NISTIR 6913 

Proposed Revisions of Part of the Test 

Procedures for Furnaces and Boilers in 

ASHRAE Standard 103-1993 

Stanley T. Liu 

U.S DEPARTMENT OF COMMERCE 

National Institute of Standard and Technology 

Building Environment Division 

Building and Fire Research Laboratory 

Gaithersburg, MD 20899-863 1 

NETNational Institute of Standards and Technology 

Technology Administration, U.S. Department of Commerce 
NISTIR 6913 

Proposed Revisions of Part of the Test 

Procedures for Furnaces and Boilers in 

ASHRAE Standard 103-1993 

Stanley T. Liu 

U.S DEPARTMENT OF COMMERCE 

National Institute of Standard and Technology 

Building Environment Division 

Building and Fire Research Laboratory 

Gaithersburg, MD 20899-863 1 

September 2002 

U.S. Department of Commerce 

Donald L. Evans, Secretary 

National Institute of Standards and Technology 

Arden 1. Bement, Jr., Director 
NISTIR 6913 

Proposed Revisions of Part of the Test 

Procedures for Furnaces and Boilers in 

ASHRAE Standard 103-1993 

Stanley T. Liu 

U.S DEPARTMENT OF COMMERCE 

National Institute of Standard and Technology 

Building Environment Division 

Building and Fire Research Laboratory 

Gaithersburg, MD 20899-863 1 

National Institute of Standards and Technology 

Technology Administration, U.S. Department of Commerce 

NISTIR 6913 

Proposed Revisions of Part of the Test 

Procedures for Furnaces and Boilers in 

ASHRAE Standard 103-1993 

Stanley T. Liu 

U.S DEPARTMENT OF COMMERCE 

National Institute of Standard and Technology 

Building Environment Division 

Building and Fire Research Laboratory 

Gaithersburg, MD 20899-863 1 

September 2002 

U.S. Department of Commerce 

DonaId L. Evans, Secretary 

National Institute of Standards and Technology 

Arden 1. Bement, Jr., Director 

PROPOSED REVISIONS OF PART OF THE TEST PROCEDURES FOR FURNACES 

AND BOILERS IN ASHRAE STANDARD 103-1993 

ABSTRACT 

The U.S. Department of Energy's (DOE) procedure for testing the Annual Fuel Utilization 

Efficiency (AFUE) of residential central furnaces and boilers references ASHRAE Standard 103- 

1993. The standard has been used by the furnaceboiler industry to obtain the AELTE values of 

their products as required by DOE. ASHRAE Standard Project Committee (SPC) 103R is in the 

process of making revisions to the standard to address some shortcomings and to improve the 

calculation procedure on the basis of comments and experiences from the industry. This report 

presents proposed revisions to certain part of the test procedure to address some of the 

comments. The proposed revisions include a new procedure to calculate the average burner 

odoff times of modulating furnaceshoilers during the cyclic operation of the burner, a revised 

method for the calculation of the annual burner operating hours, and an extension of the 

procedure to calculate the off-period flue loss of oil-tired equipment designed with a long post 

purge time. This report also provides the background information on the derivation of the 

integral functions listed in Appendix E of ASHRAE Standard 103-1993 that have not been 

formally documented. Those integral functions were used for producing the graphs in the 

standard for the hand calculation of the off-period losses with a calculator, and also in the 

numerical algorithm in the computerized calculation procedure. 

Key words: Annual energy consumption, Annual Fuel Utilization Efficiency, ASHRAE standard 

103, burner operating hours, central furnace and boiler, DOE test procedure, integral functions 

for off-period losses, step-modulating furnaceshoilers, two-stage furnaceshoilers 

... 

111 
ACKNOWLEDGMENTS 

This study was sponsored by the U.S. Department of Energy, Office of Energy Efficiency and 

Renewable Energy. 

Use of Non-SI Units in a NIST publication 

The policy of the Piational Institute of Standards and Technology is to use the International 

System of Units (metric units) in all its publications. However, in North America in the 

construction and building materials industry, certain non-SI units are so widely used instead of 

SI units that it is more practical and less confusing to include measurement values for customary 

units only. 

iv 
TABLE OF CONTENTS 

ABSTRACT 

ACKNOWLEDGMENT 

TABLE OF CONTENTS 

NOMENCLATURE 

I. INTRODUCTION 

2. ON/OFF TIMES FOR MODULATING FURNACEBOILER 

2.1 Background 

2.2 AFUE Calculated by Current Test Procedure 

2.3 Review of Current Procedure for Single Stage FurnacesBoilers 

2.4 Proposed Procedure for Modulating FurnacesBoilers 

2.5 Examples of AFUE Calculared on the Basis of Proposed Procedure 

3. ANNUAL BURNER OPERATING HOURS AND ANNUAL ENERGY 

CONSUMPTION OF MODULATING FURNACES AND BOILERS 

3.1 Background 

3.2 Review of Test Procedure for Single Stage FurnacesBoilers 

3.3 Proposed Procedure for Modulating FurnacesBoilers 

3.3.1 Annual Burner Operating Hours 

3.3.2 Annual Fuel Energy Consumption 

3.3.3 Annual Auxiliary Electrical Energy Consumption 

3.3.4 Units with Standing Pilot Light 

1. CALCULATION OF OFF-PERIOD LOSSES FOR OIL-FIRED BOILERS 

WITH LONG POST PURGE TIME 

4.1 Background 

4.2 Calculation Procedure for Losses during Post Purge Period 

4.2.1 Assumptions 

4.2.2 Calculation for Sensible Loss 

4.2.3 Calculation for Infiltration Loss 

5. INFORMATION ON THE DERIVATION OF THE INTEGRAL FUNCTIONS 

5.1 Background 

5.2 Derivation of the Integral Functions 

FOR CALCULATING THE OFF-PERIOD LOSSES IN ASHRAE 103-1993 

0. SUMMARY 

... 

