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.................................................................................................................. 7-3
 

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

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

7.2.3.3	 Assignment of Water Heater Tank Size......................................................... 7-6
 

7.2.3.4	 Determination of Hot Water Use for Instantaneous Water Heaters .............. 7-9
 

7.2.3.5	 Overview of Hot Water Use Estimates.......................................................... 7-9
 

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

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

7.2.4.3	 Description of Key Variables....................................................................... 7-15
 

7.2.4.4	 Derivation of Energy Parameters................................................................. 7-16
 

7.2.4.5	 Derivation of Temperatures ......................................................................... 7-21
 

7.2.4.6	 Overview of Approach for Calculating Water Heater Energy
 

Consumption................................................................................................ 7-26
 

7.2.5 Average Annual Energy Consumption .......................................................................... 7-27
 

7.3 DIRECT HEATING EQUIPMENT .............................................................................. 7-29
 

7.3.1 Introduction.................................................................................................................... 7-29
 

7.3.2 Household Sample ......................................................................................................... 7-30
 

7.3.3 Determining Annual House Heating Load .................................................................... 7-31
 

7.3.4 Assigning Direct Heating Equipment to Sample Households ....................................... 7-32
 

7.3.4.1	 Input Capacity of Existing Equipment......................................................... 7-33
 

7.3.4.2	 Efficiency of Existing Equipment................................................................ 7-34
 

7.3.5 Calculation of Energy Consumption.............................................................................. 7-35
 

7.3.5.1	 Fuel Consumption........................................................................................ 7-35
 

7.3.5.2	 Electricity Consumption .............................................................................. 7-35
 

7.3.6 Average Annual Energy Consumption and Energy Savings ......................................... 7-36
 

7.4 POOL HEATERS .......................................................................................................... 7-38
 

7.4.1 Household Sample ......................................................................................................... 7-38
 

7.4.2 Input Capacity and Heating Load .................................................................................. 7-39
 

7.4.3 Calculation of Energy Consumption.............................................................................. 7-40
 

7.4.4 Average Annual Energy Consumption .......................................................................... 7-41
 

LIST OF TABLES 

Table 7.2.1 Selection of RECS 2005 Records for Water Heaters........................................... 7-2
 

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

Table 7.2.3 Water Heater Tank Sizes Selected for Sample Households ................................ 7-9
 

Table 7.2.4 Average Daily Hot Water Draw Volume by Product Class and Region ........... 7-11
 

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Table 7.2.5 Assignment of RE to Gas-Fired Storage Water Heaters.................................... 7-16
 

Table 7.2.6 Assignment of RE for Electric Storage Water Heaters...................................... 7-17
 

Table 7.2.7 Distribution of RE for Oil-Fired Storage Water Heaters ................................... 7-17
 

Table 7.2.8 Assignment of RE for Gas-Fired Instantaneous Water Heaters......................... 7-18
 

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

Table 7.2.10 Distribution of Input Capacity for Electric Storage Water Heaters................... 7-20
 

Table 7.2.11 Distribution of Input Capacity for Oil-Fired Water Heaters.............................. 7-20
 

Table 7.2.12 Distribution of Input Capacity for Gas-Fired Instantaneous Water Heaters...... 7-21
 

Table 7.2.13 Water Heater Location by Product Class........................................................... 7-25
 

Table 7.2.14 Annual Energy Consumption for Gas-Fired Storage Water Heaters................. 7-28
 

Table 7.2.15 Annual Energy Consumption for Electric Storage Water Heaters .................... 7-28
 

Table 7.2.16 Annual Energy Consumption for Oil-Fired Storage Water Heaters.................. 7-29
 

Table 7.2.17 Annual Energy Consumption for Gas-Fired Instantaneous Water Heaters ....... 7-29
 

Table 7.3.1 Summary of Direct Heating Equipment Household Sample ............................. 7-31
 

Table 7.3.2 Distribution of Input Capacity for Representative Product Classes of
 

Direct Heating Equipment ................................................................................. 7-34
 

Table 7.3.3 Energy Efficiency of Existing Direct Heating Equipment ................................ 7-35
 

Table 7.3.4 Average Energy Consumption for Gas Wall Fan Direct Heating
 

Equipment.......................................................................................................... 7-36
 

Table 7.3.5 Average Energy Consumption for Gas Wall Gravity Direct Heating
 

Equipment.......................................................................................................... 7-37
 

