7-B-i

APPENDIX 7-B. DETERMINATION OF BOILER ENERGY USE IN THE LCC 

ANALYSIS 

TABLE OF CONTENTS

7-B.1 INTRODUCTION ................................................................................................. 7-B-1

7-B.2 DETERMINATION OF SPACE HEATING ANNUAL FUEL ENERGY 

CONSUMPTION................................................................................................... 7-B-1

7-B.2.1 Determination of National Average Number of Burner Operating Hours 

(BOHSS) ................................................................................................................. 7-B-2

7-B.2.2 Assigning Boiler Equipment Characteristics to Sampled Households.................. 7-B-5

7-B.2.2.1 Input Capacity of Existing and New Equipment ................................. 7-B-5

7-B.2.3 Derivation of Existing AFUE ................................................................................ 7-B-9

7-B.2.3.1 Methodology........................................................................................ 7-B-9

7-B.2.3.2 Results................................................................................................ 7-B-12

7-B.2.3.3 Adjustment of Boiler AFUE Based on Return Water Temperature 

Applications....................................................................................... 7-B-13

7-B.3 DETERMINATION OF ANNUAL ELECTRICAL ENERGY 

CONSUMPTION................................................................................................. 7-B-16

Table 7-B.2.1 Design Temperature Difference Correction Factors............................ 7-B-6

Table 7-B.2.2 Ceiling Correction Factors................................................................... 7-B-7

Table 7-B.2.3 Construction Characteristics by Vintage ............................................. 7-B-7

Table 7-B.2.4 Heat Loss by Construction Characteristics (Total BTU)..................... 7-B-7

Table 7-B.2.5 Construction Characteristics by Vintage (BTU/SQFT)....................... 7-B-8

Table 7-B.2.8 Shipment Weighted AFUE for the Year 1978 for Gas-Fired and 

Oil-Fired Hot Water Boilers ................................................................ 7-B-9

Table 7-B.2.9 Fractions of Shipments in 2001 by Efficiency Level for Boilers 

reported by AHRI .............................................................................. 7-B-10

Table 7-B.2.10 Market Penetrations of Energy Star Qualified Residential Boilers... 7-B-11

Table 7-B.2.11 DOE Estimated Market Penetrations of Energy Star Qualified 

Residential Boilers by Product Class................................................. 7-B-11

Table 7-B.2.12 Adjustment to AFUE Based on RWT Application............................ 7-B-16

Table 7-B.3.1 Inducer Motor Power Consumption by Product Type....................... 7-B-18

LIST OF FIGURES

Figure 7-B.2.1 AFUE distributions by year from 1978 to 2009 ................................ 7-B-12

Figure 7-B.2.2 Shipment Weighted AFUE (SWAFUE) for Gas-Fired Hot Boilers 

(HWGB), Gas-Fired Steam (SGB), Oil-Fired Hot Boilers (HWOB) 

and Oil-Fired Steam (SOB) ............................................................... 7-B-13

Figure 7-B.2.3 Efficiency vs. Return Water Temperature ......................................... 7-B-14
7-B-ii

Figure 7-B.2.4 Average Curve for Condensing Boilers............................................. 7-B-15

Figure 7-B.2.5 Average Curve for Non-Condensing Boilers..................................... 7-B-16
7-B-1

APPENDIX 7-B. DETERMINATION OF BOILER ENERGY USE IN THE LCC 

ANALYSIS

7-B.1 INTRODUCTION

For calculating the energy consumed by residential boilers, DOE considered the energy 

use associated with providing space heating and domestic water heating. The boiler space 

heating energy consumption methodology is based on ASHRAE Standard 103-2007 “Method of 

Testing for Annual Fuel Efficiency of Residential Central Furnaces and Boilers” to accurately 

reflect field operation. In addition, DOE developed a methodology for estimating the water 

heating energy use provided by boilers. The calculation used for the determination of the total 

energy use is as follows:

Total Energy Use = FuelUseSH + FuelUseWH + ElecUse

Eq. 7-B.1

This provides the detailed space heating and water heating energy use calculation 

methodology.

7-B.2 DETERMINATION OF SPACE HEATING ANNUAL FUEL ENERGY 

CONSUMPTION

The average annual fuel consumption (EF) is calculated in Appendix C section 2 of the 

ASHRAE 103/2007 test procedure (assuming that the equipment has no continuous pilot 

ignition):

1

EF = BOHSS * QIN, for single-stage boilers,

EF = (BOHH * QIN) + (BOHR * QIN,R), for two-stage boilers, and

EF = (BOHM * QIN,M) + (BOHR * QIN,R), for continuous modulatinga1 boilers

Eq. 7-B.2

Where:

BOHSS = national average number of burner operating hours (see derivation in section 7-B.2.1),

BOHH = national average number of burner operating hours at the maximum operating mode for 

two-stage boilers (see derivation in section 7-B.2.1),

BOHR = national average number of burner operating hours at the reduced operating mode for 

two-stage or continuous modulating boilers (see derivation in section 7-B.2.1),

BOHM = national average number of burner operating hours at the modulating operating mode 

for continuous modulating boilers (see derivation in section 7-B.2.1),

 a

 In this analysis, “continuous modulating” term is used instead of “step-modulating”. Both terms are 

interchangeably used in the literature.
7-B-2

QIN = steady-state nameplate input rate in Btu/h for single-stage boilers or steady-state 

nameplate maximum input rate in Btu/h for two-stage and continuous modulating boilers,

QIN,R = steady-state reduced fuel input rate, and

QIN,M = steady-state modulating fuel input rate.