111 

iv 

vi 

1 

2 

2 

1 

5 

7 

9 

V 

1 

7 

9 

10 

11 

11 

13 

13 

13 

15 

15 

15 

15 

15 

17 

17 

17 

18 

21 

REFERENCES 

V 

22 

NOMENCLATURE 

XFUE 

A/F 

BE 

BOH 

BOHH 

BOHM 

BOHii 

CP 

DF 

DHR 

DP 

EF 

EAE 

Effy~s 

Effyss 

Effyss .M 

CT,OFF 

F3, F4, F5 

F6, F7, F8 

HHV 

HLH 

KS.OFF 

LLOFF 

Ls.o!=F 

MF,OFF 

MF.ON 

N 

PE 

QIN 

QIN.H 

QIN.R 

QOUT 

QOUT,M 

QOUT,R 

QP 

t 

tP 

[OFF 

LON 

TC 

TD 

TF. OFF( t> 

TF. SS 

annual fuel utilization efficiency in percent 

mass ratio of stoichiometric air to fuel 

electrical power (kW) to circulation air blower or water pump 

number of annual burner operating hours 

number of burner operating hours at maximum fuel input rate 

number of burner operating hours at modulating fuel input mode 

number of burner operating hours at reduced fuel input rate 

specific heat of air 

cool-down temperature profile correction factor for the effect of cycling 

off-cycle draft factor for flue gas flow 

design heating requirement 

power burner draft factor 

annual fuel energy consumption 

annual electric energy consumption 

heating seasonal efficiency in percent 

steady-state efficiency in percent 

average steady-state efficiency operating in modulating mode in percent 

off-cycle sensible heat loss integration factors 

higher heating value of fuel 

heating load hours 

multiplication factor for sensible heat loss during the off-period 

off-cycle infiltration loss in percent 

off-cycle sensible heat loss in percent 

off-cycle flue gas mass flow rate (per minute) 

on-cycle flue gas mass flow rate (per minute) 

number of burner cycles per hour based on the response of thermostat at half load 

electrical power (kW) to burner 

steady-state rated (maximum) fuel input rate (per hour) 

steady-state maximum fuel input rate (per hour) for a modulating unit 

steady-state reduced (minimum) fuel input rate (per hour) 

heating capacity, useful heat output per hour, under steady-state rated fuel input 

average heat output rate (per hour) for modulating mode 

heating capacity, at reduced (minimum) fuel input rate 

fuel energy input per hour to pilot light 

time in minute 

power burner post-purge time period 

burner off-time per cycle 

burner on-time per cycle 

balance point temperature for modulating units 

design outdoor air temperature 

off-period flue gas temperature at time t 

steady-state flue gas temperature 

Vi 

TOA 

TOA,H 

TOA.R 

TRA 

TREF 

VF.OFF 

VF.ON 

X 

Xh 

Xr 

XH 

XR 

\ 

a 

aR 

_It 

PF 

ZOFF 

average outdoor air temperature 

average outdoor air temperature at maximum input operating mode 

average outdoor air temperature at reduced input operating mode 

laboratory room temperature 

a reference air temperature 

off-period flue gas volume flow rate in minute 

on-period flue gas volume flow rate in minute 

load factor, single-stage unit 

load factor when operating at maximum fuel input rate 

load factor when operating at reduced fuel input rate 

fraction of heating load at maximum fuel input operating mode 

fraction of heating load at reduced fuel input operating mode 

ratio of circulation air blower or pump on-time to burner on-time 

oversize factor, rated heating capacity to design heating requirement (DHR) 

oversize factor, heating capacity at reduced input rate to DHR 

time interval 

density of flue gas 

off-cycle time constant 

vii 

1. INTRODUCTION 

The U.S. Department of Energy (DOE) test procedure "I for determining the Annual Fuel 

Utilization Efficiency (AFUE) of residential central furnaces and boilers referenced the industry 

test standard ANSUASHRAE 103-1993 'I (ASHRAE 103-1993) of the American Society of 

Hearing, Refrigerating, mci Air Conditioning Engineers (ASHRAE). The standard was initially 

developed in 1982 based on the DOE test procedures for single stage furnaces and boilers 

recommended by Kelly et a1 [31. It was subsequently revised in 1988 and again in 1993 to include 

test procedures for condensing units, modulating (or multi-fire) units, and for units employing a 

short post-purge period after the burner is shut off. In 1998 ASHRAE organized the Standard 

Project Committee (SPC) 103R to begin the revision process to ASHRAE 103-1993 following 

comments from the industry on the need to address some possible shortcomings of the standard 

based on user experiences. This report documents the proposals to SPC-103R to address the 

issues raised by industry concerning the test standard. The issues are as following: 

(1) The inconsistency in the calculated AFUE values for a two-stage furnace (or boiler) 

when they are calculated on the basis of (a) the single-stage procedure and (b) the modulating 

two-stage procedure using the same test data. The first one treats the furnace as a single stage 

furnace operating at the reduced input to supply the heating load, and the second one treats the 

furnace as two-stage furnace. With a ratio of reduced input to rated input of from 60 % to 70 %, 

the standard shows that the furnace would supply almost 98 % of the annual heating load at the 

reduced input. That is, the furnace is operating as a single-stage furnace at the reduced fuel input 

rate most of the time even when it is treated as a two-stage furnace. However, the calculated 

results showed a difference in the AFUE values of iiiore than one percentage point between the 

two procedures. 

(2) In the calcularion for the number of annual Burner Operating Hours (BOH) for a 

modulating furnace (or boiler), the standard estimates the'BOH using the fuel and auxiliary 

electric energy input measured at the maximum rated input condition. For a two-stage furnace, 

the major portion of the heating load is supplied at the reduced input, with the auxiliary electrical 

components also operating under the settings for the reduced input rate. 

(3) For units employing post purge after the burner is shut off, the standard provides a 

calculation procedure that limits the post purge time to three minutes. For some oil-fired boilers 

that require a longer post purge time, the formula in the standard will produce erroneous 

(smaller) flue loss result. 

(4) The standard listed a group of integral functions named as F3 to F8 that were used to 

produce the graphs in the standard for hand calculation of the off-period losses, and in the 

numerical algorithm for the losses in a computerized calculation procedure. There is no 

documentation on the derivation of those functions. A search of the publications [3*41 where the 

functions were first presented did not show the procedure or provide information about how the 

functions were derived. 

1 
This report presents a proposed revision to the burner odoff times for a modulating appliance 

that corrected the inconsistency problem in AFUE, and a revised BOH calculation procedure for 

il modulating appliance that uses the measured fuel and auxiliary electric inputs at the actual 

operating conditions. The annual fuel and electric energy consumption are calculated on the basis 

of the BOH. A procedure for the post purge calculation beyond three minutes is derived, and the 

background information of the process in the derivation of the integral functions F3 to F6 is 

shown. 

3. ON/OFF TIMES FOR MODULATING FURNACEBOILER 

2.1 Background 

The current test procedure in ASHRAE 103-1993 prescribes the calculation of the AFUE of a 

single stage furnace by using test data from the steady state, cool down and heat up tests at the 

rated input. In the calculation procedure, the average furnace cycling odoff times are assumed 

to be 3.87 min on and 13.3 min off based on the characteristic responses of the room thermostat 

and a furnace oversize factor of 0.7 with respect to the building heating load. This results in the 

furnace operating at an average of 22.5 % of the rated capacity during the heating season. The 

AFUE of a two-stage furnace is calculated by using test data from the steady state, cool down 

and heat up tests at both the maximum (high) input rate and at the minimum (low) input rate. 