Table 7.3.6 Average Energy Consumption for Gas Floor Direct Heating Equipment ......... 7-37
 

Table 7.3.7 Average Energy Consumption for Gas Room Direct Heating Equipment........ 7-37
 

Table 7.3.8 Annual Energy Consumption for Gas Hearth Direct Heating Equipment......... 7-38
 

Table 7.4.1 Criteria for Selecting Household Sample for Pool Heaters (RECS 1993–
 

2005) .................................................................................................................. 7-39
 

Table 7.4.2 Annual Energy Consumption for Gas-Fired Pool Heaters ................................ 7-41
 

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............................................................. 7-6
 

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............................................................................... 7-22
 

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

Households by Water Heater Product Class ...................................................... 7-23
 

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

Water Heater Product Class............................................................................... 7-26
 

Figure 7.2.7 Process for Calculating Water Heater Energy Use ............................................ 7-27
 

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CHAPTER 7. ENERGY USE CHARACTERIZATION
 

7.1 INTRODUCTION
 

The life-cycle cost (LCC) and payback period (PBP) analyses described in chapter 8 

require determination of 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 the determination of 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 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 

from the Residential Energy Consumption Survey 20051 (RECS 2005), 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 2005 collected data from 4,381 

housing units that represent almost 111.1 million households. For this notice of proposed 

rulemaking, DOE updated its analysis using the RECS 2005. 

The subset of RECS 2005 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 2005 record included an indication of water heater size. 

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

DOE divided the water heater subset into four further subsets designed to include 

households that use one of the four water heater product classes (Table 7.2.1). For gas-fired 

instantaneous water heaters, DOE used the same subset as for gas-fired storage water heaters, 

with the addition of 14 households with gas-fired instantaneous water heaters in the RECS 2005 

sample. 

Table 7.2.1 Selection of RECS 2005 Records for Water Heaters 

Product Class Algorithm No. of 

Records 

No. of U.S. 

Households 

Represented 

(million) 

Natural Gas 

and LPG 

Storage 

Storage Water Heater = Yes 

Fuel Type = Natural Gas or LPG 

Shared Water Heater = No 

2166 55.2 

Electric Storage Storage Water Heater = Yes 

Fuel Type = Electricity 

Shared Water Heater = No 

1523 39.5 

Oil Storage Storage Water Heater = Yes 

Fuel Type = Fuel Oil 

Shared Water Heater = No 

100 1.8 

Natural Gas 

and LPG 

Instantaneous 

Storage/Instantaneous Water Heater = Yes 

Fuel Type = Natural Gas or LPG 

Shared Water Heater = No 

2180 55.5 

DOE used EIA’s weightings for each RECS 2005 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). 

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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, 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. 

DOE developed the hot water draw model2 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 used by a household. 

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) + (T tank _ coef ×T tank ) + (Tanksz _ coef ×Tanksz ) +

(T in _ coef ×T in air ) + (T _ coef ×T air ) + (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, o

F, 

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), 

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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 

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

distribution). 

7.2.3.2 Description of Key Variables Used in Draw Model 

The following is a description of the primary variables used in the hot water draw model. 

See appendix 7-B for criteria used to determine each draw model variable. 

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

•	 Number of Preschool Children 0–5 (age1). The total number of infants and young 

children ages 0–5. 

•	 Number of School-Age Children 6–13 (age2). The total number of children, ages 6–13. 

•	 Number of Adults 14–64 (age3). The total number of adults, ages 14 to 64. 

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

•	 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. 

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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 

Federal energy conservation standards for clothes washers that became effective in 20073 and for 

dishwashers that will become effective in 20104 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 

Occupants 

Other hot water using appliances 

Age14-64 Age Age6-13 65+ Total 

Temperature Parameters 

Presence 

during the day Senior only Doesn’t pay for water 

Household Characteristics 

Hot Water 

Use 

Dishwasher Clothes 

washer 

Tair 

Figure 7.2.1 Hot Water Draw Model Determination of Hot Water Use
 

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7.2.3.3 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 that DOE 

assigned to each size within each product class. “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 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. Figure 7.2.2 depicts the process of 

assigning tank size to the sample households. 