For the FuelUseSH,i calculation in Chapter 7, DOE used EF as the basis for determining 

energy use for each operating mode (e.g., single-stage, two-stage, or modulating) as follows:

FuelUseSH,i = BOHSH,i × QIN,i

Eq. 7-B.3

Where:

BOHSH,i = BOHSS, BOHH, BOHR, or BOHM, space heating burner operating hours (hr/yr), and

QIN,i = QIN, QIN,R, or QIN,M input capacity of existing boiler (kBtu/hr).

QIN is based on the baseline value for each product class. DOE assumed QIN,R to be 70% 

of QIN for two-stage equipment, and 40% for modulating equipment, where this value represents 

the average ratio QIN/QIN,R as derived using manufacturer product literature.

From the test procedure,1 QIN,M is calculated using QOUT,M and EffySS,M (as defined in 

section 11.4.8.10 or 11.5.8.8 in the ASHRAE SPC 103/2007 test procedure).1

7-B.2.1 Determination of National Average Number of Burner Operating Hours 

(BOHSS) 

From the ASHRAE SPC103/2007 test procedure,1 the national average number of burner 

operating hours for boilers is calculated in Appendix C section 1 (assuming that the equipment 

has no continuous pilot ignitionb):

BOHSS = 2080 * 0.77 * A * (QOUT / (1 + α)), for single-stage boilers,

BOHH = XH * (2080) * (0.77) * AH * (QOUT / (1 + α)), for two-stage boilers at the maximum 

operating mode,

BOHR = XR * (2080) * (0.77) * AR * (QOUT / (1 + α)), for two-stage and continuous modulating 

boilers operating at the reduced operating mode, and

BOHM = XH * (2080) * (0.77) * AM * (QOUT / (1 + α)), for continuous modulating boilers

operating at the modulating operating mode,

 

b For factor B, in the expanded equation, if QP = 0 (pilot flame fuel input rate in Btu/h), then B = 0, which is true for 

all boiler product classes in this analysis. 
7-B-3

Eq. 7-B.4

Where:

2080 = national average heating load hours,

0.77 = adjustment factor to adjust the calculated design heating requirements and heating load 

hours to the actual heating load experienced by the heating system,

A = 100,000/ [341300(yP * PE + yIG,R *PEIG,R + y * BE) + QIN * EffyHS]

c3

, 

AH = 100,000/ [341300(yP * PEH + yIG,R *PEIG,R + y * BEH) + QIN * EffyU,H]

a

, 

AR = 100,000/ [341300(yP,R * PER + yIG,R *PEIG,R + yR * BER) + QIN,R * EffyU,R]

a

, 

AM = 100,000/ [341300(yP * PEH + yIG,R *PEIG,R + y * BEH) + QIN,M * EffyU,M]

a

, 

QOUT = maximum fuel input rate heating capacity,

α = oversize factor set to 0.7,

XH = fraction of heating load at maximum fuel input rate operating mode,

XR = fraction of heating load at reduced fuel input rate operating mode (1-XH),

QIN = as defined in above,

QIN,R = as defined in above,

QIN,M = as defined in above, 

yP = ratio of induced or forced draft blower on-time to average burner on-time,

yP,R = ratio of induced or forced draft blower on-time to average burner on-time, measured at the 

reduced fuel input rate,

PE = burner (or draft inducer) electrical power input at full-load steady-state operation in kW,

PER = burner (or draft inducer) electrical power input at full-load steady-state operation in kW, 

measured at the reduced fuel input rate,

PEH = burner (or draft inducer) electrical power input at full-load steady-state operation in kW, 

measured at the maximum fuel input rate,

yIG = ratio of burner interrupted-ignition device on-time to average burner on-time,

yIG,R = ratio of burner interrupted-ignition device on-time to average burner on-time, measured at 

the reduced fuel input rate,

PEIG = electrical input rate to the interrupted ignition device on the burner,

PEIG,R = electrical input rate to the interrupted ignition device on the burner, measured at the 

reduced fuel input rate,

PEIG,H = electrical input rate to the interrupted ignition device on the burner, measured at the 

maximum fuel input rate,

y = pump on-time to burner on-time,

yR = pump on-time to burner on-time, measured at the reduced fuel input rate,

BE = water pump electrical energy input rate in kW,

BER = water pump electrical energy input rate in kW, measured at the reduced fuel input rate, 

BEH = water pump electrical energy input rate in kW, measured at the maximum fuel input rate.

