CHAPTER 7. ENERGY USE CHARACTERIZATION 

TABLE OF CONTENTS 

7.1 INTRODUCTION ........................................................................................................... 7-1


7.2 WATER HEATERS ........................................................................................................ 7-1


7.2.1	 Introduction.................................................................................................... 7-1


7.2.2	 Household Sample ......................................................................................... 7-1


7.2.3	 Hot Water Use................................................................................................ 7-2


7.2.3.1	 Equation for Hot Water Use .......................................................................... 7-3


7.2.3.2	 Detailed Description of Key Variables Used in Draw Model ....................... 7-4


7.2.3.3	 Impact on Hot Water Use of Technology Changes in Clothes Washers


and Dishwashers ............................................................................................ 7-4


7.2.3.4	 Assignment of Water Heater Tank Size......................................................... 7-5


7.2.3.5	 Determination of Hot Water Use for Instantaneous Water Heaters ............ 7-10


7.2.3.6	 Calculations of Hot Water Use .................................................................... 7-10


7.2.4	 Annual Energy Consumption....................................................................... 7-10


7.2.4.1	 Approach to Calculating Water Heater Energy Use .................................... 7-11


7.2.4.2	 Description of Key Variables....................................................................... 7-14


7.2.4.3	 Derivation of Energy Parameters................................................................. 7-15


7.2.4.4	 Derivation of Temperatures ......................................................................... 7-19


7.2.4.5	 Approach to Calculating Water Heater Energy Consumption..................... 7-24


7.2.5	 Average Annual Energy Consumption ........................................................ 7-25


7.2.5.1	 Gas-Fired Storage Water Heaters ................................................................ 7-25


7.2.5.2	 Electric Storage Water Heaters.................................................................... 7-26


7.2.5.3	 Oil-Fired Storage Water Heaters ................................................................. 7-26


7.2.5.4	 Gas-Fired Instantaneous Water Heaters....................................................... 7-27


7.3 DIRECT HEATING EQUIPMENT .............................................................................. 7-27


7.3.1	 Introduction.................................................................................................. 7-27


7.3.2	 Household Sample ....................................................................................... 7-28


7.3.3	 Determining Annual House Heating Load .................................................. 7-30


7.3.4	 Assigning Direct Heating Equipment to Sample Households ..................... 7-31


7.3.4.1	 Input Capacity of Existing Equipment......................................................... 7-31


7.3.4.2	 Efficiency of Existing Equipment................................................................ 7-32


7.3.5	 Calculations of Energy Consumption .......................................................... 7-33


7.3.5.1	 Fuel Consumption........................................................................................ 7-33


7.3.5.2	 Electricity Consumption .............................................................................. 7-33


7.3.6	 Average Annual Energy Consumption ........................................................ 7-34


7.4 POOL HEATERS .......................................................................................................... 7-35


7.4.1	 Household Sample ....................................................................................... 7-36


7.4.2	 Input Capacity and Heating Load ................................................................ 7-36


7.4.3	 Calculations of Energy Consumption .......................................................... 7-37


7.4.4	 Average Annual Energy Consumption ........................................................ 7-38


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LIST OF TABLES 

Table 7.2.1 Selection of RECS 2001 Records for Water Heaters........................................... 7-2


Table 7.2.2 Assignment of Water Heater Tank Sizes to Sample Households........................ 7-6


Table 7.2.3 Water Heater Tank Sizes Selected for Sample Households ................................ 7-8


Table 7.2.4 Assignment of RE to Gas-Fired Storage Water Heaters.................................... 7-16


Table 7.2.5 Assignment of RE for Electric Water Heaters................................................... 7-16


Table 7.2.6 Distribution of RE for Oil-Fired Water Heaters ................................................ 7-17


Table 7.2.7 Assignment of RE for Gas-Fired Instantaneous Water Heaters ........................ 7-17


Table 7.2.8 Distribution of Input Capacity for Gas-Fired Storage Water Heaters ............... 7-18


Table 7.2.9 Distribution of Input Capacity for Electric Storage Water Heaters................... 7-19


Table 7.2.10 Distribution of Input Capacity for Oil-Fired Water Heaters.............................. 7-19


Table 7.2.11 Distribution of Input Capacity for Gas-Fired Instantaneous Water Heaters...... 7-19


Table 7.2.12 Water Heater Location by Product Class........................................................... 7-23


Table 7.2.13 Annual Energy Consumption for Gas-Fired Storage Water Heaters................. 7-26


Table 7.2.14 Annual Energy Consumption for Electric Storage Water Heaters .................... 7-26


Table 7.2.15 Annual Energy Consumption for Oil-Fired Storage Water Heaters.................. 7-27


Table 7.2.16 Annual Energy Consumption for Gas-Fired Instantaneous Water Heaters ....... 7-27


Table 7.3.1 Summary of Direct Heating Equipment Household Sample (RECS 2001) ...... 7-29


Table 7.3.2 Distribution of Input Capacity for Representative Product Classes of


Direct Heating Equipment ................................................................................. 7-32


Table 7.3.3 Energy Efficiency of Existing Direct Heating Equipment ................................ 7-32


Table 7.3.4 Annual Energy Consumption for Gas Wall Fan Direct Heating Equipment..... 7-34


Table 7.3.5 Annual Energy Consumption for Gas Wall Gravity Direct Heating


Equipment.......................................................................................................... 7-34


Table 7.3.6 Annual Energy Consumption for Gas Floor Direct Heating Equipment........... 7-35


Table 7.3.7 Annual Energy Consumption for Gas Room Direct Heating Equipment.......... 7-35


Table 7.4.1 Criteria for Selecting Household Sample for Pool Heaters (RECS 2001)......... 7-36


Table 7.4.2 Annual Energy Consumption for Gas Pool Heaters .......................................... 7-38


LIST OF FIGURES 

Figure 7.2.1 Hot Water Draw Model Determination of Hot Water Use .................................. 7-5


Figure 7.2.2 Assignment of Water Heater Tank Sizes............................................................. 7-7


Figure 7.2.3 Range of Daily Hot Water Use in Sample Households for Each Water


Heater Product Class.......................................................................................... 7-10


Figure 7.2.4 Range of Annual Outdoor Air Temperature for Sample Households by


Water Heater Product Class............................................................................... 7-20


Figure 7.2.5 Range of Daily Average Annual Inlet Water Temperature for Sample


Households by Water Heater Product Class ...................................................... 7-22


Figure 7.2.6 Range of Annual Ambient Air Temperature in Sample Households by


Water Heater Product Class............................................................................... 7-24


Figure 7.2.7 Calculating Water Heater Energy Use............................................................... 7-25


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CHAPTER 7. ENERGY USE CHARACTERIZATION 

7.1 INTRODUCTION 

The U.S. Department of Energy (DOE) carried out the life-cycle cost (LCC) and payback 

period (PBP) calculations described in chapter 8 by determining the savings in operating cost 

consumers would realize from more energy efficient products. Energy costs are the most 

significant component of consumer operating costs, with maintenance and repair costs the other 

contributors. DOE uses annual energy use, along with energy prices, to establish energy costs at 

various energy efficiency levels. This chapter describes how DOE determined the annual energy 

use of residential gas-fired, electric, and oil-fired storage-type water heaters; gas-fired 

instantaneous water heaters; direct heating equipment; and pool heaters. 

7.2 WATER HEATERS 

The following sections describe factors that determine the amount of hot water and energy 

water heaters use, the characteristics of water heater energy efficiency, and water heater 

operating conditions. 

7.2.1 Introduction 

Water heaters may use gas, oil, or electric energy. In the engineering analysis, DOE used 

the DOE test procedure for each product to calculate the energy consumption of water heaters. 

For the LCC analysis, DOE estimated energy consumption of water heaters in actual housing 

units, as represented by the sample described in section 7.2.2. 

DOE calculated the energy use of residential water heaters in each of the four product 

classes: gas-fired storage water heaters, electric storage water heaters, oil-fired and liquefied 

petroleum gas (LPG) storage water heaters, and gas-fired instantaneous water heaters. The 

calculation considers the primary factors that determine energy use: 

• hot water use per household, 

• the energy efficiency characteristics of the water heater, and 

• water heater operating conditions other than hot water draws. 

DOE used a hot water draw model to determine hot water use for each household in the 

sample. The characteristics of each water heater’s energy efficiency were taken from the 

engineering analysis. DOE developed water heater operating conditions (other than hot water 

draws) from weather data and other relevant sources. Section 7.2.3 discusses hot water use; 

section 7.2.4 discusses the relevant characteristics of water heater energy efficiency and 

operating conditions to calculate the annual energy consumption for water heaters. 

7.2.2 Household Sample 

DOE’s calculation of the annual energy use of residential water heaters relied on data 

1 from the Residential Energy Consumption Survey 2001 (RECS 2001), which was conducted by 

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DOE’s Energy Information Administration (EIA). RECS collects energy-related data for 

occupied primary housing units in the United States. The RECS 2001 collected data from 4,822 

housing units that represent almost 107 million households. For the notice of proposed 

rulemaking stage of this rulemaking, DOE plans to update its analysis using the RECS 2005. 

The subset of RECS 2001 records used to study water heaters met all of the following 

criteria. 

• A water heater served as the primary source of heated water. 

• The water heater used one of four heating fuels (electricity, gas, LPG, or oil). 

• The water heater provided heated water for only one housing unit. 

• The RECS 2001 record included an indication of water heater size. 

• The water heater’s energy consumption was greater than zero. 

Table 7.2.1 Selection of RECS 2001 Records for Water Heaters 

Fuel Type Algorithm No. of 

Records 

No. of U.S. 

Households 

Represented 

(million) 

Natural Gas Water Heater = Yes 

Fuel Type = Natural Gas 

2167 49.4 

Propane Water Heater = Yes 

Fuel Type = Propane 

153 2.8 

Fuel Oil Water Heater = Yes 

Fuel Type = Fuel Oil 

107 1.7 

Electricity Water Heater = Yes 

Fuel Type = Electricity 

1638 36.8 

DOE used EIA’s weightings for each RECS 2001 household. The weightings indicate 

how commonly each household configuration occurs in the general population. DOE believes 

that the household records, along with their weightings, are representative of housing nationwide 

(see appendix 7-A for details). 