There the average furnace cycling odoff time are assumed to be 10 min on and 10 min off. And 

this assumes the two-stage furnace operating at an average of 50 % of the rated capacity during 

the heating season. 

During the past years, a discrepancy in the calculation procedure was discovered with respect to 

some modulating furnaces. The AFUE value for the furnaces operating as a two-stage furnace 

was lower than the value when the same test data were used in a calculation with the furnace 

operating as a single stage furnace operating under low firing rate. These furnaces were noncondensing two-stage furnaces with a low fire to high fire ratio of 0.60 to 0.70 that are typical of 

two-stage furnaces. The difference in the AFUE values between the two calculations 

(modulating procedure versus single stage procedure) can be more than 1 percentage point. 

However, based on the test procedure, a two-stage furnace with a ratio of low to high firing rates 

of 0.65 would operate in the low firing rate to meet more than 95 % of the annual heating load in 

J heating season. Therefore, the resulting AFUE was expected to be close to the value calculated 

on the basis of single-stage firing at the low rate over the entire heating season. The question was 

raised as to the reason for this one percentage point discrepancy in the AFUE values. 

2.2 AFUE Calculated bv Current Test Procedure 

Using test data of a two-stage furnace, NIST investigated the cause of this discrepancy using the 

computer program AFUEBF (a computerized version of the calculation procedure of ASHRAE 

103-1993). The original version and a slightly modified version of the program were run using 

the same test data as input. The modified version changed (only) the furnace odoff times to 

examine the effect of the changes in the odoff times. The changes made on the odoff times 

were (1) 3.87/13.3 min for both single stage and two-stage calculation, and (2) 10/10 min for 

2 
both single stage and two-stage calculation. It was found that the discrepancy in the AWE 

described above was caused by the difference in the on/off times assigned to a single stage 

lurnacc versus to a two-stage furnace. The following examples show the results (AFUE) of two 

gas-fired, two-stage non-condensing furnaces using test data from a manufacturer. Both have a 

low fire to high fire input ratio of 0.65. The maximum input of Example 1 is 80 000 Btuh The 

maximum input of Example 2 is 60 000 Btu/h. 

*Note: Current onloff times are 3.87/13.3 min for single stage furnace, 10/10 min for two-stage 

furnace. Value in bold type indicates result by current procedure for modulating furnace. 

Example 2: AFUE Value 

Using Single Stage Calculation Using Two-Stage Calculation 

On/Off High- Fire Low -Fire High-Fire Low -Fire Weighted 

Time" Average 

3.87/ 13.3 82.32 81.31 

1 0. o/ 1 0.0 8 1.50 80.33 

8 1.79 81.35 8 1.35 

80.34 80.32 80.34 

*Note: Current ordoff times are 337113.3 min for single stage furnace, 10/10 min for two-stage 

furnace. Value in bold type indicates result based on current procedure for modulating furnace. 

Example I shows that the current calculation procedure for a two-stage furnace gives a lower 

AFUE value (81.52 %) than if the furnace were operating as a single stage furnace (where the 

on/off times are set at 3.87113.3 min) at either high or low firing rates (AFUE of 82.64 % and 

8 1.99 %, respectively). When the ordoff times were changed to 10.0/10.0 min for the low-firing, 

single stage case, the calculated AFUE became 81.59 % which is closer to the 81.52 % for the 

weighted two-stage result at the same 10.0/10.0 on/off times. (From ASHRAE 103-1993, the 

weighting factor for the two-stage calculation is greater than 0.95 at low fire and less than 0.05 at 

high fire - that is, the two-stage furnace operated almost all under low fire to meet the annual 

heating load). The same close agreement happens if the ordoff times are 3.87/13.3 min for both 

single and two-stage calculation (81.99 % under single stage low fire and 81.85 % under two3 
stage). Example 2 is from test data of another (smaller capacity) furnace. The results of this 

example show that the discrepancy under the current calculation procedure could be 1 percentage 

point in favor of the single stage calculation. 

As described above, the discrepancy in the results in the current calculation procedure is caused 

by the difference in the assigned ordoff times for the two types (single stage versus modulating) 

of furnaces. A possible revision would be to change the assigned odoff times to some more 

appropriate values. The problem is to determine what should be the corrected ordoff times for a 

modulating furnace, or a modulating boiler (where odoff time are specified as 15/15 min in the 

presenr procedure). 

Section 2.3 describes the current procedures for determining odoff times. Section 2.4 describes 

the proposed procedure for determining the on/off times for a modulating furnacehoiler based 

on its minimum (low-fire) to maximum (high-fire) capacity ratio. 

2.3 Review of Current Procedure for Single Stage Furnaces/Boilers 

In the development of the original test procedure for a sin le stage unit, the on and off times of a 

furnacehoiler are calculated by the following equations L3f . 

60 X 

4NX(I-X) tolv = 

60 

4 N x (I - X) fOFF = - t0N (2 - 2) 

where 

N = number of burner cycles per hour based on the responses of thermostat at half load, 

= 5 for furnace 

= 2 for boiler 

X is the load factor and is defined as: 

where 

ToA = the average outdoor temperature. (ToA = 42 OF in the test procedure) 

TD = the outdoor design temperature. (TD = 5 % in the test procedure) 

a = oversize factor = 0.7 for a single stage unit 

For a single stage unit, X = 0.225, based on the values of TO*, TD, and a in the test procedure. 

This resulted in the values of toN/toW of 3.87A3.3 min as given in the test procedure. 

4 
2.4 Proposed Procedure for Modulating Furnaces/Boilers 

For a niodulating furnace or boiler, the current test procedure assumed a load factor of X = 0.5 

which would give the odoff times of 6/6 min (N = 5) for furnaces and 15/15 min (N=2) for 

boilers. However, the actual odoff times for furnaces in ASHRAE 103-1993 was set at 10/10 

instead of 6/6. A detailed description and the rationale for the setting of ordoff times for 

modulating furnacesboilers in the current test standard was presented in reference [5]. However, 

since a modulating furnace (or boiler) does not always have the specific minimum input rate that 

would result in X = 0.5 when the unit is operating in the minimum input rate, the odoff times 

actually vary depending on the ratio of the minimum to maximum input rates. 