EffyHS = ratio of the average length of the heating season in hours to the average heating load 

hours,

EffyU,H = average part load efficiency at the maximum fuel input rate,

 c

 The ASHRAE test procedure does not deal with ignitor energy consumption. The ratio of ignitor on-time to 

burner on-time and the ignitor power consumption variables come from the DOE test procedure.2
7-B-4

EffyU,R = average part load efficiency at the reduced fuel input rate, and

EffyU,M = average part load efficiency at the modulating fuel input rate.

The modified equations used in the LCC spreadsheet are as follows (Note that the 

BOHSH,I is the same as BOHSS, BOHH, BOHR, or BOHM depending on operating mode)d:

BOHSH,i = BHLBOILER

QUseful,i

Eq. 7-B.5

Where:

QUseful,i = A, AH, AR, or AM (as defined above), which are the total useful energy output of the 

boiler equipment, including useful heat from auxiliary electrical components for each 

operating mode (kBtu/hr), and

BHLBOILER = building heating load served by a single boiler (kBtu/yr), as described in Chapter 7.

To calculate factors A, AH, AR, AM, DOE calculated yP, yP,R, PE, PER, PEH, yIG, yIG,R, 

PEIG, PEIG,R, PEIG,H, y, yR, BE, BER, BEH, PE, yIG, PEIG, y, BE, QIN, and QP yP , as described in 

section 7-B.4 of this appendix. We calculated EffyHS, heating seasonal efficiency, as defined in 

sections 11.2.11, 11.3.11.3, 11.4.11.3, and 11.5.11.3 in ASHRAE SPC 103/2007 test procedure.

1

 

EffyHS is equal to the annual fuel utilization efficiency (AFUE) for the boilers in this analysis, 

which do not have a continuous pilot. EffySS is calculated using QOUT (as defined below), K 

(factor that adjusts the jacket losses, where K = 0 for boilers and Lj (jacket loss, where Lj = 1 is 

the default value as described in section 11.2.8.11

)

QOUT for all product classes is calculated using the following equations based on AHRI 

Directory data for Boilers:

3

For gas-fired hot water boilers: 

QOUT = QIN (AFUE + 0.29), if AFUE is between 80 and 85,

QOUT = QIN (AFUE + 0.14), if AFUE is between 90 and 91,

QOUT = QIN (AFUE – 2.53), if AFUE is above 92.

For gas-fired steam boilers: 

QOUT = QIN (AFUE + 0.31).

For oil-fired hot water boilers: 

QOUT = QIN (AFUE + 0.89).

For oil-fired steam boilers: 

QOUT = QIN (AFUE + 1.22).

 

d Maximum value for BOH is set to 8760
7-B-5

DOE calculated XH by using TC (balance-point temperature as defined in section 11.4.8.4

of ASHRAE SPC 103/2007 test procedure1

), α (the oversize factor, as calculated in Equation 

11), QOUT, and QOUT,R (reduced fuel input rate heating capacity). QOUT,R is set equal to 70% of 

QOUT for two-stage equipment and 40 percent of QOUT for modulating equipment. XR is set 

equal to 1-XH. EffyU,H = EffyU,R and therefore using the equation in section 11.5.11.3 of 

ASHRAE SPC 103/2007 test procedure is equal to EffySS. EffyU,M is calculated using the 

equation in section 11.4.9.2.3 of ASHRAE SPC 103/2007 test procedure.

1

7-B.2.2 Assigning Boiler Equipment Characteristics to Sampled Households

To estimate the heating load of each sample housing unit, DOE represented the existing 

boiler by assigning an input capacity and AFUE to the boiler in the RECS and CBECS sample 

building units. 

7-B.2.2.1 Input Capacity of Existing and New Equipment

To support this analysis, DOE had to establish a methodology for deriving the boiler 

capacities of each of the sampled buildings. Boilers are typically sized by accounting for the 

outdoor environmental conditions and specific building characteristics.e For this analysis, DOE 

established the input capacity of sampled equipment using the following equation:

Qin = AdjustedHeatLoss

AFUEbaseline

Eq. 7-B.6

Where:

AdjustedHeatLoss = estimated building heat loss adjusted for building characteristics (kBtu/h),

AFUEbaseline = AFUE of the baseline boiler for each product class AFUEex (see Appendix 7-B).

The Department calculated the input capacity for the existing boiler of each 

housing/building unit based on information derived from the RECS and CBECs data. The sizing 

approach determines the input capacity of the sampled household/building by accounting for 

building characteristics that include the building square footage, building vintage, foundation 

type, shell characteristics, number of floors, outdoor design temperature, reported insulation 

category, and two factors that correct the building heating loss. 

These characteristics are needed to estimate the household/building total heat loss. DOE 

utilized information from several boiler manufacturers and distributors to develop a methodology 

 

e For this analysis the manufacturer costs are assumed have the same input capacity regardless of efficiency level. 