7.2.3 Hot Water Use 

Hot water use differs widely among households, because it depends on characteristics of 

the household and the water heater, such as the number and ages of the people who live in the 

household and the way they consume hot water, the presence of hot water-using appliances, the 

tank size and thermostat set point of the water heater, and the climate in which the residence is 

situated. 

2 DOE developed the hot water draw model to improve on a previously used model. The 

hot water draw model accounts for the key characteristics described above to estimate the 

average daily volume of hot water various households use. 

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7.2.3.1 Equation for Hot Water Use 

The hot water draw model uses an equation expressed as follows. 

vol = {sea _ coef + ( per _ coef × per ) + (age 1_ coef × age 1) + age 2 _ ceof × age 2 +

[age 34 _ coef *(age 3 + age 4) + (Ttank _ coef ×Ttank ) + (Tanksz _ coef ×Tanksz ) +

(Tin _ coef ×Tin ) + (Tair _ coef ×Tair ) + (hom e _ coef × athome ) −

[(0.692 × per +1.335 × per ) * dw _ adj −[(1.1688 × per + 4.7737 × per ) × cw _ adj ]}* 

(senior _ mf _ coef × senior _ mf ) × (no _ pay _ coef × no _ pay ) 

Where: 

vol =	 hot water consumption, gallons per day, gal/day, 

per =	 total number of persons in household, 

age1 =	 number of preschool children, age 0–5 years, yr, 

age2 =	 number of school-age children, age 6-13, yr, 

age3 =	 number of adults, age 14–64, yr, 

age4 =	 number of adults, age 65 yr and above, yr, 

Ttank =	 thermostat setting of water heater, degrees Fahrenheit, oF, 

Tanksz =	 nominal tank size of water heater, gal, 

Tin =	 temperature of water heater inlet water, oF, 

Tair =	 outdoor air temperature, oF, 

athome =	 presence of adults at home during day, 

dw_adj =	 adjustment factor to account for the differences between the current 

energy conservation standard for dishwashers and the households that 

have no dishwasher, 

cw_adj =	 adjustment factor to account for the differences between the current 

energy conservation standard for clothes washers and the households that 

have no clothes washer, 

senior_mf =	 senior-only household in a multi-family building, 

no_pay =	 household that does not pay for hot water, 

Ttank_err =	 estimation error for the thermostat set point (normal distribution), 

sea_coef =	 coefficient for seasonal effects, 

per_coef =	 coefficient for total number of persons in household (normal distribution), 

age1_coef =	 coefficient for Aage1" (normal distribution), 

age2_coef =	 coefficient for Aage2" (normal distribution), 

age34_coef =	 coefficient for Aage3" + Aage4" (normal distribution), 

home_coef =	 coefficient for Aathome@ (normal distribution), 

Tanksz_coef =	 coefficient for water heater tank size (normal distribution), 

Ttank_coef =	 coefficient for thermostat set point (normal distribution), 

Tinlet_coef =	 coefficient for water inlet temperature (normal distribution), 

Tair_coef =	 coefficient for average outdoor temperature (normal distribution), 

senior_mf_coef =	 coefficient for senior-only household in a multi-family building (normal 

distribution), and 

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no_pay_coef =	 coefficient for household that does not pay for hot water (normal 

distribution). 

7.2.3.2 Detailed Description of Key Variables Used in Draw Model 

The following is a detailed description of the primary variables used in the hot water 

draw model. 

• Number of Persons in Household (per). The total number of household members. 

• Number of Preschool Children 0–5 (age1). The equation calls for the number of infants 

and young children ages 0–5. However, RECS 2001 bins ages of household members. 

The closest age bin was infants 0–1, so that is what was used. This will underestimate hot 

water usage due to this age category. 

• Number of School-Age Children 6–13 (age2). The equation calls for the number of 

children, ages 6–13. The closest age bin available in RECS 2001 was used, children 1– 

12. Hot water usage due to this category will therefore be somewhat overestimated. 

• Number of Adults 14–64 (age3). The equation calls for the number of adults, ages 14 to 

64. DOE used the closest RECS 2001 bin—ages 13–64. 

• Number of Adults 65+ (age4). The equation calls for number of adults, age 65 or older. 

There is a RECS 2001 age category that corresponds to this. 

• Number of Thermostat Set point (Ttank). The thermostat setting of the water heater. 

• Water Heater Tank Size (Tanksz). The nominal size of the water heater tank. 

• Outdoor Air Temperature (Tair). The average annual outdoor air temperature. 

• Inlet Water Temperature (Tin). The temperature of the water entering the water heater. 

• Household Member (athome). The presence of an adult household member at home 

during the day. 

• Senior Only (senior_mf). A senior-only (age 65 or above) household in a multi-family 

building. 

• No-Pay Household (no_pay). A household that does not pay to heat water. 

• Coefficient for Seasonal Effects (sea_coef). Variable that adjusts hot water use for winter, 

spring, summer, and fall seasons. 

• Dishwasher (dw_adj). Adjustment factor to account for the differences due to the current 

energy conservation standard for dishwasher and for the cases of households with no 

dishwasher. 

• Clothes Washer (cw_adj). Adjustment factor to account for the differences between the 

current energy conservation standard for clothes washers and households that have no 

clothes washers. 

7.2.3.3 Impact on Hot Water Use of Technology Changes in Clothes Washers and 

Dishwashers 

The hot water draw model incorporates parameters that represent clothes washers and 

dishwashers as they existed in the 1990s. Since that time, the considerable changes made to 

clothes washer and dishwasher technologies have lowered hot water use. In particular, updated 

3 Federal energy conservation standards for clothes washers that became effective in 2007 and for 

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4 dishwashers that will become effective in 2010 have a significant impact on hot water use by 

clothes washers and dishwashers. To account for changes in hot water use by clothes washers 

and dishwashers, DOE adjusted the parameters in the hot water draw model. See appendix 7-B, 

hot water draw model, for further discussion of the adjustment factors. 

Water Use Chart. DOE applied the same equation for hot water use to all households, 

regardless of water heater fuel type. Figure 7.2.1 illustrates how the hot water draw model 

determines household hot water use. 

RECS 

Tank Size 

Age0-5 

Weather Data 

Ttank Tin 

Dishwasher Clothes 

washer 

Occupants 

Other hot water using appliances 

Age14-64 Age Age6-13 65+ Total 

Tair 

Temperature Parameters 

Presence 

during the day Senior only Doesn’t pay for water 

Household Characteristics 

Hot Water 

Use 

Figure 7.2.1 Hot Water Draw Model Determination of Hot Water Use 

7.2.3.4 Assignment of Water Heater Tank Size 

Calculating hot water use for each sample household requires assigning the water heater a 

specific tank size (referred to as rated volume). For each household, RECS reports one of three 

water heater tank sizes, small, medium, or large, as well as giving the size range in gallons. Table 

7.2.2 shows RECS data on water heater tank size and the fraction of households associated with 

each size. 

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Table 7.2.2 Assignment of Water Heater Tank Sizes to Sample Households


Fuel 

Type 

RECS Data Standard 

Sizes Used 

in LCC 

Analysis 

(gal) 

Percent of 

Historical 

Shipments* 

(%) 

Percent 

Assigned to 

Sample 

Households 

(%) 

Size 

Class 

Size 

Range 

(gal) 

Percent of 

Sample 

Households 

(%) 

Small ≤30 12 30 17 12 

Natural 45 

Gas and Medium 31-49 61 40 41 

42 LPG 50 40 Large ≥50 27 75 2 2 

Small ≤30 17 30 22 17 

40 33 36 

Electric 

Medium 31-49 52 

50 35 38 

Large ≥50 31 66 3 

75 10 6 

Small ≤30 25 

32 90 90 Oil Medium 31-49 44 

Large ≥50 31 50 10 10 

5 *Sources: Natural gas and LPG: 1988–1995 data from the Gas Appliance Manufacturers Association (GAMA) , 

6 2004–2006 shipments data from the Air Conditioning, Heating, and Refrigeration Institute (AHRI). Electric: U.S. 

7 8 Census Bureau data (2002–2006). Oil: 2001 DOE Water Heater Rulemaking. 

“Typical” water heater sizes, which are those most common for each fuel type, have the 

minimum energy factor allowed by current energy conservation standards. These “typical” units 

have the largest market share in their product class (50 gal/190 liter for electric, 40 gal/150 liter 

for natural gas and LPG, and 32 gal/120 liter for oil). In the LCC analysis, DOE expanded the 

range of sizes considered to include all those listed in Table 7.2.2 so that the analysis would 

accurately reflect the broad range of water heater sizes that consumers may encounter. The sizes 

in the expanded repertoire are referred to as “standard” sizes. 

DOE applied a two-step process to assign water heater sizes to the RECS households: (1) 

for each fuel type, a fraction of the RECS households was assigned to each standard size; and (2) 

water heater sizes were assigned to specific households based on RECS data on the number of 

bathrooms in the house. Figure 7.2.2 illustrates the process. 

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Figure 7.2.2 Assignment of Water Heater Tank Sizes 

Step 1. Assignment of Water Heater Sizes. Table 7.2.3 shows the percentage of each 

standard size that contained in recent shipments of water heaters. (See chapter 5, engineering 

analysis, for a discussion of how DOE chose standard sizes.) The percentages of standard sizes 

in shipments provide insight into the percentages one would expect to find in the housing stock. 

Lastly, the table shows the percentages of RECS households that were assigned to each standard 

size for the LCC analysis. 

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Table 7.2.3 Water Heater Tank Sizes Selected for Sample Households


Fuel Type RECS Size No. of Water Heater Percent 

Class Bathrooms Size (gal) Assigned to 

Size Class 

Small Any # BR 30 100% 

Any # BR 40 74% 

Natural Gas 

or LPG 

Medium Any # BR 50 26% 

2 BR 50 100% 

50 58% 

Large ≥ 3 BR 75 42% 

Small Any # BR 30 100% 

Any # BR 40 69% 

Electric Medium Any # BR 50 31% 

< 2 BR 50 100% 

50 62% 

66 22% 

2 BR 75 16% 

Large ≥ 3 BR 75 100% 

Small Any # BR 32 100% 

Oil Medium Any # BR 32 100% 

Large 

Any # BR 32 69% 

Any # BR 50 31% 

The 1995 GAMA shipments data show that 17 percent of shipments of gas-fired water 

heaters were the 30-gallon size. Historical RECS data1, 9, 10 show that households having a small 

9 1 water heater declined from 20 percent in RECS 1993 to 12 percent in RECS 2001.