RECS Engineering

Data Shipments AHRI Directory

Number of

Bathrooms Size Classes Standard Sizes

Households Size

Assignments

Household Tank

Size Fractions in

2015

Figure 7.2.2 Assignment of Water Heater Tank Sizes 

Step 1. Assignment of Water Heater Sizes. Table 7.2.2 shows the percentage of each 

standard size 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. The table also 

shows the percentages of RECS households that were assigned to each standard size for the LCC 

analysis. 

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Table 7.2.2 Assignment of Water Heater Tank Sizes to Sample Households
 

Fuel 

Type 

RECS 2005 Data Standard 

Sizes Used in 

LCC 

Analysis 

(gal) 

Percent of 

Historical 

Shipments* 

(%) 

Size Assigned 

to Sample 

Households 

(%) 

Size Class Size Range 

(gal) 

Percent of 

Sample 

Households 

(%) 

Natural 

Gas and 

LPG 

Small ≤30 13.7 30 16.9 13.7 

Medium 31-49 58.2 40 45.4 45.4 

50 35.7 37.1 

Large ≥50 28.1 66 1.9 1.9 

75 1.9 1.9 

Electric 

Small ≤30 23.3 30 22.5 23.3 

Medium 31-49 48.8 40 33.9 33.4 

50 34.6 34.2 

Large ≥50 27.9 66 

9.0 

3.4 

80 4.2 

119 1.4 

Oil 

Small ≤30 21.5 32 67.0 67.0 

Medium 31-49 54.0 

50 33.0 33 Large ≥50 24.5 

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

2004–2008 gas and electric storage shipments data from the Air Conditioning, Heating, and Refrigeration Institute 

6 7 8 (AHRI). Electric: U.S. Census Bureau data (2002–2007). 2007-2008 water heater shipments data from AHRI 

9, 10 1998–2007 oil storage shipments data from AHRI.

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

1, 11, 12, 13, 14 water heaters were the 30-gallon size. Historical RECS data show that households 

having a small water heater declined from 21 percent in RECS 199014 to 14 percent in RECS 

20051. Furthermore, only 5 percent of RECS 2005 “new construction households” (households 

built in 2000–2005) have a small water heater. DOE assigned a 30-gallon water heater to all 

households having a small water heater (14 percent), based on this declining market share. DOE 

used the 1995 GAMA shipment data to assign 75-gallon water heaters to 2 percent of sample 

households. DOE assumed the share of the 66-gallon water heater to be equal to the 75-gallon 

water heater (i.e., 2 percent). 

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

a 40- or a 50-gallon water heater. Based on the 2007–2008 AHRI shipments data,6, 8 DOE 

estimated that 45.4 percent of the total are 40-gallon. This leaves the 50-gallon size with the 

remaining 37.1 percent of the total. DOE assigned either a 40- or a 50-gallon water heater to all 

RECS 2005 households having medium water heaters, which resulted in assigning 40-gallon size 

to 78 percent of these households and 50-gallon size to 22 percent. All RECS households having 

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a large water heater (after subtracting those assigned 75-gallon water heaters) were assigned a 

50-gallon water heater. 

For electric water heaters, 2002–2005 U.S. Census shipments data show that 22.5 percent 

of shipments 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). In the RECS 2005 sample, 

16.9 percent of households have a small water heater. DOE assigned a 30-gallon water heater to 

all households having small water heaters, and estimated that the rest of the shipments consisted 

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

DOE assigned 66-, 80-, or 119-gallon water heaters to 9 percent of households, based on 

2002–2007 U.S. Census shipments data. Based on the March 2008 AHRI directory, DOE 

divided this 9 percent fraction as follows: 37 percent have 66-gallon water heaters, 47 percent 

have 80-gallon water heaters, and 16 percent have 119-gallon water heaters. 

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 49.4 percent 

of the total is 40-gallon, and 50.6 percent is 50-gallon. Applying this 49/51 split to the remaining 

RECS 2005 households having medium and large water heaters resulted in assigning a 40-gallon 

size to 69 percent and a 50-gallon size to 31 percent. All RECS 2005 household having large 

water heaters (after subtracting those assigned 66- and 75-gallon sizes) were assigned a 50­

gallon water heater, and all RECS 2005 households having medium water heaters were assigned 

either a 40- or a 50-gallon water heater. 