For modulating furnaceshoilers, ASHRAE 103-1993 defined a balance point temperature Tc as 

the change over temperature for the unit to switch from the minimum input rate to the 

modulating input or the maximum input rate. The balance point temperature is defined as the 

value of the outdoor temperature above which the modulating equipment is capable of providing 

the building heating load requirement at the minimum or low input rate. For outdoor 

temperature below the balance point temperature, the unit will switch to its modulating mode of 

operation for step-modulating units or to its maximum input rate for a two-stage unit. In 

ASHRAE 103- 1993, The balance point temperature is calculated as, 

Q0lJT.R 

Q0"T 

T, = 65 - (60)(1+ a)------ 

Where QoUT, R/QOUT isthe minimum to maximum capacity ratio of the modulating unit. a = 0.7 

is the oversize factor at the maximum input rate of the modulating unit. It is seen that the value 

of Tc depends on the ratio of the minimum to maximum capacity ratio. With a = 0.7 and a 

capacity ratio of approximately 0.5884, Tc would be equal to approximately 5 OF, the average 

outdoor design temperature. This means that for a furnaceboiler with a capacity ratio of greater 

than or equal to 0.5884, the furnacehoiler will operate in the minimum (low) firing rate for the 

entire heating season. For values of the capacity ratio less than 0.5884, Tc would have a value of 

between 5 OF and 65 OF. In this range, the modulatin unit will operate in the low firing rate 

when the outdoor temperature is between Tc and 65 F. The unit will switch to the maximum 

(high) firing rate (two-stage control) or to the intermediate (step-up) firing rates (step-modulating 

control) when the outdoor temperature is between the design temperature of 5 OF and Tc. 

ASHRAE 103-1993 provides graphs to determine the fractions of the annual heating load XR and 

XH (as functions of Tc) that will be supplied by the modulating equipment operating at the 

minimum (low) and the maximum (high) firing rateb, respectively. The standard also gives the 

values of the average outdoor tem eratures TO*. R (between Tc and 65 OF in low firing rate 

operation) and ToA. H (between 5 F and Tc in high firing rate operation), as function of Tc. 

These remperatures are equivalent to the TO* of 42 OF for the single stage equipment. 

8 

8 

For the values of the capacity ratio (low fire to high fire) greater than approximately 0.59, the 

modulating furnacehoiler operates mostly in the low firing rate. If, as assumed, the oversize 

5 
factor a of the unit operating at the high firing rate is at 0.7. an oversize factor U.R at the 

minimum firing rate with respect to the building design heating load can be defined as, 

Qom,R l+a, =- 

DHR 

where, 

a~ = the oversize factor at the reduced input rate, 

DHR = Design Heating Requirement. 

As stated before, at the maximum firing rate, the oversize factor a is defined as, 

%LIT l+a=- 

DHR 

Combining the above two expressions results in, 

Q0uT.R l+a, =(l+a)- 

QouT 

(2 - 5) 

It was noted at the meetings of the multi-fire working group of ASHRAE SPC 103R that at the 

values of a = 0.7 and QO~T.R / Qou~ of approximately 0.59, the unit is no longer oversized at the 

minimum firing rate (CXR 0). Therefore, equation (2-5) holds for values of a and the capacity 

ratio that give a value of a~ equal or greater than 0. 

With the calculated a~ from equation (2-5), equation (2-3) for the load factor X can be used to 

calculate a load factor Xr at the minimum input and a load factor Xh at the maximum input rate, 

as, 

It was noted at the meetings of the multi-fire working group of ASHRAE SPC 103R that the 

minimum outdoor temperature for the load factor Xr is Tc, not the design temperature TD, since 

Tc is the limiting low outdoor temperature for the modulating unit to operate in low firing rate. 

In ASHRAE 103-1993 only the odoff times based on the low firing rate is used in the 

calculation procedure. This is because that, for the majority of modulating units, the requirement 

of optimum fuel to air ratio under all firing rates results in the capacity ratio of low to high firing 

of from 0.4 to 0.7. In this range, from 80 9% to 99% of the annual heating load are met by the 

6 
modulating unit operating at low fire. Employing the same approach, equation (2-6), in place of 

equation (2-3), can be used to calculate the load factor X. The odoff times of equations (2-1) and 

(2-2) for the modulating unit can then be calculated based on the value of X from equation (2-6). 

The calculated ordoff times are then used in the calculating procedure for modulating furnaces or 

boilers. The following table shows the calculated ordoff times for a modulating furnace (with 

N=5) and a modulating boiler (N=2), at capacity ratios of 0.4 to 0.75, where 0.75 is the highest 

ratio defined in ASHRAE 103-1993 for modulating units. The two columns in the table labeled 

"(N=3)" showed the values of toN and tom for the case that the value of N is assumed to be 3 

instead of 5 for modulating furnaces. The value N=3 resulted from the approximation of the 

on/off of 10/10 for modulating furnaces in ASHRAE 103-1993: 

Modulated Furnace OdOff Times Based on QOUT. R OUT -

Capacity Tc 1+UR TOA,R* TOA.H* XR* X=Xr [ON tom [ON tom tON tow 

Ratio OF OF OF Load (min) (min) (min) (min) (min) (rmn) 

Factor (N= 5) (N = 3) (N = 2) .-______-____-__--_----------------------------------------------

0.40 24.2 1.0 43.5 19 0.83 0.527 6.34 5.69 10.57 9.49 15.85 14.23 

0.45 19.1 1.0 42.6 15 0.89 0.488 5.86 6.15 9.77 10.25 14.65 15.37 

0.50 14.0 1.0 41.8 11 0.94 0.455 5.50 6.59 9.17 10.99 13.76 16.19 

0.55 8.9 1.0 41.0 7.5 0.97 0.428 5.24 7.01 8.74 11.69 13.11 17.53 

0.60 5 1.02 40.7 5 0.99 0.397 4.98 7.56 8.29 12.59 12.44 18.89 

0.65 5. 1.11 40.7 5 0.99 0.367 4.74 8.19 7.89 13.64 11.84 20.46 

0.70 5 1.19 40.7 5 0.99 0.340 4.55 8.81 7.58 14.69 11.37 22.04 

0.75 5 1.26 40.7 5 0.99 0.318 4.40 9.44 7.33 15.74 10.99 23.61 

Single Stage (specified in ASHRAE 103) 0.225 3.87 13.33 -- -- 9.68 33.26 

*Note: The values of TOA, R (average outdoor temperature at minimum input), TO*, H (average 

outdoor temperature at maximum input), and XR (the fraction of annual heating load supplied at 

the minimum input) as function of Tc are from Figures 23 and 24 of ASHRAE 103-1993 ['I. 

For comparison, the last row in the above table listed the values of the load factor X and the 

burner ordoff times for a single stage furnace (with N = 5) and a single stage boiler (with N = 2) 

as specified in ASHRAE 103-1993. 