Furthermore, only 4 percent of RECS 2001 “new construction households” (households built 

1996–2001) have a small water heater. DOE assigned a 30-gallon water heater to all households 

having a small water heater (12 percent), and estimated that the rest of the shipments (5 percent) 

represented larger water heaters. DOE used the shipment data to assign 75-gallon water heaters 

to 2 percent of sample households. 

DOE assigned the remaining households that have a gas-fired water heater either a 40- or 

6 a 50-gallon water heater. Based on the AHRI shipments data, DOE estimated that 52 percent of 

the that total are 40-gallon, and 48 percent are 50-gallon water heaters. Applying this 52/48 split 

to the remaining RECS 2001 households having medium and large water heaters resulted in 

assigning a 40-gallon size to 45 percent of households and a 50-gallon size to 42 percent. All 

RECS households having large water heater (after subtracting those assigned 75-gallon water 

heaters) were assigned a 50-gallon water heater. All RECS 2001 households having medium 

water heaters were assigned either a 40- or a 50-gallon water heater. 

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U.S. Census shipments data show that 22 percent of shipments of electric water heaters 

were the 30-gallon size or smaller (including sizes smaller than 20 gallons and tabletop models, 

both of which are not subject of this rulemaking). Of RECS 2001 sample households, 16 percent 

have a small water heater. DOE assigned a 30-gallon water heater to all household having small 

water heaters, and estimated that the rest of the shipments (6 percent) consisted of water heaters 

having less than a 20-gallon volume and tabletop water heaters. 

DOE assigned 66- or 75-gallon water heaters to 10 percent of households, based on the 

U.S. Census shipments data. DOE estimated that 70 percent of those water heaters were 75 

gallons, and 30 percent 66 gallon, based on AHRI’s shipments data for 1988–1995. 

DOE assigned the remaining households having electric water heaters either a 40- or a 

50-gallon water heater. Based on U.S. Census shipments data, DOE estimated that 49 percent of 

the total are 40-gallon, and 51 percent are 50-gallon water heaters. Applying this 49/51 split to 

the remaining RECS 2001 households having medium and large water heaters resulted in 

assigning a 40-gallon size to 36 percent and a 50-gallon size to 38 percent. All RECS 2001 

household having large water heaters (after subtracting those assigned 66- and 75-gallon sizes) 

were assigned a 50-gallon water heater, and all RECS 2001 households having medium water 

heaters were assigned either a 40- or a 50-gallon water heater. 

The distribution of sizes for oil-fired water heaters in the shipments data taken from the 

technical support document for DOE’s 2001 water heater rulemaking differs greatly from that in 

the RECS 2001 data. DOE believes the shipments data are likely to be more accurate than the 

RECS 2001 data, because the RECS sample is very small. Based on the shipments data, DOE 

assigned a 32-gallon water heater to 90 percent of the RECS 2001 households and a 50-gallon 

water heater to the remaining 10 percent. DOE assigned a 32-gallon water heater to all RECS 

2001 households having small and medium water heaters, and split the RECS 2001 households 

having large water heater between 32- and 50-gallon sizes. DOE assigned a 50-gallon water 

heater to one-third of the households having a large water heater, and assigned a 32-gallon water 

heater to the remaining two-thirds of households that have a large water heater. 

Step 2. Assignment of Household-Specific Water Heater Sizes. The preceding section 

described how DOE assigned standard water heater sizes to the households in each RECS water 

heater size class. In exploring the correlation between water heater size and several other 

variables, DOE found that the highest correlation with the number of bathrooms. DOE further 

therefore refined the assignment of water heater sizes to specific sample households by 

considering RECS data on the number of bathrooms per house. Table 7.2.3 shows how DOE 

combined the RECS data on water heater size and number of bathrooms to assign household 

specific standard water heater sizes. See appendix 7-B, hot water draw model, for further 

discussion of the assumptions used. 

A few sample houses reported an “unknown” water heater tank size. DOE randomly 

assigned tank sizes to those households based on the percentages assigned to households that 

reported a tank size. 

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7.2.3.5 Determination of Hot Water Use for Instantaneous Water Heaters 

DOE developed the hot water draw model for storage-type water heaters. Instantaneous 

water heaters have no “size” in terms of storage volume, so DOE did not explicitly model their 

hot water use. A feature of instantaneous water heaters is their potential to deliver “unlimited” 

hot water during times of high household demand. However, DOE did not find any usable data 

showing higher or lower hot water use for instantaneous water heaters, so it estimated that a 

household having an instantaneous water heater uses the same volume of hot water as it would 

with a storage-type water heater. 

7.2.3.6 Calculations of Hot Water Use 

DOE calculated average daily hot water use to be 43.0 gallons for households having 

electric storage water heaters, 45.5 gallons for households having gas-fired storage water heaters, 

43.0 gallons for households having oil-fired water heaters, and 45.8 gallons for households 

having gas-fired instantaneous water heaters. The variations primarily reflect differences in the 

average number of people in the households that use each type of fuel. Figure 7.2.3 shows the 

range in hot water use among sample households. 

Hot Water Use (gallons) Ranges by Efficiency Level 

Median; Box 25%-75%; Whisker: 5%-95% 

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

100 

Gas WH Electric WH Oil WH Instantaneous Gas WH 

median 

Average 

Figure 7.2.3	 Range of Daily Hot Water Use in Sample 

Households for Each Water Heater Product Class 

7.2.4 Annual Energy Consumption 

DOE calculated the energy use of water heaters using a simplified energy equation, the 

11 water heater analysis model (WHAM). WHAM accounts for a range of operating conditions 

and energy efficiency characteristics of water heaters. To describe energy efficiency 

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characteristics of water heaters, WHAM uses three parameters that also are used in the DOE test 

12 procedure: recovery efficiency (RE), standby heat-loss coefficient (UA), and rated input power 

(PON). Water heater operating conditions are indicated by the daily hot water draw volume, inlet 

water temperature, thermostat setting, and air temperature around the water heater (ambient air 

temperature). 

The current version of WHAM is appropriate for calculating the energy use of electric 

resistance storage water heaters. To account for the characteristics of other types of water 

heaters, energy use must be calculated using modified versions of the WHAM equation. These 

modified versions are discussed below. 

7.2.4.1 Approach to Calculating Water Heater Energy Use 

DOE utilizes the WHAM equation, which yields average daily water heater energy 

consumption (Qin). The equation is expressed as follows. 

⎛

⎜

⎝

∗ 1− ⎞

⎟

⎠

vol × den CP (Ttank − Tin ) UA (Ttank − Tamb × × × ) Q
in = + 24 ×UA × (Ttank − Tamb ) RE
 Pon


Where: 

Qin = total water heater energy consumption, British thermal units per day 

Btu/day, 

RE = recovery efficiency, %, 

PON = rated input power, Btu/h, 

UA = standby heat-loss coefficient, Btu/h-o

F, 

Ttank = thermostat set point temperature, oF, 

Tin = inlet water temperature, oF, 

Tamb = temperature of the ambient air, o

F, 

vol = volume of hot water drawn in 24 hours, gal/day, 

den = density of stored water, set constant at 8.29 pounds per gallon, lb/gal, and 

CP = specific heat of stored water, set constant at 1.000743, Btu/lb-oF. 

Additional details about the WHAM equation are provided in appendix 7-C, LCC energy 

use calculations. 

WHAM provides total water heater energy consumption (Qin). For gas-fired storage 

water heaters, gas-fired instantaneous water heaters, and oil-fired storage water heaters, the 

values for electricity and fuel consumption must be disaggregated. Qin is the sum of fuel and 

electricity consumption. DOE calculated electricity consumption as follows. 

in Qelectricity = Q × (Paux − Pstandby )− Pstandby × 24 

PON 

Where: 

7-11 

EE-2006-STD-0129 and EERE-2006-BT-STD-0129 COMMENT 32.2
Qelectricity = electricity consumption, kWh/day,


Qin = total water heater energy consumption, kWh/day,


PON = rated input power, kW,


Paux = electricity demand when burner is on, kW, and


Pstandby = electricity demand when burner is off, kW.


DOE calculated gas consumption by subtracting electricity consumption from the total 

energy consumption for the water heater (Qin). 

For heat pump water heaters, energy efficiency and consumption are dependent on 

ambient temperature. To account for this factor, DOE expanded the WHAM to utilize the 

calculation approach used in the New York State Energy Research and Development Authority 

(NYSERDA) heat pump water heater site screening tool and DOE’s weatherization assistance 

program. 13, 14 The equation for determining the energy consumption of heat pump water heaters 

is similar to WHAM equation, but a performance adjustment factor that is a function of the 

average ambient temperature is applied to adjust RE. A heat pump water heater operates either in 

heat pump or in electric resistance mode. DOE assumed that the electric resistance mode of 

operation is used 100 percent of the time when the monthly ambient temperature is less than 32 

o

F. A heat pump water heater also operates in the electric resistance mode for part of the time 

even when the monthly ambient temperature exceeds 32 oF, because this product has a slower 

recovery rate than an electric resistance water heater. DOE determined that, depending on 

household hot water consumption patterns, the electric resistance mode of operation occurs for as 

much as 10 percent of the unit’s operating time. (See appendix 7-C for a full description of the 

method). The equation is: 

⎡ HP frac
 ⎛ UA ⎞ ⎤ (Ttank − Tamb × ) ⎢

⎢

⎢

⎢

⎢

_ 

RE × PA HPWH 

(1 − HP _ frac ) 

⎥

⎥

⎥

⎥

⎥

*
 1 − +

P
ON ,HPWH 

⎜

⎜

⎝

⎟

⎟

hpwh ⎠ (T in Q vol den C
 = × × − T
 )*
 + 24
×UA
× (T − T
 ) in P tank tank amb ⎛

⎜

⎜

⎞

⎟

⎟

UA (Ttank − Tamb × ) *
 1 − RE
 P
 ⎣ ⎝ ON ,ER ⎠ ⎦ ER 

Where: 

Qin = total water heater energy consumption, Btu/day, 

vol = daily draw volume, gal/day, 

den = density of water, lb/gal, 

Cp = specific heat of water, Btu/lb-o

F, 

Ttank = set point temperature for tank thermostat, oF, 

Tin = temperature of inlet water, o

F, 

RE = recovery efficiency, %, 

UA = stand-by heat-loss coefficient, Btu/h-o

F, 

Tamb = temperature of the ambient air, o

F, 

PON = rated input power, Btu/h, 

PAhwh = performance adjustment factor, and 

7-12 

EE-2006-STD-0129 and EERE-2006-BT-STD-0129 COMMENT 32.2
HP_frac = fraction of water heating load that is satisfied by heat pump mode. 