For oil-fired water heaters, AHRI estimated that 67 percent of shipments are 32-gallon 

9, 10 water heaters and 33 percent are 50-gallon water heaters. DOE assigned a 32-gallon water 

heater to all RECS 2005 households having a small water heater and assigned a 50-gallon water 

heater to all RECS 2005 households having a large water heater. DOE split the RECS 2005 

households having medium water heater between 32- and 50-gallon sizes, assigning 84 percent 

of these households a 32-gallon water heater and the remaining 16 percent a 50-gallon water 

heater. 

Step 2. Assignment of Household-Specific Water Heater Sizes. In exploring the 

correlation between water heater size and several other variables, DOE found that the highest 

correlation was with the number of bathrooms. Therefore, DOE further 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. 

For the small number of households with gas-fired instantaneous water heaters in the gasfired instantaneous water heater sample, DOE randomly assigned tank sizes based on the 

percentages assigned to all households that reported a tank size. 

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Table 7.2.3 Water Heater Tank Sizes Selected for Sample Households
 

Fuel Type RECS Size 

Class 

No. of 

Bathrooms 

Water Heater 

Tank Size (gal) 

Percent 

Assigned to 

Size Class 

Natural Gas 

or LPG 

Small Any # BR 30 100% 

Medium Any # BR 40 77.9% 

50 22.1% 

Large 

2 BR 50 100% 

≥ 3 BR 

50 45.5% 

66 27.2% 

75 27.2% 

Electric 

Small Any # BR 30 100% 

Medium Any # BR 

40 68.6% 

50 31.4% 

Large 

< 2 BR 50 100% 

2 BR 

50 64.6% 

66 24.6% 

80 10.8% 

≥ 3 BR 80 65.3% 

119 34.7% 

Oil 

Small Any # BR 32 100% 

Medium Any # BR 32 84.3% 

50 15.7% 

Large Any # BR 50 100% 

7.2.3.4 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 DOE 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. 

7.2.3.5 Overview of Hot Water Use Estimates 

Using the hot water draw model, DOE calculated average daily hot water use to be 41.5 

gallons for households having electric storage water heaters, 44.3 gallons for households having 

gas-fired storage water heaters, 45.0 gallons for households having oil-fired water heaters, and 

44.5 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 

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15, 16 type of fuel. These results are similar to recent hot water draw studies. Figure 7.2.3 shows 

the range in hot water use among sample households. 

Hot Water Use 

Average; Median; Box25%-75%; Whisker: 5%-95% 

9.7 8.7 10.5 9.9 

93.9 

88.9 89.7 

94.0 

44.3 41.5 

45.0 44.5 

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

100 

Oil WH Instantaneous Gas 

Hot Water Use (gallons) 

median 

Average 

Gas WH ElectricWH 

WH 

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 

water heater analysis model (WHAM).17 WHAM accounts for a range of operating conditions 

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

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

procedure:18 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). Daily hot water draw volumes for each region and product class are listed in Table 

7.2.4. More information on daily hot water draw volumes can be found in appendix 7-B. 

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Table 7.2.4 Average Daily Hot Water Draw Volume by Product Class and Region
 

Region 

Gas and 

LPG 

(gal/day) 

Electric 

(gal/day) 

Oil 

(gal/day) 

Gas 

Instantaneous 

(gal/day) 

1 32.8 34.1 42.3 37.8 

2 39.4 44.1 44.2 44.0 

3 37.2 40.5 N/A 40.6 

4 37.0 40.6 N/A 40.4 

5 42.9 44.9 41.5 45.1 

6 41.1 44.8 N/A 44.1 

7 38.4 43.8 N/A 43.9 

8 46.3 45.1 N/A 45.0 

9 44.5 52.7 N/A 47.4 

10 37.4 43.7 49.4 41.5 

11 32.9 49.7 N/A 48.2 

12 48.7 48.3 N/A 50.3 

13 44.5 49.3 N/A 54.4 

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. 

7.2.4.2 Approach to Calculating Water Heater Energy Use 

The WHAM equation yields average daily water heater energy consumption (Qin). The 

equation is expressed as follows. 

⎛

⎜

⎝

UA
 −
 Tamb )
⎞

⎟

⎠

(T
tank −
 Tin vol den C
 ×
 ×
 × )
 ×
 (T
 P tank ∗
 1
−
 24 (T
tank −
 T
amb Q
 )
 in =
 +
 ×
 UA
 ×
 RE
 Pon
 

Where: 

Qin = total water heater energy consumption in 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, oF,
 

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-o

F.
 

Additional details about the WHAM equation are provided in appendix 7-C. 