25 Examples of AFUE Calculated on the Basis of Proposed Procedure 

The following are two examples of using the calculated ordoff times (with N = 5) in the AWE 

calculation for the two-stage furnace examples shown previously. In place of the fixed ordoff 

values of 10/10 (minutes) for the furnace, the values were calculated using equations 2-1,2-2, 

and 2-6) on the basis of the capacity ratio QOUT, R /QOUT. The capacity ratio was computed based 

on data from the steady states test. 

7 
_---__-_-------~~~ 

OrdOff High-Fire Low-Fire High-Fire Low-Fire Weighted 

Times (min) Test Data Test Data Test Data Test Data Average 

3.8711 3.3 82.64 8 1.99 

10.01 1 0.0 8 1 SO 8 1.53 8 1.52 

4.W8.67 (Calculated) 8 1.90 8 1.70 81.71 

Example 2: AFUE Value 

3.87/ 13.3 82.32 81.31 

1 o.o/ 10.0 80.34 80.32 80.34 

In example 1, the calculated AFUE is 8 1.99 % based on single-stage at low fire at the existing 

ordoff times, and 8 1.7 1 % based on two-stage with the new ordoff times. For the second furnace, 

the comparison is 8 1.3 1 % to 80.94 5%. It is noted that even though the differences are smaller, 

the results still do not agree with each other exactly. This is because that for single-stage lowfire, the on/off times really should not be 3.87/13.3 but instead should be approximately 4.6/8.7 

since at the low firing rate, the oversize factor is not 0.7, but a smaller value of approximately C~R 

= 0.1 1 (see previous table on the value of (1+ CXR = 1.1 1) at QOUT,R / QOUT of 0.65). If the odoff 

values of 4.6B.7 are used in the single-stage calculation, the result would be very close to the 

two-stage procedure since in the two-stage operation, 99 % of the annual heating load is met by 

the furnace operating under low-fire. 

Lastly, it is noted that in the above proposal, the odoff times based on the low-fire condition are 

used for the high-fire portion of the calculation also. This is a simplification based on the reason 

that the fraction of high fire condition is small in the practical range of the capacity ratios from 

0.40 to 0.75. If more precision in the result is desired, separate load factors for Xh and Xr can be 

calculated from equations (2-6) and (2-7), and they in turn can be used to calculate the separate 

ordoff times for low fire and high fire conditions from equations (2- 1) and (2-2). However, as 

stated before, it is considered at the present time to be not necessary for the range of capacity 

ratios encountered in practice. 

8 
3. ANNUAL BURNER OPERATING HOURS (BOH) AND ANNUAL ENERGY 

CONSUMPTION OF MODULATING FURNACES AND BOILERS 

3.1 Background 

In the existing ASHRAE 103-1993DOE test procedure [1*21, an equivalent number of burner 

operating hours (BOHss) based on operation condition at the maximum rated input was defined 

to estimate the annual fuel energy consumption of both the single-stage and the modulating 

furnacesboilers. For single stage units, the formulation is straightforward. The number of burner 

operating hours is calculated as the annual building load divided by the useful outputs of the 

furnaceboiler delivered to the conditioned space. The useful output energy from the 

furnacehoiler composed of the heat output from the burning of fuel at the heating seasonal 

efficiency plus the heat from the electrical power input to the auxiliary electric equipment. The 

annual fuel energy consumption is calculated as the number of burner operating hour times the 

rated energy input plus the off-period pilot light consumption if employed. For modulating 

furnacehoiler, the same formulation is retained. However, in the BOHss formula, a 

inultiplication factor R = 2.3 or 3.0 (depending on the capacity ratio of minimum (low-fire) to 

maximum (high-fire)) is introduced in the auxiliary electrical power term. The heat energy 

output from the fuel is calculated as the maximum rated input at the weighted heating seasonal 

efficiency of the modulating furnaceboiler. The purpose of the factor R was to account for the 

greater on-time fraction per cycle (0.50) for modulating unit as compared to the on-time fraction 

per cycle (0.225) for single stage unit. The ratio of 0.50 to 0.225 results in R as 2.30. The 

assumption was that the auxiliary electrical energy consumption of the modulating furnaceboiler 

due to the assumed larger fraction on time per cycle would be approximated by the 

multiplication factor 2.3 to the auxiliary electrical power input of air circulation blowers and 

power burners, etc. Since the maximum fuel input rate and the steady-state auxiliary electrical 

power input measured at the maximum firing rate are used to compute the BOHss, the term no 

longer represents the actual annual burner operating hours of the equipment which mostly 

operated in the reduced or modulating input mode. Rather, it is used as an equivalent BOHss 

term to approximate or estimate the annual fuel energy consumption of the modulating 

equipment during a heating season. (Appendix C of ASHRAE 103-1993.) However, these 

assumptions ignored the effects of the possible different values of the electrical power input at 

the maximum and reduced fuel input rates for furnaceshoilers with a variable speed circulating 

Liir blower and combustion blower. Also, the use of the maximum fuel input rate in the 

calculation formula for the BOHss causes confusion to users of the standard since for a 

modulating furnaceboiler, the design purpose is for the equipment to operate at the reduced 

input rate most of the heating season. 

To improve upon the above-mentioned assumptions in the calculation procedure, the proposed 

calculation procedure would use the actual power inputs of a variable speed blower at the firing 

rates it is operating under. The actual burner operating hours at the maximum input and the 

reduced (minimum) input rates are calculated to determine the annual energy consumption of the 

equipment operating under both modes. The procedure would also use both the maximum and 

9 
the reduced fuel input rates in the calculation of the burner operating hours under each firing rate 

to eliminare the approximation of an equivalent BOHss. 

3.2 Review of Test Procedure for Single Stape FurnacedBoilers 

From Appendix A of NBSIR 78-1543 ['I or Appendix C of ASHRAE 103-1993 [21, the number of 

Burner Operating Hours (BOH) and the annual fuel consumption EF for a single stage furnace 

dre calculated ab, for indoor installation, 

BOH = [Heating Season Fuel Consumed - (Qp)(Heating Season Hour)] / Qm, 

EF =Annual Fuel Energy Consumption 

= Heating Season Fuel Consumed +, (Qp)(Non-Heating Season Hours), 

Heating Season Fuel Consumed = [Annual Building Load - (PE + y BE)(BOH)] / EffyHs, 

Annual Building Load = (C)(HLH)(DHR, 

where, 

QIN 

Qp 

BE 

PE 

Y 

EffyHs = heating seasonal efficiency 

HLH = Heating Load Hours 

DHR = design heating requirement of a building 

C = 0.77, an adjustment factor defined in Appendix C of ASHRAE 103-1993. 

