performance. The predicted steady-state and partload efficiencies from the heat loss method were found to be within three percent of

the performance determined using the input/output method.

17. KEY WORDS - Boilers; central heating; condensing boilers; efficiency, part load;

fossil-fuel heating systems; gas-fired boilers; hydronic heating; pulse combustion.

19. SECURITY CLASS 21. NO. OF

(THIS REPORT) PRINTED PAGES

UNCLAS'IFIED 33

20. SECURITY CLASS 22. Price (THIS PAGE)

$6 . CC) UNCLASSIFIED

18. AVAILABILITY |X3 Unlimited

I I For Official Distribution. Do Not Release to NTIS

I

' Order From Sup. of Doc., U.S. Government Printing Office, Washington, DC

20402, SD Stock No. SN003-003-

Order From National Technicai Information Service (NTIS), Springfield, VA. 22161 USCOMM-DC
9 *

i B

«0IIA«H*TI 10 UA?ftf« iAWJITAH

'**' *mF ^

iJ» lOfHOU U

nr ^r;jTt«ii9r

"ascot-'

’

,.

, 1* . .

' - . Jt

'

- 'mwnngir*^ ' —

•,.7 f »

''\'r

IK,; '* yx^ r;rrt,TSoTq' i io' ', lo,' J/-:

ri>3t iar«*ifc*5»'^Kf 3«*w ' CS^)' . iwlJ

,0i r,t*H_ laky*h^.-i>*h i;^(f»lid[|it<}^

..T- y*^*^*"*-^^ pwttjrjMS/JiJi »90«X'Xa|^^

W !>«#«, ,

. ;i*d bit><»'3 tt ;,q<y£«V»fc i

4jinJra^w*ri,

•' y4a;o?lo arfa a-i»^ »' aa "fwa ijoJfc'Ttaf »aw ijlrfs anlsnf'baoji^

‘l

- ' ', ' ^ 5J t ' I'ff ',’.' '' , - - _ - _ - . * " --^ba^aa ipafkn;# ,'3^0':

bip aif>9^w' .lw>Ti'55»4aaf aifS-4-

4-1:91/ 1o'1b4»93’

,ei

»oo«M«o!H>»q .tisjjooiiae-.ort'i iioniwl' 96l4X»9' bcUiUbM^A Uf"

8^aoJtl<wf 9it3 ;'S0' -loon k t9mXi^rH\ 4'miMoq ^__^J,s;'‘^, 4#:y'3'w9l?!l'5'tojfW'«Kl^

lUi, 4triMDMM|' 9011^4 nXit4ln#0d'' 03'‘^J^IS9I^

^b4o^ .lia« ,yofl!»loiaia ;-a!a.'»llo<f4‘iii;4'So«-Wo3.; JfCWQJ»''Vy3CI«''

^fioWwiJatoty i-fn"t4a»d ‘twTaOiJd

• “ ' — ' ’-'••• •’ ?J|f;^»n ')£' .. ^i-..Ti ....^jr^. urW /' „ , .• r. I. • ,v., ucAOHiEn . _^.,. W

. w 'lltd tyft^

ys

tsamart

m’ySWmBWii

£f;:*tT»0H31i?.WTt,

m > a3t1i'M43*W,.

ism:

tifxAM

j?r

iA

.«- wbiWo'wa^

00. i^

<£ftii tti «a

Ifk,

' .-..USf • pjUWfl'bi^^ .8.U, ,.M>0 *«

. »•<!(<

jlferfS'- H .. t"%„: ,xH^...-3i r. iAir^.....«a.. .-itiS-Ait^-

• ’

'".1 :si' ,7ffi’ IIIS«s(.