Heat pump water heaters draw heat from the space in which they are located. Thus, when 

such a water heater is located in a conditioned space, its use affects the heat load that the home’s 

space heating and air conditioning equipment must meet. When the home is being heated, use of 

the heat pump water heater increases the heating load, and when the house is being cooled, its 

use decreases the cooling load. 

To account for the indirect effects of heat pump water heaters on home energy use, DOE 

estimated the impact on space heating and air conditioning energy consumption for each home in 

the RECS subsample for electric water heaters that has the water heater in the conditioned space. 

For each such home, DOE estimated the impact on space heating in heating months only (when 

indoor temperature is 10 degrees greater than the average outdoor temperature), and the impact 

on air conditioning for cooling months only (when indoor temperature is 5 degrees less than the 

average outdoor temperature). DOE calculated the amount of cooling added (heat removed) by 

the heat pump water heater as follows: 

CoolingInput = CoolingCap HPWH ×

vol × den ×Cp (Ttan k − Tin )

P × RE × PA ON ,HPWH HPWH HPWH 

Where:


CoolingInput = amount of cooling added by the heat pump water heater, Btu/h,


vol = daily draw volume, gal/day,


den = density of water, lb/gal,


Cp = specific heat of water, Btu/lb-oF,


Ttank = set point of tank thermostat, oF,


Tin = indoor air temperature, oF,


PON,HPWH = rated input power, Btu/h,


RE = recovery efficiency, %,


PAHPWH = performance adjustment factor, and


CoolingCapHPWH = cooling capacity of heat pump water heater.


14 PON for heat pump water heaters is 500 watts (W) (1,704 Btu/h), and 

13 Cooling_Cap_hpwh is 3,500 Btu/h. DOE used RECS data for each household to determine the 

primary heating system, fuel used to heat the house, and the type of cooling system. DOE then 

assigned an energy efficiency to each type of equipment to determine the energy loss (from 

heating) or gain (from cooling). DOE based the energy efficiency of the heating and air 

conditioning equipment on the stock-average values for 2015 given in EIA’s Annual Energy 

Outlook 2008. To account for the times during the summer months when the air conditioner is 

7-13 

EE-2006-STD-0129 and EERE-2006-BT-STD-0129 COMMENT 32.2
not operating, DOE reduced the estimated cooling benefits from the water heater by one-third.a 

See appendix 7-C for further details. 

For instantaneous water heaters, DOE utilized the approach used for storage water 

heaters to calculate the energy use, modified to account for the absence of storage tank. DOE 

applied a performance adjustment factor to account for evidence that the rated energy efficiency 

of instantaneous water heaters overstates actual performance. Preliminary results from a study of 

instantaneous water heater installations conducted for the California Energy Commission (CEC) 

15 indicate a performance drop-off at small draw volumes. To account for this phenomenon, the 

study performed for the CEC recommends reducing the listed Energy Factor (EF) for gas-fired 

instantaneous water heaters by 8.8 percent. Thus a gas-fired instantaneous water heater rated at 

0.82 EF would be treated as if rated at 0.748 EF. (Note that the study does not suggest that the 

CEC’s directory rate the instantaneous water heater at the lower EF). To incorporate this 

recommendation, DOE applied an adjustment factor by that effectively increases 8.8 percent the 

calculated energy use of a gas-fired instantaneous water heater at each EF. The resulting 

equation is: 

⎛

⎜

⎜

⎝

Where: 

Qin = total water heater energy consumption, Btu/day,


vol = daily draw volume, gal/day,


den = density of water, lb/gal,


Cp = specific heat of water, Btu/lb-oF,


Ttank = set point of tank thermostat, oF,


Tin = inlet water temperature, oF,


RE = recovery efficiency, %,


PAiwh = performance adjustment factor,


Qp = pilot input rate, Btu/h,


Qout = heat content of water drawn from the water heater, Btu/h, and


PON = rated input power, Btu/h.


7.2.4.2 Description of Key Variables 

The following is a description of the key variables for calculating energy use by gas-fired 

instantaneous water heaters: 

a DOE believes it would be impractical for the occupants of the few households that use wood, coal, or kerosene as 

primary heating fuels to supplement heat produced by heat pump water heaters. Because DOE would find it difficult 

to quantify energy costs, DOE assumed that occupants would use an electric resistance heater to satisfy the 

additional heating requirements produced by the heat pump water heater. 

⎞

⎟

⎠

⎟ + (T in ) Q


P
ON 

vol den C
 × × P k − T
 p (T amb ) tan Q
 = × 1− 24 ×Q
 × − T
 in p Tank RE PA × hpwh 

7-14 

EE-2006-STD-0129 and EERE-2006-BT-STD-0129 COMMENT 32.2
• Recovery Efficiency (RE). The recovery efficiency (RE) is the ratio of energy added to 

the water compared to the energy input to the water heater. It represents how efficiently 

energy is transferred to the water when the heating element is on or the burner is firing. 

RE covers steady-state energy efficiency only. It accounts for the amount of energy lost 

through the water heater jacket, the flue, and fittings while the burner is firing. 

• Rated Input Power (PON). Rated input power is the nominal power rating the 

manufacturer assigns to a particular design of water heater expressed in kW for electric 

water heaters and Btu/h for gas-fired or oil-fired water heaters. For gas-fired water 

heaters the rated input power includes the pilot light. 

• Standby Heat-Loss Coefficient (UA). The standby heat loss coefficient (UA) indicates 

the water heater hourly standby energy losses, expressed in Btu/h-oF. UA, which is 

calculated by the DOE test procedure, is reported in terms of the energy input required to 

maintain the water at the set point temperature. It represents the rate at which energy 

must be added to the water heater when it is not heating water for delivery. 

• Set Point of Thermostat (Ttank). The thermostat set point is the desired delivery 

temperature of the hot water. 

• Inlet Water Temperature (Tin). The inlet water temperature is the temperature of the 

water supplied to the water heater. 

• Temperature of the Air Surrounding the Water Heater (Tamb). The temperature 

surrounding the water heater is the ambient air temperature of the space where the water 

heater is located. 

• Volume of Hot Water Drawn in 24-Hour Period (vol). The estimated daily household 

use of hot water. This volume is calculated in the hot water draw model. 

• Density of Water (den). The density of hot water at the average of the set point and inlet 

temperatures (8.29 lb/gal). The density is mass per unit volume, expressed as lb/gal 

(kg/l). 

• Specific Heat of Water (Cp). The specific heat of water at the average of the set point and 

inlet temperatures (1.000743 Btu/lb-oF). The specific heat is the amount of heat needed to 

increase or decrease the temperature of 1 pound mass of water by 1 °F (1 kJ/kg - Kelvin). 

7.2.4.3 Derivation of Energy Parameters 

Key parameters in DOE’s calculation of water heater energy consumption are the 

recovery efficiency (RE), the standby heat loss coefficient (UA), and the rated input power (Pon). 

16 DOE’s test procedure for water heaters provided the definitions for these parameters. DOE 

developed the parameters for selected EF as described below. (See chapter 5, engineering 

analysis, for a discussion of DOE’s selection of EFs.) 

7-15 

EE-2006-STD-0129 and EERE-2006-BT-STD-0129 COMMENT 32.2
Determining RE. For each energy efficiency level used in the LCC analysis, DOE 

determined typical values of RE. For gas-fired and oil-fired storage water heaters, those values 

were based on the design features at each energy efficiency level and the distribution of models 

17 at each EF, as given in the March 2007 GAMA Directory. This approach captures the variation 

of RE among models in the market. 

Table 7.2.4 shows the RE values for gas-fired storage water heaters having a 40-gallon 

rated volume. The same RE values apply to the other three size categories (30, 50, and 75 

gallons), because the design characteristics at each energy efficiency level are the same across 

the size categories. 

Table 7.2.4 Assignment of RE to Gas-Fired Storage Water Heaters 

Efficiency 

Level 

EF (40 

gallon) 

RE (%) 

0 0.59 76 

1 0.62 76 

2 0.63 76 

3 0.64 78 

4 0.65 78 

5 0.66 82 

6 0.77 89 

DOE estimated that the recovery efficiency for electric water heaters having immersed 

heating elements is 98 percent for most energy efficiency levels (Table 7.2.5). For heat pump 

water heaters, DOE calculated RE by using the UA value reported for the heat pump water 

14 heater manufactured by ECR.

Table 7.2.5 Assignment of RE for Electric Water Heaters 

Efficiency 

Level 

EF (50 

gallon) 

RE (%) 

0 0.90 98 

1 0.91 98 

2 0.92 98 

3 0.93 98 

4 0.94 98 

5 0.95 98 

6 2.20 242 

The RE values for 32-gallon size oil-fired water heaters also apply to the 50-gallon size 

(Table 7.2.6). 