= steady-state rated fuel energy input 

= pilot light fuel consumption rate 

= electric power to circulation blower or water pump 

= electric power to burner 

= ratio of circulation blower or pump on-time to burner on-time in a burner cycle 

For uniform rating purpose, ASHRAE 103- 1993 established the following national average 

values for some of the weather-related parameters: 

Non-Heating Season Hours = 4600, 

Heating Season Hours= 4160, 

Heating Load Hours (HLH) = 2080. 

For simplicity, let Ep = [(PE) + (y)(BE)], qu = EffyHs, and using the national average values, 

BOH = HLH [(C)(A)(DHR) - B] = 2080 [(C)(A)(DHR) - B], 

A = 1 / [Ep + (QIN - QP) q,l, 

B = (Qp)(q,)(A)(Heating Season Hours) / HLH = 2 QP 7" A 

(3-1) 

The above equations are the same as those given in Appendix C of ASHRAE 103-1993, except 

that for simplicity, the percent and unit conversion factors (100 in percent and 3413 W = 1 Btu/h) 

spelled out in the equations in ASHRAE 103-1993 are not included here. (For use in actual 

10 
calculation with numbers, consistency in units with the appropriate unit conversion factors is 

needed and included in the equations in the standard.) 

3.3 Proposed Procedure for Modulating FurnacesBoilers 

3.3.1 Annual Burner Operating Hours (BOH): 

The following nomenclature is used in the derivation of the proposed procedure: 

BOHH = annual BOH of a two-stage unit at the maximum rated input Q~,H 

Q,N.H = fuel input rate at the maximum firing rate 

E~H = electric power consumption measured under steady state at input Q1N.H 

q,,H = part-load efficiency of a two-stage unit operating at input Qw.H 

BOHM = annual burner operating hours of a step-modulating unit at modulating mode 

QIN.M = fuel input rate in the modulating mode of a step-modulating unit as calculated in sec. 1, 

EpM = electric power input of a step-modulating unit operating in the modulating mode and 

q,,M = average part-load efficiency of a step-modulating furnacehoiler operating in the 

Appendix C of ASHRAE 103-1993 

measured under steady state at input QIN,H 

modulating mode (Sec. 11.4.9.2.3, ASHRAE 103-1993) 

BOHR = annual burner operating hours at the minimum input Q~,R 

QIN,R = fuel input rate at the minimum firing rate 

EPR = electric power input measured under steady state at input Q~.R 

q".~ = part-load efficiency operating at input Q1N.R 

It is assumed that for the modulating furnaceboiler operating at either the maximum or 

minimum input, or at modulating mode, the Design Heating Requirement (DHR) of the building 

is not changed and therefore the required furnaceboiler heat output to meet the DHR of the 

building is not changed. 

For simplicity in showing the derivation process, it is assumed that the modulating unit has an 

electric ignition device so that the pilot QP = 0 (extension of results to include QP will be shown 

later in this report). With this assumption, the general equations for the BOH (Eq.3-1) and the 

\,ariables A and B of Section 3.2 above are reduced to: 

BOH = 2080 [(C)(A)(DHR)]. (3-2) 

With the above assumptions, the burner operation hours in meeting the total annual heating load 

under the maximum firing rate is, 

11 
BOHH* = HLH [(C)(AH)(DHR)], 

AH = 1 / [EPH i- (QIN.H) qu.H1 

where ihc asterisk '* indiutes the told BOH if the appliance operates at a particular (maximum 

here) firing rate for the entire heating season. If only a fraction of the total annual heating load is 

met by the appliance operating at the maximum firing rate, the actual BOH at the maximum 

firing rate (for a two-stage unit) during the heating season will be, 

where XH is defined in Sec. 11.4.8.6 of ASHRAE 103-1993 as the fraction of the total annual 

heating load that is provided by a two-stage appliance at the maximum firing rate. 

Similarly, the total burner operating hours to satisfy the total annual heating load when operating 

under the minimum firing rate for the entire heating season is, 

BOHRi = HLH [(C)(AR)(DHR)], 

AR = 1 / [EPR -t (QIN.R) qu,R]. 

And, if only a fraction of the total annual heating load is met by the appliance operating at the 

minimum firing rate, the BOH at the minimum firing rate during the heating season will be, 

BOHR = (X,)(BOHR*) = (X~)[2080 (C)(AR)(DHR)], (3-4) 

where XK is defined in Ssc. 11.4.8.7 of ASHRAE 103-1993 as the fraction of the total annual 

heating load that is provided by a modulating appliance at the minimum firing rate. 

For a step-modulating furnaceboiler, the fuel input rate at the modulating mode is not measured 

in ASHRAE 103-1993 since the firing rate at the modulating mode varies and cannot be 

determined a priori. Instead, ASHRAE 103-1993 calculates a Q~,M as, 

QIN.M = QOUT,M / Effyss,M, (From Sec. 1 of Appendix C, ASHRAE 103-1993) 

where 

EffysS.M= average steady-state efficiency calculated in Sec. 11.4.8.8 of ASHRAE 103-1993 

QOUT,M = average heat output rate calculated in Sec. 11.4.8.10 of ASHRAE 103-1993 

The fraction of the total annual heating load that is provided by a step-modulating appliance in 

the modulating mode, XM, is assumed in ASHRAE 103-1993 to equal (1- XR). That is, the 

modulating mode is at any input rate greater than the minimum input rate. Also, the electrical 

power inputs to the step-modulating appliance in the modulating mode (BEM and PEM) are not 

measured in ASHRAE 103-1993 and are assumed to be those measured during the steady-state 

test at the maximum fuel input rate, that is, EPM = EPH. Therefore, the burner operating hours for 

a step-modulating appliance operating in the modulating mode during the heating season is, 

12 
BOHM = (X~)[2080 (C)(AM)(DHR)], (3-5) 

In the above formulation, the subscripts H, M and R denote the values at the maximum firing 

rate, the modulating mode (step-modulating), and the minimum firing rate, respectively. The 

variables EPH, E~R, QIN.H, QIN.R, QIN.M, T,,,H, %,R and T~,M are defined previously. 

3.3.2 Annual Fuel Energy Consumption 

FoI two-stage modulating appliances, the annual heating load is met by the appliance operating 

at either the minimum firing rate or at the maximum firing rate. Therefore, the annual fuel energy 

consumption for a two-stage appliance is, 

For step-modulating appliance, the total annual heating load is met by the appliance operating at 

either the minimum firing rate or at the modulating mode (up to the maximum firing rate). 

Therefore, the annual fuel energy consumption for a step-modulating appliance is, 

3.3.3 Annual Auxiliary Electrical Energy Consumption 

The annual auxiliary electrical energy consumption for a two-stage appliance is, 

The annual auxiliary electrical energy consumption for a step-modulating appliance is, 

In the general form without the subscript, Ep = (BE) + (y)(PE), as defined previously. 