Therefore, DOE assumed that the household would have one and the same input capacity regardless of efficiency 

level. This might lead to some households being significantly oversized for higher efficiency equipment.
7-B-6

to derive the heat loss per square foot (see Table 7-B.2.4 and Table 7-B.2.5). Using the total heat 

loss value, DOE identified boiler models with an output capacity meet the calculated heat loss of 

the home. The following steps describe how the boiler input capacity was determined:

1) We started with determining the vintage of the sampled building using RECS/CBECS

and applying building shell and construction characteristics to each vintage.f Using these 

characteristics, DOE selected a heat loss per square foot value based on a design 

temperature difference of 70°F.g Details of the relationship developed between building 

vintage, shell, and construction characteristics can be found in the Table 7-B.2.3.

2) Next using information gathered from RECS/CBECs, for each sampled building, we

determined the number of floors (including heated basements), the total square footage, 

and the outdoor design temperature. 

3) Using the heat loss values and building and environmental characteristics determined

above, we calculated the heat loss per floor including any necessary correction factors 

and then summed the results of all floor to establish the total heat loss of the sampled

building. DOE assumed that boiler output capacity was equal to the total heat loss of the 

buildings.

To adjust the building total heat loss value to account for the design temperature

difference and the first floor heat loss in multiple story building, we applied the Design 

Temperature Correction Factor and Ceiling Correction Factor from Weil-Mclain’s Boiler 

Replacement Guide (see Table 7-B.2.1 and Table 7-B.2.2).

8

Table 7-B.2.1 Design Temperature Difference Correction Factors

DESIGN 

TEMP. 

DIFFERENCE FACTOR

DESIGN 

TEMP. 

DIFFERENCE FACTOR

DESIGN 

TEMP. 

DIFFERENCE FACTOR

25°F 0.35 55°F 0.78 90°F 1.29

30°F 0.42 60°F 0.85 95°F 1.36

35°F 0.5 65°F 0.92 100°F 1.43

40°F 0.57 75°F 1.07 105°F 1.5

45°F 0.64 80°F 1.15 110°F 1.57

50°F 0.71 85°F 1.2 115°F 1.64

 

g The design temperature difference is equal to the indoor temperature setting (70°F) minus the outdoor design 

temperature (0°F). 
7-B-7

Table 7-B.2.2 Ceiling Correction Factors

CONSTRUCTION 

NUMBER FACTOR

1 .82

2 .79

3 .88

4 .91

Table 7-B.2.3 Construction Characteristics by Vintage

FRAME CONTRUCTION CHARACTERISTICS

CONSTRUCTION 

VINTAGE

CONSTRUCTIO

N DESIGN 

NUMBER

WEATHERSTRIPPE

D GLASS

INSULATION 

THICKNESS

WALL CEILING

<1960 1 Single 1" 2"

1961-1975 2 Double 1" 2"

1976-2000 3 Double 3" 6"

2001-2009 4 Double 3" 9"

Table 7-B.2.4 Heat Loss by Construction Characteristics (Total BTU)

HEAT LOSS (BTU)

FLOOR 

AREA CONSTRUCTION DESIGN NUMBER

SQ. FT. 1 2 3 4

500 28,750 23,900 17,910 15,460

600 32,300 27,030 20,460 17,520

700 35,860 30,150 23,010 19,580

800 39,840 33,590 25,760 21,840

900 43,340 36,680 28,270 23,860

1000 46,890 39,800 30,820 25,920

1100 50,450 42,920 33,370 27,980

1200 54,000 46,040 35,920 30,040

1300 56,640 48,470 38,030 31,660

1400 60,200 51,590 40,580 33,720

1500 62,830 54,030 42,690 35,340

1600 65,530 56,490 44,840 37,000

1700 68,590 59,250 47,140 38,810

1800 71,720 62,040 49,490 40,670

1900 73,930 64,150 51,400 42,090

2000 77,050 66,940 53,750 43,950

Table 7-B.2.4 contains the estimated heat loss by square foot bin for all design numbers. 
7-B-8

Table 7-B.2.5 Construction Characteristics by Vintage (BTU/SQFT)

HEAT LOSS (BTU/SQFT)

FLOOR 

AREA CONSTRUCTION DESIGN NUMBER

SQ. FT. 1 2 3 4

500 57.5 47.8 35.8 30.9

600 53.8 45.1 34.1 29.2

700 51.2 43.1 32.9 28.0

800 49.8 42.0 32.2 27.3

900 48.2 40.8 31.4 26.5

1000 46.9 39.8 30.8 25.9

1100 45.9 39.0 30.3 25.4

1200 45.0 38.4 29.9 25.0

1300 43.6 37.3 29.3 24.4

1400 43.0 36.9 29.0 24.1

1500 41.9 36.0 28.5 23.6

1600 41.0 35.3 28.0 23.1

1700 40.3 34.9 27.7 22.8

1800 39.8 34.5 27.5 22.6

1900 38.9 33.8 27.1 22.2

2000 38.5 33.5 26.9 22.0

Table 7-B.2.5 contains the estimated heat loss per square foot for each size bin and for all 

design numbers. 

Finally, to approximate output capacity values similar to shipment weighted values or 

from number of available models, DOE used an oversize factor of 1.7 (based on the ASHRAE 

103-2007 oversize factor). The resulting baseline output capacity values are shown in table 7-

B.2.6. Table 7-B.2.7 shows the comparison of these results to shipment weighted data and data 

from number of currently available boiler models.