7-16 

EE-2006-STD-0129 and EERE-2006-BT-STD-0129 COMMENT 32.2
1

2

3

4

5

6

7

1

2

3

4

5

6

7

Table 7.2.6 Distribution of RE for Oil-Fired Water Heaters


Efficiency 

Level 

EF (32 

gallon) RE (%) 

0 0.53 76 

0.54 76 

0.56 76 

0.58 76 

0.60 76 

0.62 78 

0.66 80 

0.68 82 

DOE based RE values for instantaneous water heaters at each energy efficiency level on 

the design features at each level and on the distribution of models at each EF, as given in the 

18 Federal Trade Commission (FTC) 2007 directory. This approach captures the variability of RE 

among the models in the market. The same RE values apply to all input capacities, because the 

design characteristics at each energy efficiency level are the same across the size categories. The 

RE for each energy efficiency level is shown in Table 7.2.7. 

Table 7.2.7 Assignment of RE for Gas-Fired Instantaneous Water Heaters 

Efficiency 

Level 

EF RE (%) 

0 0.62 72 

0.69 78 

0.78 78 

0.80 80 

0.82 82 

0.84 84 

0.85 85 

0.92 93 

Determining UA. DOE used the following equation to calculate values for stand-by heat 

loss coefficient (UA). 

⎛

⎜

⎝

⎞

⎟

⎠

1 1 − EF RE UA = ⎛ 24 1
 ⎞ (T k − Tamb ) − Qout RE × Pon 

× ⎜

⎜

⎝

⎟

⎟

⎠ tan 

Where: 

UA = standby heat loss coefficient, Btu/h-oF, 

EF = energy factor, 

RE = recovery efficiency; %. 

7-17 

EE-2006-STD-0129 and EERE-2006-BT-STD-0129 COMMENT 32.2
Ttank = temperature of the air surrounding the water heater, o

F;


Tamb = thermostat set point temperature, oF;


Qout = heat content of the water drawn from the water heater, Btu/h, and


PON = the rated input power, Btu/h.


Appendix 7-C, LCC energy use calculations, describes the derivation of the above 

equation. For calculating UA, DOE used values for tank and ambient temperatures from the DOE 

test procedure. 

For heat pump water heaters, DOE estimated UA equals that of a 50-gallon electric 

storage water heater having a 0.95 EF. The additional heat losses due to the plumbing 

connections between the storage tank and the heat pump components are offset by the 

significantly higher RE of the heat pump water heater. 

Determining PON. For each category of rated volume within each product class, DOE 

determined appropriate bins for rated input power (PON) based on the models listed in the March 

17 2007 GAMA Directory, then calculated the percentage of models in each bin. This approach 

captures the variability of input capacity among the products models in the market. The 

percentage of gas-fired water heaters in each volume category is shown in Table 7.2.8. 

Table 7.2.8 Distribution of Input Capacity for Gas-Fired Storage Water Heaters 

Input Capacity Rated Volume (gallons) 

(PON) 

(kBtu/h) 

30-gal 

(%) 

40-gal 

(%) 

50-gal 

(%) 

75-gal 

(%) 

26 3.59 0.00 0.00 0.00 

30 58.08 5.92 1.45 0.00 

35 23.35 37.06 15.55 0.00 

40 14.97 48.90 47.02 0.00 

50 0.00 8.12 17.72 1.37 

60 0.00 0.00 6.33 9.59 

65 0.00 0.00 11.93 9.59 

70 0.00 0.00 0.00 54.79 

75 0.00 0.00 0.00 24.66 

The percentage of electric water heaters in each volume category are shown in Table 

19 7.2.9. For heat pump water heaters, the PON of 0.5 kW was based on ECR’s Watt Saver model.

7-18 

EE-2006-STD-0129 and EERE-2006-BT-STD-0129 COMMENT 32.2
Table 7.2.9 Distribution of Input Capacity for Electric Storage Water Heaters


Input Capacity Rated Volume (gallons) 

(PON) 

(kW) 

30-gal 

(%) 

40-gal 

(%) 

50-gal 

(%) 

66-gal 

(%) 

75-gal 

(%) 

3.0 5.74 0.54 0.97 0.00 2.06 

3.8 27.05 22.04 16.91 10.67 12.37 

4.5 65.57 74.73 77.78 86.67 79.38 

5.5 1.64 2.69 4.35 2.67 6.19 

Table 7.2.10 shows the percentage of oil-fired water heaters in each volume category. 

Table 7.2.10 Distribution of Input Capacity for Oil-Fired Water Heaters 

Input Capacity Rated Volume (gal) 

(PON) 

(kBtu/h) 

32-gal 

(%) 

50-gal 

(%) 

90 50.0 0.0 

105 50.0 100.0 

For gas-fired instantaneous water heaters, which have no rated volume, DOE estimated 

the percentage of each of the three input capacities using RECS data on the size of gas-fired 

storage water heaters in households. To households having a small tank (<=30 gallon) DOE 

assigned a gas-fired instantaneous water heater having 140 kBbtu/h input capacity. To 

households having a large tank (>=50 gallon) tank DOE assigned a gas-fired instantaneous water 

heater having 199 kBtu/h input capacity. To households having a medium tank (31 to 49 

gallons), DOE assigned some a gas-fired instantaneous water heater having 180 kBtu/h input 

capacity. DOE assigned the rest of the households a gas-fired instantaneous water heater having 

199 kBtu/h input capacity (see Table 7.2.11). 

Table 7.2.11 Distribution of Input Capacity for Gas-Fired Instantaneous Water Heaters 

Range of Input 

Capacity 

(kBtu/h)* 

Assigned Input 

Capacity 

(kBtu/h) 

Percent Assigned to 

Sample Households 

(%) 

140-169 140 11 

170-185 180 16 

186-199 199 73 

* DOE did not include units having a capacity <140 kBtu/h, because such units are considered unsuitable for whole20 house water heating. 

7.2.4.4 Derivation of Temperatures 

The temperatures for thermostat set point, inlet water, and the ambient air are derived 

from the average annual outdoor air temperature for each sample household. 

Outdoor Air Temperature. RECS 2001 provides data on heating and cooling degreedays, but not on air temperatures for each household in the sample. To each RECS 2001 

7-19 

EE-2006-STD-0129 and EERE-2006-BT-STD-0129 COMMENT 32.2
household DOE assigned a physical location from which outdoor air temperatures could 

be derived as follows: 

• DOE assembled weather data from 282 weather stations that provide 30-year averages for 

21, 22 annual average outdoor air temperatures. DOE also gathered the heating and cooling 

degree-days at a base temperature of 65 oF for 2004 for those weather stations.23 The 

2004 heating and cooling degree-days match the period used to determine the degreedays in RECS 2001. 

• RECS 2001 reports both heating and cooling degree-days to base temperature 65 oF for 

each housing record. DOE assigned each RECS 2001 household to one of the 282 

weather stations by calculating which station (within the appropriate census region or 

large state) gave the best linear least-squares fit of RECS 2001 data to weather data. 

Details about the derivation of the annual average outdoor air temperature for the RECS 

2001 water heater sample are provided in appendix 7-D, weather data and temperature 

parameters for LCC analysis. DOE’s analysis produced average outdoor temperatures of 59.1 oF 

for electric storage water heaters, 56.3 o

F for gas-fired storage water heaters and gas-fired 

instantaneous water heaters, and 52.4 oF for oil-fired water heaters. Figure 7.2.4 shows the range 

of average annual outdoor air temperatures among sample households. 

Average Annual Outdoor Temperature (° F) Ranges by Efficiency Level 

Median; Box 25%-75%; W hisker: 5%-95% 

40 

45 

50 

55 

60 

65 

70 

75 

80 

Gas WH Electric WH Oil WH Instantaneous Gas WH 

median 

Average 

Figure 7.2.4	 Range of Annual Outdoor Air Temperature for 

Sample Households by Water Heater Product Class 

Inlet Water Temperature. Daily hot water and water heater energy use depend on 

average inlet water temperature. The inlet water comes to the water heater either from a 

municipal treatment plant or from ground well sources. DOE determined the water source using 

7-20 

EE-2006-STD-0129 and EERE-2006-BT-STD-0129 COMMENT 32.2
RECS 2001, which enables one to identify the households in the sample that have a water well. 

Households that don’t have a water well presumably use water from municipal treatment plants. 

DOE derived inlet water temperature using an approach developed by the National 

Renewable Energy Laboratory.24, 25 This approach accounts for seasonal variations in inlet water 

temperature as a function of annual average outdoor air temperature. The monthly average inlet 

water temperature varies directly with the average annual outdoor air temperature corrected by 

an offset term. The equation for inlet water temperature has the following form: 

TIN = Tair ,avg + offset + lag 

The calculation details and the parameter definitions are described in appendix 7-D. 

26 Institute to calculate the offset for the households that have water wells. This method suggests 

adding two degrees to the average annual outdoor air temperature to calculate the inlet water 

temperature for ground water sources. The offset for households using municipal treatment 

plants was calculated using typical average long-term water temperatures for households having 

26 water that comes from municipal treatment plants. The derivation of those values is described 

in appendix 7-D. 

The analysis identified an average inlet water temperature of 62.6 o

F for electric storage 

water heaters, 59.9 oF for gas-fired storage water heaters and gas-fired instantaneous water 

heaters, and 55.7 oF for oil-fired water heaters. Figure 7.2.5 shows the range of inlet water 

temperatures among sample households. 

7-21 

EE-2006-STD-0129 and EERE-2006-BT-STD-0129 COMMENT 32.2
Average Annual Inlet Water Temperature (° F) Ranges by Efficiency Level 

Median; Box 25%-75%; Whisker: 5%-95% 

40 

45 

50 

55 

60 

65 

70 

75 

80 

85 

Gas WH Electric WH Oil WH Instantaneous Gas 

WH 

median 

Average 

Figure 7.2.5 Range of Daily Average Annual Inlet Water 

Temperature for Sample Households by Water 

Heater Product Class 

Water Heater Thermostat Settings. DOE assigned water heater thermostat settings to 

RECS 2001 households based on a 2006 survey commissioned by Plumbing & Mechanical 

27 magazine. The information about thermostat settings reflects the results from a survey of 343 

plumbing/hydronic heating contractor firms that install water heaters throughout the United 

States in new and replacement markets. 