3.3.4 Units with Standing Pilot Light 

It is assumed previously that the modulating appliance employs interrupted electronic ignition 

and no pilot light is needed. If the modulating appliance has a continuing burning pilot light with 

fuel input rate QP, the above formulation should be modified by setting the parameters AH, AR, 

AM and BH, BR, BM as 

13 
And the burner operation hours will be modified as 

The annual fuel energy consumption will be modified as, 

For two-stage modulating appliance, 

(3-10) 

(3-1 1) 

(3-12) 

(3-13) 

For step-modulating appliance, 

It is assumed that the value of Qp is the same for all modes of operations. The general equation 

forms of the parameters A and B and the BOH are shown in section 3.2 previously. 

Note that the above formulation is for furnacehoiler installed indoors. For furnacehoiler 

installed in an unconditioned space as an isolated combustion installation or installed outdoors, 

the current DOE test procedure ['I specified the procedure for modifying the calculation of the 

burner operating hours. The modification involved the accounting of the portions of the electric 

energy input to the blower motors that contribute to the heating of the conditioned space and the 

portions that are considered as lost to the ambient. 

4. CALCULATION OF OFF-PERIOD LOSSES FOR OIL-FIRED BOILERS WITH 

LONG POST PURGE TIME 

4.1 Background 

As discussed previously, the post purge period in the test and calculation procedures of 

ASHRAE 103-1993 was limited to three minutes after the burner is shut off. This limit was 

considered adequate for most fossil-fueled applianct.5. However, for certain oil-fired boilers the 

operating requirement is such that the post purge period runs longer than three minutes. Since the 

longer post purge period resulted in greater off-period losses, it was deemed appropriate that 

calculiirion for the longer post purge period should be specified to account for the greater losses. 

14 
4.2 Calculation Procedure for Losses during Post Purge Period 

4.2.1 Assumptions 

The following assumptions were made during the derivation of the post purge period losses in 

ASHRAE 103- 1993. They are used in the present derivation also. 

(a) Constant volumetric flue gas flow rate during post purge to equal to the on-cycle rate. 

(b) Flue gas temperature approximated by piecewise linear variation over small time interval At. 

(c) Measurement data of the flue gas temperature in the off-period, TF,OFF(t) at time interval At 

following burner shut-off. 

4.2.2 Calculation for Sensible Loss 

(a) Assuming constant combustion blower or draft inducer volumetric flow rate during the post 

purge period to equal the steady state on-period rate results in: 

where VF,ON and MF,ON are the volumetric and mass flow rate of the flue gas during the onperiod, respectively. VF,OFF is the off-period volumetric flow rate during post purge, and p~,~~ and 

~F,OFF are the density of flue gas at temperatures TF.SS (steady state) and TF,Om(t), respectively. 

ivF,Om(t) and TF,OFF(t) are the flue gas mass flow rate and temperature at time t during the post 

purge period, respectively. 

From the above equation, and assumin? the flue gas as a perfect gas, 

(b) Assuming piecewise linear flue gas temperature variation over time interval At, (t, S t < t,+,), 

during the post-purge period, gives 

where TF.OFF, , = TF,On(t,) = flue gas temperature at time ti in post purge period. 

Applying the factor CT,OFF for cycling effect to (TF.OFF(t) - TR~) as in ASHRAE 103-1993: 

15 
(4 - 3) 

where CT,~FF = the cool-down temperature profile correction factor for the effect of cycling "*'I 

TKA = laboratory room air temperature. 

The total sensible heat loss during the post-purge period is, 

where tp = total length of post purge time interval. 

N = tp / At = number of intervals of length At within tp 

Cp = specific heat capacity of flue gas 

The percent sensible loss (as percent of total input during the on-cycle) during the post purge 

period is, by substituting the expressions for (TF.ow(t) - TRA) and TF,OFF(t) above into Ls, purge, 

and integrate: 

(4 - 5) 

where CTS = CT,O~; for indoor combustion air, 

= CS' CT,OFF for outdoor combustion air, 

CS' = off-period correction factor for outdoor unit as defined in ASHRAE 103-1993, 

toN = burner on-time per cycle, 

Q~J = fuel input rate. 

100 = conversion factor from decimal to percent 

4.2.3 Calculation for Infiltration Loss 

The toLal infiltration hear loss, Ll,purgz, during post purge period is, by the same way, 

16 
where TREF = a reference temperature defined to be 70 "F in ASHRAE 103-1993, 

TOA = heating season average outdoor temperature as defined in ASHRAE 103-1993. 

The percent infiltration loss (as percent of total input during the on-cycle) during the post purge 

period IS. by substituting the expression for TF.OFF(t) in equation (4-3) into L1, purge, and integrate: 

It is noted that when N = 1; At = tp, and the expressions for the sensible and infiltration losses 

during post purge reduce to the ones in ASHRAE 103-1993. 

5. BACKGROUND OF THE INTEGRAL FUNCTIONS FOR CALCULATING THE 

OFF-PERIOD LOSSES IN ASHRAE STANDARD 103-1993 

5.1 Background 

In ASHRAE 103-1993, the off-cycle sensible and infiltration losses (see sections 11.2.10.6 and 

11.2.10.8 of the Standard) are calculated through the pair of functions (F3, F4); (F5, F6); or (F7, 

F8) depending on furnacehoiler location (indoor, outdoor, or isolated combustion system) and 

\tack damper installation (with or without). The functions are presented in graphical forms 

(Figures 13 through 19 of the Standard) for manual calculation with an electronic calculator as 

well as in formula forms (in terms of definite integrals) presented in Appendix E of the standard. 

In the formulas for the functions F4, F6 and F8, there is a number 100 appearing as a divisor as 

well as an addition in the numerator and denominator of the first integral of each function (see 

Appendix E of ASHRAE 103-1993 for the exact locations). The same 100 also appears in the 

formula for the stack darnper effectiveness factor Do and in the defining equations for the terms 

G3 and G4 in the formula for Do (see section 11.2.9.18 of ASHRAE 103-1993). In ASHRAE 

103-1993 the term 100 was not defined. This brief report shows the origin of the term and the 

derivations of the integral functions. 

5.2 Derivation of the Intemal Functions 

17 
For illustration purpose, the following derivations use the case of a furnacehoiler using indoor 

air for combustion and installed indoors without a stack damper. The applicable function par is 

F3 and F4. 

ln Appendix A of the report NBSIR 78-1543 13', the off-cycle sensible heat loss is given as: 

where 

I3 'r0,t 

WF.0 = ~F.0.x C1.oFF = CI.oFF [TF,oFF (f: -TF.OFF (-)I e 

I,!/,- = T,,,,, (-) - Tm , (t = - denotes reaching equilibrium condition during off - period) 

TF.oFF (t) = WF.0 

t? = a specific time in the off-period at which the flue gas temperature is measured, 

TOFF = off-period time constant of the unit. 