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

heaters, gas-fired instantaneous water heaters, and oil-fired storage water heaters, Qin is the sum 

of fuel and electricity consumption, and the values for electricity and fuel consumption must be 

disaggregated. DOE calculated electricity consumption as follows. 

Q = Qin × (P − P )− P × 24 electricity aux standby standby PON 

Where: 

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 

19, 20 program. The equation for determining the energy consumption of heat pump water heaters 

is similar to the 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 or more than 100 oF. A heat pump water heater also operates in the electric resistance mode 

for part of the time even when the monthly ambient temperature is between 32 oF and 100 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 ⎞ ⎤ (T − T ) × tank amb 
 ⎢

⎢

⎢

⎢

⎢

_ 

RE HPWH × PA 

(1 − HP _ frac ) 

⎥

⎥

⎥

⎥

⎥

*
 1
 − +
 P
 

⎜

⎜

⎝

⎟

⎟

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

⎜

⎜

⎞

⎟

⎟

(T − T ) UA
 × tank amb 
 *
 1
 − RE
 P
 ⎣ ⎝ ⎠ ⎦ ER ON ,ER 

Where: 

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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, oF,
 

PON = rated input power, Btu/h,
 

PAhwh = performance adjustment factor, and
 

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 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. 

In some cases where there is a significant effect, a household would likely choose to 

install a venting system to exhaust cooled air. DOE estimated that those households for which 

the heat pump cooling load is greater than 10 percent of the household heating load would have a 

venting system installed. Using calculations specific to each household in the subsample for 

electric water heaters, DOE estimated that 40 percent of replacement installations would incur 

this cost, which is described in chapter 8. 

In the remaining cases, DOE believes the household would not want to incur the cost of a 

venting system, and would instead operate their heating and cooling systems to compensate for 

the effects of the heat pump water heater. To account for these indirect effects of heat pump 

water heaters on home energy use, DOE estimated the impact on space heating and air 

conditioning energy consumption for some of the homes in the RECS electric water heater 

subsample that have 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: 

vol × den ×C (T − T ) p tan k in CoolingInput = CoolingCap HPWH ×

P × RE × PA ON ,HPWH HPWH HPWH 

Where: 

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

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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.
 

For the PON and cooling capacity of the heat pump water heater, DOE created 

19, 20, 21 distributions that apply for a range of possible heat pump water heater designs. 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 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 results from studies of instantaneous 

22, 23, 24, 25 water heater performance under field conditions. Preliminary results from a study of 

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

under field conditions at a single-family two-occupant residence indicated higher energy use than 

under the test procedure conditions. The report concludes that the discrepancy is due to the extra 

losses at small draw volumes.22 The other studies have shown a similar effect. DOE’s approach 

used data from Gas Technology Institute (GTI) and CEC studies to derive the adjustment factor 

as a distribution of values as a function of the household hot water use. Appendix 7-B provides 

details about the method. 

The resulting equation is: 

(T in ) ⎛ p ⎞

⎟

⎠

⎟ + 24
 ×Q
p 

×
 vol den C
 ×
 ×
 −T
 Q
 (T amb ) P tan k Qin =
 ×⎜1
−
 −T
 ⎜

⎝ Tank RE ×(1+ PA iwh ) PON 

Where: 

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

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. 

7-14 

EE-2006-STD-0129 COMMENT 56.10
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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,
 

PON = rated input power, Btu/h.
 

7.2.4.3 Description of Key Variables 

The following is a description of the key variables for calculating energy use by water 

heaters. 

•	 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. 

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•	 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). 

7.2.4.4 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). 

DOE’s test procedure for water heaters provided the definitions for these parameters.26 DOE 

developed the parameters for selected energy efficiency level as described below. (See chapter 5 

for a discussion of DOE’s selection of energy efficiency levels.) 

Determining RE. For gas-fired and oil-fired storage water heaters, DOE based the RE 

values on the design features and the distribution of models at each energy efficiency level as 

given in the March 2008 AHRI Directory.27 This approach captures the variation of RE among 

models in the market. 

Table 7.2.5 shows the distribution of RE values for gas-fired storage water heaters by 

efficiency level. The same RE values apply to all four rated volume size categories (30, 40, 50, 

and 75-gallons), based on the assumption that the design characteristics at each energy efficiency 

level are the same across the size categories. DOE estimated the recovery efficiency for the 

condensing water heater to be 90 percent. 