The magazine survey indicated that 41 percent of responding contractors always install a 

water heater with a set point temperature of 120 o

F; 20 percent always install the water heater 

with a set point temperature higher than 120 oF; and the other 39 percent that they usually install 

the water heater with a thermostat at 120 o

F. Based on this information, DOE estimated that half 

of the latter water heater installations have a set point temperature of 120 o

F, leading to a total of 

61 percent of water heaters set to 120 oF, with 39 percent uniformly distributed between 121 oF 

and 140 oF.b This approach resulted in a mean temperature set point of 124.2 oF for the RECS 

2001 water heater household sample. 

Although water heaters are shipped having the thermostat set to 120 °F, several factors 

may cause contractors and/or household occupants to increase the set-point temperature, such as: 

• High hot water draws: Increasing the set point temperature decreases the likelihood of 

running out of hot water. 

b 140 o

F is the maximum allowed to avoid scalding. 

7-22 

EE-2006-STD-0129 and EERE-2006-BT-STD-0129 COMMENT 32.2
• Cold inlet water: Increasing the set point temperature can help compensate for the 

mixture produced by very cold water and hot water. 

• Small water heaters: A smaller water heater might be set to a higher set-point temperature 

to be more responsive to hot water needs. 

Ambient Air Temperature. Ambient air temperature (the temperature of the air 

surrounding the water heater), which is used to calculate heat losses, differs based on where the 

water heater is installed. Water heaters most commonly are installed in garages, in basements, in 

crawlspaces, or inside the house.8, 13 For single-family houses, RECS 2001 reports whether the 

house has a basement and whether the basement is heated or unheated. It also reports the 

presence or absence of a garage or crawlspace. For houses that have a basement only, DOE 

assumed that the water heater was located in the basement. For houses that have a basement and 

garage, DOE estimated that half the water heaters are in the basement and half in the garage. For 

houses that have a garage only, DOE assumed that the water heater was in the garage. For houses 

that have a crawlspace only, DOE assumed that half the water heaters were in the crawlspace and 

half inside the house. In the absence of a basement, garage, or crawlspace, DOE assumed that the 

water heater was inside the house (in a laundry room, kitchen, or utility closet). DOE further 

assumed that 50 percent of manufactured homes and 25 percent of multi-family houses in 

climates where the heating degree days are less than 4,000 would have the water heater installed 

in an outdoor closet. The rest of the manufactured houses and multi-family houses were assumed 

to have the water heater installed indoors. 

DOE assumed that oil-fired water heaters were not installed indoors. Therefore, if the 

above approach designated an oil-fired water heater as indoors, the water heater was assumed to 

be in a space similar to a garage or crawlspace. Table 7.2.12 shows the percentages of the 

various water heater locations as well as the ambient air temperature derivation for each 

installation location. 

Table 7.2.12 Water Heater Location by Product Class 

Location Temperature Derivation GasFired (%) 

Electric 

(%) 

Oil-Fired 

(%) 

Heated basement Avg. Indoor Temperature 24.3 13.6 53.2 

Unheated Basement (Avg. Indoor Temp + Avg. 

Outdoor Temp) / 2 

8.5 8.1 21.5 

Garage, Crawlspace, 

or Outdoor closet 

Avg. outdoor Temp + 5 °F 36.9 37.0 25.3 

Inside house Avg. Indoor Temperature 30.3 41.3 0.0 

Total 100 100 100 

7-23 

EE-2006-STD-0129 and EERE-2006-BT-STD-0129 COMMENT 32.2
DOE determined ambient air temperatures based on information about typical locations 

of water heaters as follows: 

• For houses having heated basements, DOE assumed the ambient temperature equaled the 

home’s average house indoor temperature given by RECS 2001. 

• For houses having unheated basements, DOE calculated the ambient temperature as the 

mean of the average house outdoor and indoor temperatures. 

• For houses having a garage, crawlspace, or outside closets, DOE assigned an ambient 

temperature that was 5 oF (2.8 oCelsius) higher than the outdoor air temperature for that 

8 location.

• For inside-the-house installations, DOE assumed the ambient temperature equaled the 

home’s average house indoor temperature given by RECS 2001. 

This analysis identified in average annual ambient temperatures of 68.7 o

F for electric 

storage water heaters, 67.0 o

F for gas-fired storage water heaters and gas-fired instantaneous 

water heaters, and 64.5 oF for oil-fired water heaters. Figure 7.2.6 shows the range of annual 

ambient air temperature among sample households. 

Average Annual Ambient Air Temperature (° F) Ranges by Efficiency Level 

Median; Box 25%-75%; W hisker: 5%-95% 

50 

55 

60 

65 

70 

75 

80 

Gas WH Electric WH Oil WH Instantaneous Gas 

WH 

median 

Average 

Figure 7.2.6	 Range of Annual Ambient Air Temperature in 

Sample Households by Water Heater Product Class 

7.2.4.5 Approach to Calculating Water Heater Energy Consumption 

Figure 7.2.7 provides an overview of DOE’s approach to calculating water heater energy 

use for each sample household. 

7-24 

EE-2006-STD-0129 and EERE-2006-BT-STD-0129 COMMENT 32.2
RECS Sample 

Energy Use 

Draw Model Water Use WHAM 

Tamb 

Engineering 

Analysis 

Contractor 

Survey Weather Data 

Ttank Tin RE 

UA 

EF 

Operating Conditions Efficiency 

Characteristics 

Figure 7.2.7 Calculating Water Heater Energy Use 

7.2.5 Average Annual Energy Consumption 

The following sections describe the derivation of average annual energy use and the 

average energy savings for each considered energy efficiency level compared to the baseline 

energy efficiency for gas-fired storage, electric storage, oil-fired storage, and gas-fired 

instantaneous water heaters. 

7.2.5.1 Gas-Fired Storage Water Heaters 

Table 7.2.13 lists the average annual energy use for gas-fired storage water heaters and 

the average energy savings for each considered energy efficiency level compared to the 

associated baseline water heater. The energy savings include the higher use of electricity at an 

EF of 0.64 and above. 

7-25 

EE-2006-STD-0129 and EERE-2006-BT-STD-0129 COMMENT 32.2
Table 7.2.13 Annual Energy Consumption for Gas-Fired Storage Water Heaters


Energy 

Efficiency Level 

Average Energy 

Consumption 

Average Energy 

Savings 

(EF) Gas (MMBtu/yr) 

(MMBtu/yr) 

Electricity 

(kWh/yr) 

0.59 (baseline) 16.7 0 --

0.62 15.6 0 1.1 

0.63 15.3 0 1.4 

0.64 14.8 69 1.8 

0.65 14.5 68 2.2 

0.66 14.2 67 2.5 

0.77 12.1 61 4.6 

7.2.5.2 Electric Storage Water Heaters 

Table 7.2.14 lists average annual energy use for electric storage water heaters and 

average energy savings for each energy efficiency level evaluated in the LCC analysis, compared 

to the baseline water heater. 

Table 7.2.14 Annual Energy Consumption for Electric Storage Water Heaters 

Energy 

Efficiency Level 

(EF) 

Average Energy 

Consumption 

(kWh/yr) 

Average Energy 

Savings 

(kWh/yr) 

0.90 (baseline) 2,703 -

0.91 2,657 46 

0.92 2,612 91 

0.93 2,568 135 

0.94 2,525 177 

0.95 2,483 220 

2.20 1,342 1,361 

7.2.5.3 Oil-Fired Storage Water Heaters 

Table 7.2.15 shows the average annual energy use for oil-fired water heaters and the 

average energy savings for each energy efficiency level compared to the baseline. The energy 

savings include the lower use of electricity at an EF of 0.54 and above. 

7-26 

EE-2006-STD-0129 and EERE-2006-BT-STD-0129 COMMENT 32.2
Table 7.2.15 Annual Energy Consumption for Oil-Fired Storage Water Heaters


Energy 

Efficiency Level 

Average Energy 

Consumption 

Average 

Energy Savings 

(EF) Fuel Oil (MMBtu/yr) 

(MMBtu/yr) 

Electricity 

(kWh/yr) 

0.53 (baseline) 19.4 56 --

0.54 18.9 55 0.5 

0.56 18.0 53 1.4 

0.58 17.2 50 2.2 

0.60 16.4 48 2.9 

0.62 15.7 46 3.7 

0.66 14.7 43 4.7 

0.68 14.2 41 5.2 

7.2.5.4 Gas-Fired Instantaneous Water Heaters 

Table 7.2.16 lists the average annual energy use for gas-fired instantaneous water heaters 

and the average energy savings for each energy efficiency level compared to the baseline water 

heater. 

Table 7.2.16 Annual Energy Consumption for Gas-Fired Instantaneous Water Heaters 

Energy 

Efficiency Level 

Average Energy 

Consumption 

Average 

Energy Savings 

(EF) Gas (MMBtu/yr) 

(MMBtu/yr) 

Electricity 

(kWh/yr) 

0.62 (baseline) 17.0 0 --

0.69 16.0 0 1.0 

0.78 12.5 26 4.5 

0.80 12.2 29 4.8 

0.82 11.9 29 5.1 

0.84 11.6 29 5.4 

0.85 11.5 29 5.6 

0.92 10.6 29 6.4 

7.3 DIRECT HEATING EQUIPMENT 

The following sections describe DOE’s analysis of energy consumption for direct heating 

equipment. 

7.3.1 Introduction 

Direct heating equipment utilizes both gas and electric energy. In the engineering 

analysis, DOE calculated energy consumption of baseline direct heating equipment using the 

7-27 

EE-2006-STD-0129 and EERE-2006-BT-STD-0129 COMMENT 32.2
DOE test procedure for those products. For the LCC analysis, DOE estimated energy 

consumption of direct heating equipment in actual housing units. 

To represent actual households likely to purchase and use direct heating equipment, DOE 

1 developed a household sample from RECS 2001. For each household in the sample, DOE used 

RECS 2001 reported heating energy consumption (based on the existing heating system) to 

calculate the heating load of each household. The heating load represents the amount of heating 

required to keep a housing unit comfortable throughout an average year. DOE estimated the 

heating loads from the reported heating energy consumption and the estimated energy efficiency 

of the direct heating equipment in each sample housing unit. DOE assigned the energy efficiency 

of existing systems based on the distribution of energy efficiencies for direct heating equipment 

17 provided in the 2007 GAMA directory. The estimation of heating loads also required 

calculating the electricity consumption of the blower (when applicable), because heat from the 

blower contributes to heating the housing unit. To complete the analysis, DOE calculated the 

energy consumption of alternative (more energy efficient) products if they replaced existing 

systems in each housing unit. 