Table 7-B.2.6 Range of Derived Input Capacity for Each Boiler Product Class, 

MMBtu/year

Product Class Min Max Average

Percentiles

5% 25% 50% 75% 95%

Gas-Fired Hot Water Boilers 30 300 164 50 90 140 240 300

Gas-Fired Steam Boilers 60 300 179 60 100 170 280 300

Oil-Fired Hot Water Boilers 80 300 176 80 110 160 230 300

Oil-Fired Steam Boilers 90 300 179 90 110 160 250 300

Electric Boilers 30 300 148 40 80 120 200 300
7-B-9

Table 7-B.2.7 Comparison of Derived Input Capacity to Shipment Weighted Data and 

Model Data 

Product Class

DOE Derived 

Average

2013 DOE Boiler 

Models*

Shipment Weighted Output 

Capacity for Cast Iron Boilers**

Input 

Capacity

Output 

Capacity

Input 

Capacity

Output 

Capacity 1970 1980 1990 2000

Gas-Fired Hot Water Boilers 164 145 151 131 124 122 113 116

Gas-Fired Steam Boilers 179 135 172 142 153 153 138 139

Oil-Fired Hot Water Boilers 176 144 158 137 148 154 147 144

Oil-Fired Steam Boilers 179 148 190 162

1 1 1 1

Electric Boiler 164 147 NA NA NA NA NA NA

* Based on October 2013 AHRI directory as well as other sources (see Appendix 7-D.

** Based on AHRI (formerly GAMA) shipment data provided in April 2002.

7-B.2.3 Derivation of Existing AFUE

As described above, the AFUE of existing equipment that has already been installed is 

required in order to calculate the building heating load. DOE developed the historical 

distributions of efficiencies for existing boilers based on historical shipment data by efficiency 

from the Air-Conditioning, Heating, and Refrigeration Institute (AHRI, formerly the Gas 

Appliance Manufacturers Association: GAMA), the distribution of models in historical versions 

of the AHRI certification directory, and fraction of Energy Star shipments.

7-B.2.3.1 Methodology

Efficiency distributions have been determined from 1978 to 2009. For the year 1978, 

DOE used average AFUE data that was provided for both gas-fired hot water boilers and oilfired hot water boilers.5 The distribution of AFUE for 1978 was designed so that the average 

AFUE correspond to the value indicated by the aforementioned data. Table 7-B.2.8 shows the 

shipment-weighted efficiencies available for 1978. 

Table 7-B.2.8 Shipment Weighted AFUE for the Year 1978 for Gas-Fired and Oil-Fired 

Hot Water Boilers

Gas-Fired Hot Water Oil-Fired Hot Water

65% 76%

The efficiency distributions for the years 1991 and 1992 were assessed using data from 

GAMA.6 The AFUE distributions correspond to the ratio of models by efficiency levels.
7-B-10

For the year 2001, shipment data were available.7 The distributions of the efficiencies for 

which shipment data were not available were based on GAMA data.6 Table 7-B.2.9 shows the 

historical shipment data of residential boilers by efficiency level in 2001 collected by AHRI 

(formerly GAMA).7

Table 7-B.2.9 Fractions of Shipments in 2001 by Efficiency Level for Boilers reported 

by AHRI 

Gas-Fired Hot Water Gas-Fired Steam Oil-Fired Hot Water Oil-Fired Steam

AFUE Shipment AFUE Shipment AFUE Shipment AFUE Shipment

80 35% 78-79 33% 80-82 16% 80-82 24%

81 22% 80 46% 83-85 70% 83-85 76%

82 14% 81-83 21% 86-87 14%

83 15%

84 8%

85 1%

87-95 6%

Market penetration data were available for Energy Star-compliant models from 2003 to 

2009. For 2009, the efficiency distribution was originally assessed using data from GAMA,

which was then scaled to take into account the portion of the market that was Energy Star 

compliant.6 The distributions from 2003 to 2008, are based on a linear interpolation between 

2002 and 2009, and were scaled to take into account the portion of the market that was Energy 

Star compliant for each year.

The Energy Star program for residential boilers started on April 1, 20028 and 

performance criteria for Energy Star boilers was set at 85% AFUE. Table 7-B.2.10 shows the 

market penetrations and shipments of Energy Star qualified residential boilers from 2003-2012 , 

as well as the fraction of manufacturers reporting Energy Star shipment data. Some of the growth 

in higher efficiency boilers has been due to incentive programs (e.g. Federal tax incentives, State 

and utility incentives). Federal tax credits started in 2005 and remain in effect until 12/31/2013.