+WF.-. +TR, 

e --I ' TIM 

With the above defining equations for ~F.O and WF,-; L~.OFF can be written as, 

It is seen that in the above expression for Ls.0~ is a function oft with the ~F.O and WF,- as 

parameters whose values are determined from test data for a particular furnacehoiler. 

During the 1970s when the furnace test procedure was developed, the accurate numerical 

evaluation of the integral in the above expression was a major problem without using a central 

computer. Therefore, it was decided that graphs would be developed to facilitate the calculation 

with the use of an electronic calculator. However, since both WF,O and WF,- are related to a 

specific furnacehoiler and will have different values for different furnaceboiler, and a threedimensional graph with both as parameters on a graph was not practical, a large number of 

graphs would be required. That is, a number of graphs with WF,O as the parametric variable, 

similar to the two graphs for the functions F3 and F4 in ASHRAE 103-1993, would be needed 

for each value of WF,-- One way to reduce the number of graphs required was to take the 

parameter WF,- out of the integral. During the evaluation process, it was found that the function 

in the integral, F(t, WF.O, WF,~), can be approximated as a linear function of 

values oft between 0 and tow (13.3 min for furnaces and 33.3 min for boilers as assumed in the 

test procedure) and for constant WF,O. That is, it can be assumed that 

for constant 

F(t,VF, 0 7 WF, m) = '1 VF. - + c'2 (5 - 3) 

18 
with the conditions F = F(t, WF.O, 0) for WF,.. = 0; and F = F(t, WF,O, 100) for WF,- = 100, where the 

point WF,- = 100 was arbitrarily chosen as one possible value of off-cycle flue gas temperature 

(differential) at equilibrium. Usually, WF.- = TF,oFF(~) - TRA would be much smaller than 100. 

Figure 5- I shows a plot of the linear variation of the function F as a function of WF,- between the 

values of 0 to 100 for ranges oft = 0 min to 30 min after the burner was off for a boiler operating 

indoors with the value of ~F,O determined by test data based on ASHRAE 103-1993. It is seen 

that the linear approximation is very good. 

3 

U. + 

7 

0 

25 

2 

1.5 

1 

0.5 

0 

Figure 5-1 Variation of the function F versus vF, 

With the above boundary conditions (F = F(t, WF,O, 0) for WF,.-. = 0; and F = F(t, WF,O, 100) for 

WF,.. = loo), the functional form of C1 and CZ in the linear approximation can be determined as, 

Or, explicitly, 

19 
Substituting the expressions of CI and C? into the function F inside the integral for the loss Ls.0~ 

and changing the variable of integration from t to t/TOFF, result in the expression for the 

mproximation of LS.OFF by the functions F3 and F4 as shown in section 11.2.10.6 of ASHRAE 

103- 1993. 

‘ON 0 ‘ON 0 

where 

(5-5) 

F4(WF.0,r0FF “OFF 

It is noted that since the parameter WF,.. is a constant, it is taken out of the integral involving F4 

in the expression for LS.OFF. 

The function F5, F6, F7, and F8 in ASHRAE 103- 1993 were derived the same way. It is noted 

here that in ASHRAE 103-1993, the derivation of the damper effectiveness factor Do and the 

defining equations for the terms G3 and G4 in the formula for Do (see section 11.2.9.18 of 

ASHRAE 103-1993) used the same approximation for the off-cycle losses (and the same term 

‘100’). Details of that derivation is presented in the ASHRAE Transaction paper titled “The 

Effect of a Time Delayed Stack Damper on Off-Cycle Heat Losses for Residential Heating 

Equipment” by C. Park, et a1.@] 

As mentioned before, the value 100 (as one possible temperature difference between the flue gas 

temperature and room temperature at equilibrium during cool down) was selected arbitrarily to 

determine the slope of the straight line in the linear approximation of the function F(t, WF,~, WF,~). 

For furnaces the actual temperature difference would be fairly small for unit with pilot light (and 

20 
0 for unit with interrupted electronic ignition as specified in the test procedure), and for boilers 

the actual temperature difference would be generally much less than 100. 

6. SUMMARY 

This report documented cht: developments of proposed revisions to the ASHRAE standard 103- 

1993 on the method of testing for residential central furnaces and boilers. The changes include 

the following: 

( 1) A revised procedure to calculate the average burner odoff times of modulating furnaces and 

boilers during the cyclic operation of the burner. The procedure used the same cycles per hour at 

half load as were assumed for a single stage unit (5 cycles per hour for furnaces and 2 cycles per 

hour for boilers), thus gives a more consistent comparison with a single stage unit. 

(2) A revised method for the calculation of the annual burner operating hours using the measured 

electric power input at the actual fuel input rate to determine the annual burner operating hours 

and the annual fuel and electric energy consumption at the actual burner firing rates of a 

modulating unit. 

( 3) An extension of the current procedure to calculate the off-period flue loss of oil-fired 

equipment with a longer post purge time. The procedure will more accurately determine the offperiod ilue loss that is higher when the boiler is operating with a longer than 3 min of post purge 

time. 

This report also provides the background information on the derivation of the integral functions 

listed in Appendix E of ASHRAE standard 103-1993 that were not being formally documented. 

Those integral functions were used for producing the graphs in the standard for the hand 

calculation of the off-period losses with a calculator, and also in the numerical algorithm in the 

computerized calculation procedure. 

21 
REFERENCES 

I. Code of Federal Regulations, 10 CFR Part 430, Subpart B, Appendix N, "Uniform Test 

Method for Measuring the Energy Consumption of Furnaces (May 1997)", January 1,2002 

2. ANSYASHRAE standard 103-1993, "Methods of Testing for Annual Fuel Utilization 

Efficiency of Residential Central Furnaces and Boilers", ASHRAE, Atlanta, GA 

3. 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, March 1978. 

4. Chi. J.. Kelly, G.E., "A Method for Estimating the Seasonal Performance of Residential Gas 

and Oil-Fired Heating Systems", ASHR.-\E Transactions, Vol. 84, Part 1, 1978 

5. Kweller, E., "An Analysis of Burner On and Off Periods and Their Effect on Part-Load 

Efficiency for Furnaces and Boilers Equipped with modulating Controls", Letter Report 

prepared for the Department of Energy, January 1983 

6. Park, C., Didion, D.A., Kelly, G.E., "The Effect of a Time Delayed Stack Damper on OffCycle Heat Losses for Residential Heating Equipment", ASHRAE Transactions, Vol. 89, 

1983 

22