7.3.2	 Household Sample 

The subset of RECS 2001 households used for evaluating the energy use of direct heating 

equipment met all of the following criteria: 

• The household employed a pipeless floor or wall furnace, fireplace, or heater as the 

primary or secondary source of heat. 

• The heating fuel was natural gas or LPG. 

• The equipment heated only one housing unit. 

• Primary fireplace equipment had a thermostat or secondary fireplace equipment had a 

flue vent. 

• The equipment had energy consumption greater than zero. 

The subset of household using direct heating equipment represents 578 housing records, 

representing 12.3 million households (11.5 percent of the total weighted RECS sample). Table 

7.3.1 lists the criteria used to develop the household sample and shows the number of households 

included in the analysis. Appendix 7-A, RECS 2001 values and variables, describes the subset of 

households. 

7-28 

EE-2006-STD-0129 and EERE-2006-BT-STD-0129 COMMENT 32.2
Table 7.3.1 Summary of Direct Heating Equipment Household Sample (RECS 2001)


Category Selection Criteria Fuel Type No. of 

Records 

No. of US 

Households 

Represented 

(million) 

Primary Equipment 

Floor/Wall 

Furnace 

Primary Equipment Type = Floor/Wall 

Pipeless Furnace 

Number of Housing Units Heated = 1 

Natural Gas 134 3.1 

LPG 11 0.2 

Room Heater Primary Equipment Type = Heater 

Number of Housing Units Heated = 1 

Natural Gas 136 2.8 

LPG 53 1.1 

Fireplace Primary Equipment Type = Fireplace 

Number of Housing Units Heated = 1 

Fireplace has Thermostat = Yes 

Natural Gas 5 0.1 

LPG 3 0.0 

Secondary Equipment 

Floor/Wall 

Furnace 

Secondary Equipment Type = Floor/Wall 

Furnace 

Natural Gas 7 0.1 

LPG 9 0.1 

Room Heater Secondary Equipment Type = Heater Natural Gas 47 0.9 

LPG 46 0.9 

Fireplace Secondary Equipment Type = Fireplace 

Fireplace is vented = Yes 

Natural Gas 106 2.6 

LPG 21 0.4 

Each RECS heating equipment category is matched to the direct heating equipment 

product classes using several assumptions. For gas wall fan direct heating equipment, DOE used 

a household sample that includes the RECS 2001 categories of floor/wall furnace and fireplaces. 

17 Based on the percentage of fireplace models in the GAMA directory and GAMA’s shipments 

28 data, DOE sampled 80 percent from the RECS 2001 floor/wall furnace category and 20 percent 

from the fireplace category. For gas wall gravity direct heating equipment, DOE used a 

household sample that includes only the RECS 2001 floor/wall furnace categories. For gas floor 

direct heating equipment, DOE used a household sample that includes only the RECS 2001 

category for primary floor/wall furnaces.c For gas room direct heating equipment, DOE used a 

household sample that includes the RECS 2001 room heater and fireplace categories. Based on 

17 28 the percentage of fireplace models in the GAMA directory and GAMA’s shipments data,

DOE sampled 20 percent from the RECS 2001 room heater categories and 80 percent from the 

RECS fireplace categories. 

RECS 2001 does not distinguish between wall and floor furnaces. Thus the samples for wall furnace, gravity 

furnace, and floor furnace direct heating equipment utilize the same household sample (not including fireplaces). 

7-29 

c 

EE-2006-STD-0129 and EERE-2006-BT-STD-0129 COMMENT 32.2
7.3.3 Determining Annual House Heating Load 

The annual house heating load (HHL) is the total amount of heat output from the direct 

heating equipment during the heating season. This amount includes heat from both the burner 

and the blower. DOE determined the HHL for each sampled housing unit based on the household 

heating fuel consumption and the characteristics of the systems. DOE used the following 

12 calculations, which are based on the DOE test procedure. Details are presented in appendix 7­

C. 

QYR ,RECS − 8760 ×QP HHL = + 4160 ×QP ×η U A× (QIN − QP ) 

Where: 

QYR,RECS = annual fuel consumption for heating the housing unit, from RECS 2001, 

kBtu/yr, 

QP = pilot input rate, Btu/h, 

QIN = rated input capacity of direct heating equipment, Btu/h, 

8,760 = hours in a year, h, 

4,160 = average heating season hours according to test procedure, h, 

A = TP factor, h/kBtu, 

100,000 = , 341,000 × PE + (Qin − QP )×η U 

PE = power consumption of the direct heating equipment while the burner is on, 

W, and 

ηU = efficiency of part-load utilization (%). 

The average pilot input rate for the above calculation is 400 Btu/h for gas wall fan and 

gas wall gravity direct heating equipment, 450 Btu/h for gas floor direct heating equipment, and 

29 350 Btu/h for gas room direct heating equipment. Section 7.3.4.1 describes the values for input 

capacity (QIN ). 

The PE value reflects a design that incorporates a draft inducer, electronic ignition, and 

air circulation fan components. To calculate PE, DOE used values given in the 1993 technical 

29 support document. For room heaters that operate at higher energy efficiency levels, PE also 

includes the power consumption of the air circulation fan. Note that the efficiency of part-load 

utilization (ηU) depends on AFUE, the assignment of which is explained in Section 7.3.4.2. 

For households for which the natural gas use for heating clearly is associated solely with 

use of the direct heating equipment as primary or secondary heating equipment, DOE used the 

annual fuel consumption for heating the housing unit, from RECS 2001. DOE adjusted the house 

heating load for households that used a gas furnace as the primary heating equipment and the 

direct heating equipment as secondary heating equipment. RECS 2001 reports the percentage of 

heating energy consumption attributable to secondary products. DOE reduced the house heating 

7-30 

EE-2006-STD-0129 and EERE-2006-BT-STD-0129 COMMENT 32.2
load applicable to the direct heating equipment by subtracting the estimated amount of heat 

provided by the secondary system. Details are presented in appendix 7-E, derivation of heating 

load for pool heaters and direct heating equipment, for details. 

7.3.4 Assigning Direct Heating Equipment to Sample Households 

As explained in the framework document, DOE proposes to maintain the sixteen product 

classes for direct heating equipment established in the Energy Policy and Conservation Act of 

1975 (EPCA). DOE, however, performed the LCC analysis using a single representative product 

class for each product category (i.e., for gas wall fan, gas wall gravity, gas floor, and gas room 

equipment). DOE extrapolated to the other classes the analytical results for the representative 

product class in each category of direct heating equipment. 

DOE selected the class having the greatest percentage of shipments to serve as the 

representative product class. Based on shipment data from the 1993 technical support 

29 document for gas wall fan, gas wall gravity, gas floor, and gas room direct heating equipment, 

and the number of available models listed in GAMA March 30, 2007, Residential Direct Heating 

Directory, DOE chose the following product classes to represent each direct heating equipment 

category: 

• gas wall fan direct heating equipment using more than 42,000 Btu/h; 

• gas wall gravity direct heating equipment using more than 27,000 and up to 46,000 Btu/h; 

• gas floor direct heating equipment using more than 37,000 Btu/h; and 

• gas room direct heating equipment using more than 27,000 and up to 46,000 Btu/hour. 

7.3.4.1 Input Capacity of Existing Equipment 

Table 7.3.2 shows the distribution of input capacities for the representative product 

classes listed above, based on GAMA March 30, 2007, Residential Direct Heating Directory. 

Gas wall fan direct heating equipment includes vented gas fireplace heaters that are listed as fan 

wall heaters. Gas room direct heating equipment includes the rest of the vented gas fireplace 

heaters as well as gas stoves and other hearth heaters. 

7-31 

EE-2006-STD-0129 and EERE-2006-BT-STD-0129 COMMENT 32.2
Table 7.3.2 Distribution of Input Capacity for Representative Product Classes of Direct 

Heating Equipment 

Input Capacity Percentage of Models 

(kBtu/h) Gas Wall Fan Gas Wall Gravity Gas Floor Gas Room 

27.5 0.0% 1.8% 

28 8.3% 0.0% 

30 16.7% 52.7% 

31 0.0% 1.8% 

31.5 0.0% 3.6% 

32 16.7% 0.0% 

33 20.8% 1.8% 

35 33.3% 16.4% 

36 0.0% 1.8% 

37 0.0% 7.3% 

38 4.2% 10.9% 

40 0.0% 1.8% 

45 0.0% 0.0% 16.7% 0.0% 

50 14.3% 22.2% 

55 45.2% 0.0% 

60 4.8% 11.1% 

62 0.0% 22.2% 

62.5 19.0% 0.0% 

65 16.7% 5.6% 

75 0.0% 22.2% 

Average (kBtu/h) 57.6 32.8 59.3 32.6 

7.3.4.2 Efficiency of Existing Equipment 

To represent the energy efficiency of existing direct heating equipment, DOE used the 

current Federal minimum energy conservation standards for such equipment that took effect in 

30 1990. This choice was based on product ages from RECS 2001 and the belief that energy 

efficiency changed little during the 1980s and 1990s. DOE assumed that existing direct heating 

equipment is equipped with a continuous pilot light. Table 7.3.3 shows the energy efficiency 

levels for the representative product classes. 

Table 7.3.3 Energy Efficiency of Existing Direct Heating Equipment 

Representative Product Class Efficiency 

(AFUE) 

Gas Wall Fan More Than 42,000 Btu/h 74% 

Gas Wall Gravity More Than 27,000 up to 46,000 Btu/h 64% 

Gas Floor Product Classes More Than 37,000 Btu/h 57% 

Gas Room More Than 27,000 up to 46,000 Btu/h 64% 

7-32 

EE-2006-STD-0129 and EERE-2006-BT-STD-0129 COMMENT 32.2
7.3.5 Calculations of Energy Consumption 

Knowing the heating load of each sample housing unit, one can estimate the energy 

consumption if more energy efficient products, rather than the existing baseline products, were 

used in each household. 