9

As shown in table 7-B.2.11, DOE used total shipment estimates by product class (derived 

from the shipment model described in chapter 9) to calculate the Energy Star fractions used in 

the analysis.
7-B-11

Table 7-B.2.10 Market Penetrations of Energy Star Qualified Residential Boilers

Year

Original Energy 

Star Market

Share

Energy Star 

Shipments

Fractions 

Reporting

Gas

Boiler

Oil

Boiler

Gas

Boiler

Oil

Boiler

Gas & Oil

Boiler

2003 31% 75% 48,769 89,441 79%

2004 41% 71% 96,000 115,000 90%

2005 25% 51% 55,091 83,434 81%

2006 34% 67% 68,102 84,668 81%

2007 39% 61% 76,309 99,226 100%

2008 57% 62% 109,605 75,151 95%

2009 46% 62% 88,000 76,000 100%

2010 52% 61% 100,000 75,000 98%

2011 42% 52% 82,000 64,000 93%

2010 57% 47% 109000 57,000 98%

Table 7-B.2.11 DOE Estimated Market Penetrations of Energy Star Qualified 

Residential Boilers by Product Class

Year

DOE Estimated Shipments

(see Chapter 9)

DOE Estimated Fraction of 

Energy Star Qualified Boilers

By Product Class

HWGB SGB HWOB SOB HWGB SGB HWOB SOB

2003 239,843 38,105 169,680 17,706 26% 0% 63% 35%

2004 264,588 36,846 167,444 19,460 40% 0% 72% 36%

2005 252,465 37,127 152,742 17,256 27% 0% 64% 32%

2006 229,231 30,243 127,531 16,016 37% 0% 78% 35%

2007 232,969 33,932 130,301 15,494 33% 0% 72% 35%

2008 259,742 32,374 132,785 11,061 44% 0% 56% 39%

2009 204,370 23,540 98,626 9,616 43% 0% 73% 43%

2010 208,711 21,957 106,340 9,951 49% 0% 68% 42%

2011 218,112 25,615 99,611 9,084 40% 0% 65% 41%

2010 246,590 28,516 84,952 7,483 45% 0% 65% 42%

5-year average 44% 0% 65% 41%

The distributions for the years between 1978 and 1991, 1992 and 2001, and eventually 

2002, are based on a linear interpolation.

Further refinements include the utilization of shipment data to derive the efficiency 

distribution of the year 1985.
7-B-12

7-B.2.3.2 Results

Figure 7-B.2.1 shows the distribution of AFUE from 1978 to 2009 by AFUE bins. The 

rapid drop of AFUE bellow 80 between 1991 and 1992 is explained by the passage of a new 

efficiency standard. 

Figure 7-B.2.1 AFUE distributions by year from 1978 to 2009

Figure 7-B.2.2 shows the shipment weighted AFUE for gas-fired hot boilers, gas-fired 

steam, oil-fired hot boilers, and oil-fired steam.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

1978198019821984198619881990199219941996199820002002200420062008

>=90

85-89

80-84

65-79

AFUE Bins
7-B-13

Figure 7-B.2.2 Shipment Weighted AFUE (SWAFUE) for GasFired Hot Water Boilers (HWGB), Gas-Fired Steam 

(SGB), Oil-Fired Hot Water Boilers (HWOB) and 

Oil-Fired Steam (SOB)

7-B.2.3.3 Adjustment of Boiler AFUE Based on Return Water Temperature 

Applications

The residential boilers considered in this rulemaking utilize a variety of distribution

systems, the designs of which depend on the age and design of the home, the type of boiler 

equipment used, and building and climate characteristics. 

Most of the hydronic heat distribution systems include boiler equipment, distribution 

piping, and heat emitters (finned-tube convective baseboards, panel radiators, in-floor radiant 

tubing and fan-coil convectors).

9 Each system is designed to accommodate a specific supply 

water temperature which can vary depending on the heat emitter type. For instance, fan-coil units 

are recommend to operate with supply water at 180-200°F, 140-190°F for finned-tube 

baseboards, and 100-140°F for in-slab radiant floors.10 This analysis assumes that a temperature 

difference between supply and return water temperature of 20°F is typical of most of the emitter 

described above. Boiler thermal efficiency decreases for both condensing and non-condensing 

equipment as return water temperatures increases.11

Several manufacturers publish a relationship between boiler thermal efficiency and the 

return water temperature (RWT) for specific boiler series.

12,13,14,15,16 This information was used

to develop an average efficiency vs. RWT relationship representative of boiler performance for a 

range of return water temperatures. The analysis assumes that the AFUE scales according to the 

60.0

65.0

70.0

75.0

80.0

85.0

90.0

1975 1980 1985 1990 1995 2000 2005 2010

AFUE

SWAFUE for HWGB

SWAFUE for SGB

SWAFUE for HWOB

SWAFUE for SOB
7-B-14

relationship reported for the thermal efficiency. Figure 7-B.2.3 presents the information obtained 

from these sources and the derived average efficiency curve.

h

Figure 7-B.2.3 Efficiency vs. Return Water Temperature

Based on the average efficiency versus RWT relationship shown in Figure 1, DOE

developed separate values for condensing and non-condensing equipment to adjust boiler 

efficiency based on the high, medium, and low RWT applications experienced in the field. For 

both condensing and non-condensing boilers, the medium water temperature point reflects the 

reported boiler performance as rated according to the DOE test procedure (RWT of 120-124°F). 