7.3.5.1 Fuel Consumption 

DOE calculated the fuel consumption (FuelUse) for single-stage direct heating equipment 

31 using the following formula, which is taken from the current revision of DOE’s test procedure.

FuelUse = BOH × (QIN − QP ) + 8760 ×QP 

Where:


FuelUse = fuel consumption, kBtu/year,


BOH = burner operating hours, h,


Qin = input capacity, kBtu/h (at the max input rate),


8,760 = hours per year, and


Qp = pilot light input rate, kBtu/h.


The value assigned to burner operating hours (BOH) is a function of the heating load, 

average heating season hours, efficiency of part-load utilization, and pilot light input rate. The 

BOH accounts for the contribution the electrical components make to heating the house. For 

each considered energy efficiency level, BOH has a different value. Calculating energy 

consumption is described in detail in appendix 7-C. 

7.3.5.2 Electricity Consumption 

DOE calculated the electricity consumed by direct heating equipment for the draft 

inducer, igniter, and air circulation fan and during stand-by mode.e The current DOE test 

procedure accounts for only the total electricity demand when the product is firing (PE), but 

some components consume electricity when the unit is not firing. This standby consumption 

includes use by the ignition device, by the controls when the unit is off, and when the circulation 

fan is operating while the burner is off. DOE calculated the electricity consumption for singlestage direct heating equipment as follows. 

ElecUse = BOH × PE + (8760 − BOH ) × Pstby

Where: 

ElecUse = electricity consumed by direct heating equipment, kWh/year; 

BOH = burner operating hours, h; 

e The DOE test procedures do not measure the electricity used by controls when the direct heating equipment is not 

firing. 

7-33 

EE-2006-STD-0129 and EERE-2006-BT-STD-0129 COMMENT 32.2
PE = electricity demand when the equipment is firing, kW; and 

Pstb y = stand-by power consumption (when the burner is off), kW. 

The details for calculating electricity consumption are described in appendix 7-C. 

7.3.6 Average Annual Energy Consumption 

The average annual energy use and energy savings for each considered energy efficiency 

level are shown in Tables 7.3.4 through 7.3.7 for each representative product class. The energy 

savings reflect the changes in electricity use for different efficiency levels. 

Table 7.3.4 Annual Energy Consumption for Gas Wall Fan Direct Heating Equipment 

Efficiency 

Level 

Average Energy 

Consumption 

Average Energy 

Savings 

(AFUE) Gas (MMBtu/yr) 

(MMBtu/yr) 

Electricity 

(kWh/yr) 

74 (baseline) 27.1 35 --

75 25.6 42 1.5 

76 25.2 42 1.9 

77 24.9 42 2.2 

80 23.9 61 3.2 

Table 7.3.5 Annual Energy Consumption for Gas Wall Gravity Direct Heating 

Equipment 

Efficiency 

Level 

Average Energy 

Consumption 

Average Energy 

Savings 

(AFUE) Gas (MMBtu/yr) 

(MMBtu/yr) 

Electricity 

(kWh/yr) 

64 (baseline) 29.6 0 --

66 28.7 0 0.8 

68 27.9 0 1.6 

71 26.8 0 2.7 

72 25.6 18 4.0 

75 24.4 58 5.2 

7-34 

EE-2006-STD-0129 and EERE-2006-BT-STD-0129 COMMENT 32.2
Table 7.3.6 Annual Energy Consumption for Gas Floor Direct Heating Equipment


Efficiency 

Level 

Average Energy 

Consumption 

Average Energy 

Savings 

(AFUE) Gas (MMBtu/yr) 

(MMBtu/yr) 

Electricity 

(kWh/yr) 

57 (baseline) 30.4 0 --

58 29.9 0 0.5 

61 28.5 0 1.9 

62 28.1 0 2.3 

65 25.5 12 4.9 

78 21.0 54 9.3 

Table 7.3.7 Annual Energy Consumption for Gas Room Direct Heating Equipment


Efficiency 

Level 

Average Energy 

Consumption 

Average Energy 

Savings 

(AFUE) Gas (MMBtu/yr) 

(MMBtu/yr) 

Electricity 

(kWh/yr) 

64 (baseline) 22.0 0 --

65 21.7 0 0.3 

66 21.4 0 0.6 

67 21.1 0 0.9 

68 20.8 0 1.2 

70 19.4 12 2.6 

78 17.2 75 4.9 

93 14.4 65 7.6 

7.4 POOL HEATERS 

In the engineering analysis, DOE calculated energy consumption of pool heaters using 

the DOE test procedure for those products. For the LCC analysis, DOE estimated energy 

consumption of pool heaters in actual housing units. To represent actual households likely to 

1 purchase and use pool heaters, DOE used a household sample from RECS 2001.

For each household, DOE used RECS-reported energy consumption for pool heating 

(based on existing systems) to calculate the pool-heating load of each sample household. The 

pool heater heating load represents the amount of heating required for a pool throughout a year. 

DOE estimated the pool-heating load from the reported pool-heating energy consumption and the 

estimated energy efficiency of the existing pool heater in each sample housing unit. To complete 

the analysis, DOE calculated the energy consumption of alternative (more energy efficient) 

products if they were installed in each housing unit in place of existing products. 

7-35 

EE-2006-STD-0129 and EERE-2006-BT-STD-0129 COMMENT 32.2
7.4.1 Household Sample 

The subset of records used to select a RECS 2001 household sample for pool heaters met 

all of the following criteria (see appendix 7-A, RECS 2001, values and variables, for details). 

• The household had a swimming pool. 

• A pool heater was used to heat pool water. 

• The pool heater used natural gas or liquefied petroleum gas (LPG) as a heating fuel. 

• The heater heated pool water for only one housing unit. 

Table 7.4.1 Criteria for Selecting Household Sample for Pool Heaters (RECS 2001) 

Fuel Type Algorithm No. of 

Records 

No. of US 

Households 

Represented 

(million) 

Natural Gas Have Swimming Pool? = Yes 

Is it heated? = Yes 

Fuel Type = natural gas 

30 0.8 

Propane Have Swimming Pool? = Yes 

Is it heated? = Yes 

Fuel Type = propane 

11 0.2 

7.4.2 Input Capacity and Heating Load 

DOE assigned all sample households a 250-kBtu/h pool heater, the most common rating 

32 for all covered models listed in the May 2007 FTC pool heater directory. The annual pool 

heater heating load (PHHL) is the total heat output from the pool heater during a year. DOE used 

the following equation to determine PHHL for each sampled household. 

PHHL = (QRECS − Qp × POH ) × Et 

Where:


QRECS = pool heater annual fuel consumption (kBtu/yr);


Qp = pilot light input rate (kBtu/yr);


POH = average number of pool operating hours (h/yr); and


Et = thermal efficiency of the existing pool heater associated with the


household (%). 

The annual fuel consumption for the pool heater (QRECS) for each household having a 

pool heater was taken from RECS 2001. 

For units that have a pilot light, DOE assigned a pilot light input rate of 1,000 Btu/h.29, 33 

DOE assigned pilot lights to 29 percent of the households that have pool heaters based on the 

7-36 

EE-2006-STD-0129 and EERE-2006-BT-STD-0129 COMMENT 32.2
percentage of such models in the 2007 FTC directory. DOE assumed the remaining 71 percent 

were equipped with electronic ignition. 

The DOE test procedure uses an average value for pool heater operating hours (when the 

pilot light is on) of 4,464 hours per year. DOE used a distribution around this average to assign 

values for pool heater operating hours to the sample households. The distribution ranges from 

235 h/yr to 8,760 h/yr, where the upper limit represents a year-around operation. The value of 

235 h/yr is the minimum value of the triangular distribution used to calculate pool operating 

hours. 

DOE assigned all sample households a pool heater having a 78 percent thermal 

efficiency. This efficiency was selected based on a maximum lifetime for pool heaters of 12 

years. Thus all pool heaters associated with RECS 2001 households were installed after 1989, 

which was after EPCA mandated the 78-percent energy conservation standard. 

7.4.3 Calculations of Energy Consumption 

Knowing the pool heater heating load for each sample housing unit, one can estimate 

what the energy consumption would be if more energy efficient products, rather than the existing 

products, were used in each household. DOE used the method stipulated in the DOE test 

procedure to calculate the fuel consumption at the considered energy efficiency levels for each 

household. 

EF = BOH ×QIN + (POH − BOH ) ×Qp 

Where:


EF = average annual fuel energy for pool heater, kBtu/yr,


BOH = burner operating hours, h/yr,


Qin = pool heater input rate, kBtu/h,


POH = pool operating hours, h/yr, and


Qp = pilot light input rate, kBtu/h.


DOE calculated the electricity consumption at the considered energy efficiency levels for 

each household using the method provided in the DOE test procedure and adding stand-by 

electricity consumption. 

EAE = BOH × PE − (POH − BOH ) × PE standby 

Where: 

EAE = average annual auxiliary electrical energy for pool heater (including standby electrical consumption), kWh/yr; 

PE = electrical power when the burner is on, kW; and 

PEstandby = electrical power when the burner is off, kW. 

7-37 

EE-2006-STD-0129 and EERE-2006-BT-STD-0129 COMMENT 32.2
The calculations for fuel and electricity consumption are detailed in appendix 7-C. 

7.4.4 Average Annual Energy Consumption 

The average annual energy consumption for each considered energy efficiency level for 

pool heaters is shown in Table 7.4.2. 

Table 7.4.2 Annual Energy Consumption for Gas Pool Heaters 

Efficiency Level 

(Thermal Efficiency in 

Percent) 

Average Energy 

Consumption 

(MMBtu/year) 

Average Energy 

Savings 

(MMBtu/year) 

78 (baseline) 53.6 --

79 52.9 0.7 

81 51.6 1.9 

82 51.0 2.5 

83 49.2 4.4 

84 48.6 5.0 

86 47.5 6.1 

90 45.4 8.2 

95 43.0 10.6 

7-38 

EE-2006-STD-0129 and EERE-2006-BT-STD-0129 COMMENT 32.2
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EE-2006-STD-0129 and EERE-2006-BT-STD-0129 COMMENT 32.2