The reported AFUE was the basis for adjustment. For non-condensing boilers, only two RWT 

applications, medium and high, were used, as non-condensing boilers are not typically used in 

low temperature applications.

To establish the representative conditions for low temperature applications, we assumed 

that a RWT of 100°F was typical of annual operations of in-slab radiant systems. For the high 

temperature applications, we assumed that 150°F was typical of annual operations of finned-tube 

baseboards and fan-coil units. Initially, both the low and high annual operating temperatures 

were approximated to be 110°F and 160°F; however, after accounting for the use of an automatic 

means for adjusting water temperature, the initial temperatures were reduce by 10°F.

For condensing boilers, the analysis used the average relationship shown in Figure 7-

B.2.3 to establish the magnitude of the efficiency adjustment required for the high and low 

temperature applications. As shown in Figure 7-B.2.4, the relationship indicates an approximate 

 

h The average efficiency curve represents the mean efficiency at each return water temperature reported in the 

manufacturers’ information. 

80

85

90

95

100

60 80 100 120 140 160 180

Thermal Efficiency (%)

Return Water Temperature

Boiler Efficiency vs Return Water Temperature

(Condensing)

Hydrotherm

Hydrotherm 2

Aerco

Burderus

ASHRAE

Deppman

Crest

Average Curve
7-B-15

change of 3% thermal efficiency from 100°F (low) to 120°F (med) and from 120°F (low) to 

150°F (high). 

Figure 7-B.2.4 Average Curve for Condensing Boilers.

For non-condensing boilers, the analysis used the average efficiency/RWT relationship at 

the 140-180°F temperature range to establish the efficiency adjustment required for the high 

temperature applications. The data for temperatures between 140°F and 180°F was used to 

extrapolate the efficiency at 120°F using linear regression. As shown in Figure 7-B.2.5, the 

relationship resulted in an approximate change of 0.6% efficiency from 120°F (med) to 150°F 

(high).

80.0

82.0

84.0

86.0

88.0

90.0

92.0

94.0

96.0

98.0

Thermal Efficiency (%)

Return Water Temperature

Boiler Efficiency vs Return Water Temperature

(Condensing)

Average Curve

93.3

90.2

87.0
7-B-16

Figure 7-B.2.5 Average Curve for Non-Condensing Boilers.

Table 7-B.2.12 summarizes the AFUE adjustments used in the analysis. The adjusted 

AFUE values reflect the efficiency of boilers for the fraction of households and buildings that 

utilize hydronic heat distribution systems which operate at return water temperatures different 

from 120°F.

Table 7-B.2.12 Adjustment to AFUE Based on RWT Application

Low RWT (100°F) Medium RWT (120°F)i High RWT (150°F)

Non-Condensing - As Report -0.6%

Condensing +3% As Report -3%

7-B.3 DETERMINATION OF ANNUAL ELECTRICAL ENERGY 

CONSUMPTION 

Using the ASHRAE SPC 103/2007 test procedure,1 the average annual auxiliary 

electrical energy consumption is calculated in Appendix C section 3:

EAE = BOHSS (yP * PE + yIG *PEIG + y * BE), for single-stage boilers,

and

 

y = -0.0204x + 90.492

R2 = 0.9695

80

82

84

86

88

90

Thermal Efficiency (%)

Return Water Temperature

Boiler Efficiency vs Return Water Temperature

(Non-Condensing)

Average Curve Linear (Average Curve)

88.0%

87.4%
7-B-17

EAE = BOHR (yP,R*PER + yIG,R *PEIG,R + yR*BER) + BOHH or M (yP*PEH + yIG *PEIG,H + 

y*BEH)

j2

, for two-stage and continuous modulating boilers,

Eq. 7-B.7

Where:

BOHss = as defined in section 0,

BOHH = as defined in section 7-B.2.1,

BOHM = as defined in section 7-B.2.1,

BOHR = as defined in section 7-B.2.1,

yP = as defined in section 7-B.2.1,

yP,R = as defined in section 7-B.2.1,

PE = as defined in section 7-B.2.1,

PER = as defined in section 7-B.2.1,

PEH = as defined in section 7-B.2.1,

yIG = as defined in section 7-B.2.1,

yIG,R = as defined in section 7-B.2.1,

PEIG = as defined in section 7-B.2.1,

PEIG,R = as defined in section 7-B.2.1,

PEIG,H = as defined in section 7-B.2.1,

y = as defined in section 7-B.2.1,

yR = as defined in section 7-B.2.1,

BE = as defined in section 7-B.2.1,

BER = as defined in section 7-B.2.1, and

BEH = as defined in section 7-B.2.1.

During manufacturer interviews, manufacturers reported that the majority of their units 

are shipped with a pump. The most common pump for residential applications is the Taco 007 

which uses approximately 80 watts and is used on boiler capacities ranging from 45,000 to 

140,000 Btu/h. Condensing boilers have tighter heat exchangers and generally require a more 

powerful pump motor depending on the pressure drop (such as the Taco 0013). For this analysis, 

DOE assumed that non-condensing and condensing units use the same pump motor type, which 

on average uses 80 watts. 

DOE is aware that some consumers use circulator pumps with ECM motor that allow for 

variable speed operation. Such designs use approximately 38 watts while fully operating and less 

when at lower demand, but for this analysis DOE did not consider pumps with ECM motors.

DOE calculated the ratio of pump on-time to burner on-time y and yR using t+ (pump ondelay) and t- (pump off-delay). Manufacturers commented that the on-cycle times of the draft 

inducer vary considerably. For example, high efficiency products and copper products have a 

shut-down time delay of up to 30 seconds to protect the heat exchanger from overheating. For 

 j

 The ASHRAE test procedure does not deal with ignitor energy consumption. The ratio of ignitor on-time to 

burner on-time and the ignitor power consumption variables come from the DOE test procedure.2
7-B-18

this analysis, DOE assumed the shut-down time delay t

+ = 0.5 min and the start-up time delay t-

= 0 min, based on stakeholder comments and boiler model data.

Stakeholders commented that the draft inducer had an average electrical consumption of 

85 to 150 watts (90 watts for 100 kBtu/hr). For these values it is assumed that the inducer motor 

is a PSC type, which is mostly used in non-condensing models. Comments also stated that 

condensing products are forced-draft and use ECM motors in their draft inducers. The typical 

size of the ECM motor is approximately 60 watts. In order to assess the inducer power 

consumption (PEH), DOE used average values derived from the models in the AHRI directory, 

which reflects different motor types, as shown in Table 7-B.3.1.

Table 7-B.3.1 Inducer Motor Power Consumption by Product Type

Gas-Fired Hot 

Water Boiler

Gas-Fired 

Steam Boiler

Oil-Fired Hot 

Water Boiler

Oil-Fired 

Steam Boiler

Non-Condensing 

(natural-draft) 44 66 236 264

Non-Condensing 

(induced-draft) 153 106 180 175

Condensing 95 - 266 -

In addition, for modulating and two-stage units, DOE assumed that the reduced inducer 

power (PER) would be 70% compared to the PEH values indicated in Table 7-B.3.1 (for this 

analysis this assumption was applied to gas-fired hot water boilers above 90% AFUE).

The ratio of draft inducer on-time to burner on-time yP and yP,R are calculated using tP

(post-purge time). Generally the induced- and forced-draft fans operate during post-purge, as 

well as during the burner on-cycle. In general, the post-purge can range from 0 to 30 seconds. 

For this analysis, DOE used a draft inducer post-purge time (tP) of 30 seconds based on 

stakeholder input.

Based on manufacturers’ input, boilers utilize spark ignition, hot surface ignition, and 

direct spark to pilot. Spark ignition is generally used in condensing products while the others are 

used in non-condensing products. In this analysis, DOE assumed spark ignition for all products 

with PEIG equal to 25 watts and tIG equal to 5 seconds.
7-B-19

REFERENCES

1. American Society of Heating Refrigerating and Air-Conditioning Engineers Inc., 

ASHRAE Standard: Method of Testing for Annual Fuel Utilization Efficiency of 

Residential Central Furnaces and Boilers, 2007. Report No. ANSI/ASHRAE 103-2007. 

2. U.S. Department of Energy-Office of Energy Efficiency and Renewable Energy, Title 10, 

Code of Federal Regulations, Chapter II Part 430 Appendix N, Subpart B-Uniform Test 

Method for Measuring the Energy Consumption of Furnaces, January 1, 2013. 

3. Air Conditioning Heating and Refrigeration Institute, Consumer's Directory of Certified 

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<http://www.ahridirectory.org/ahridirectory/pages/home.aspx>

4. Energy Information Administration, Annual Energy Outlook 2013 with Projections to 

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Clothes Dryers, Water Heaters, Room Air Conditioners, Home Heating Equipment, 

Kitchen Ranges and Ovens, Central Air Conditioners and Furnaces, June, 1980. U.S. 

Department of Energy,. Report No. DOE/CS-0166. 

6. Gas Appliance Manufacturers Association, GAMA Directory Database, 1991, 1992, 2001 

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7. Gas Appliance Manufacturers Association, "GAMA Shipment Data," DOE Docket No. 

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8. Weil-McLain, Boiler Replacement Guide, 1997. Report No. C-654 R7(999). 

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Buildings. 3 ed. 2012. Delmar Cengage Learning.

10. U.S. Boiler Company, Outdoor Reset IQ Option Card, p/n 102723-01 Instruction Sheet, 

2012. U.S. Boiler Company. (Last accessed October 14, 2013.) 

<http://cdn.usboiler.net/products/boilers/mpo-iq/assets/outdoor-reset-instructionsheet.pdf>
7-B-20

11. American Society for Heating Refrigeration and Air-Conditioning Engineers (ASHRAE), 

ASHRAE Handbook: HVAC Systems and Equipment. 2008.

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<http://mesteksa.com/fileuploads/Literature/Hydrotherm%20Boilers/KN%20Series%20B

oiler/KN%20Series%20Catalog%20%28KNAP-3%291.pdf>

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<http://www.deppmann.com/2009/10/19th/>

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