Table 7.2.5 Assignment of RE to Gas-Fired Storage Water Heaters 

RE 

Efficiency 

Level 0 

Efficiency 

Level 1 

Efficiency 

Level 2 

Efficiency 

Level 3 

Efficiency 

Level 4 

Efficiency 

Level 5 

70% 0.4% - - - - -

71% 2.0% - - - - -

73% 2.7% - - - - -

74% 4.2% - - - - -

75% 1.2% - - - - -

76% 52.1% 67.3% 76.5% 48.4% 36.0% 72.7% 

77% 16.6% 8.9% - - - -

78% 7.8% 1.9% 2.4% 6.3% 13.5% -

79% 9.9% 11.7% 10.6% 5.6% 22.5% -

80% 3.1% 6.5% 4.1% 28.6% 2.2% 1.5% 

81% - 3.7% 3.5% 9.5% 14.6% 9.1% 

82% - - 2.9% 1.6% 11.2% 16.7% 

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

heating elements is 98 percent for most energy efficiency levels (Table 7.2.6). For heat pump 

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

manufactured by ECR.20 

Table 7.2.6 Assignment of RE for Electric Storage 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 0.95 985 

6 2.00 223% 

7 2.20 240% 

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

(Table 7.2.7). 

Table 7.2.7 Distribution of RE for Oil-Fired Storage Water Heaters 

Efficiency 

Level 

EF 

(32-gallon) RE 

0 0.53 76% 

1 0.54 76% 

2 0.56 76% 

3 0.58 76% 

4 0.60 76% 

5 0.62 76% 

6 0.66 80% 

7 0.68 82% 

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

28, 29 the design features at each level and on the distribution of models at each EF. 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 7.2.8. 

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Table 7.2.8 Assignment of RE for Gas-Fired Instantaneous Water Heaters
 

Efficiency 

Level 

EF RE 

0 0.62 78% 

1 0.69 78% 

2 0.78 78% 

3 0.80 80% 

4 0.82 82% 

5 0.84 84% 

6 0.85 85% 

7 0.92 92% 

8 0.95 95% 

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
tan k −
 Tamb )
 ×
 −
 Qout RE × Pon 

⎜

⎜

⎝

⎟

⎟

⎠

Where: 

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

EF = energy factor,
 

RE = recovery efficiency; %.
 

Ttank = temperature of the air surrounding the water heater, oF;
 

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 that 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 

2008 AHRI Directory,27 and 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 

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percentage of PON values in each gas-fired storage water heater volume category is shown in 

Table 7.2.9. 

Table 7.2.9 Distribution of Input Capacity for Gas-Fired Storage Water Heaters 

Input Capacity 

(PON) 

(kBtu/h) 

Rated Volume (gallons) 

30-gal (%) 40-gal (%) 50-gal (%) 60-gal (%) 75-gal (%) 

26 2.5 - - - -

28 - 0.3 - - -

29 9.6 - - - -

30 37.7 2.3 0.4 - -

31 5.3 - - - -

32 22.4 1.0 0.4 - -

33 - 2.8 - - -

34 0.7 9.5 0.5 - -

35 1.8 - - - -

35.5 7.5 3.0 1.5 - -

36 - 17.6 10.6 - -

37 5.0 2.5 3.9 - -

37.5 - 1.1 1.2 - -

38 - 9.8 5.9 3.1 -

40 7.5 42.4 33.1 1.3 -

42 - 2.6 10.8 0.6 15.1 

45 - - 0.1 3.8 -

46 - 0.2 0.2 - -

47 - 1.1 2.8 - -

48 - 0.9 2.3 - -

50 - 3.0 9.7 21.9 -

52 - - - 4.4 

53 - - 0.7 - -

55 - - 1.6 9.4 10.8 

57 - - 6.5 

60 - - 4.7 6.9 6.5 

61 - - 0.6 - -

62.5 - - 0.5 6.3 -

63 - - 0.7 6.9 -

65 - - 7.1 33.8 15.1 

67 - - 0.4 - -

70 - - - 1.9 20.4 

73 - - - - 2.2 

75 - - - - 23.7 

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The percentages of PON values in each electric water heater volume category are shown in 

Table 7.2.10. For heat pump water heaters, DOE created distributions that apply for a range of 

19, 20, 21 possible heat pump water heater designs. See appendix 7-C for further details. 

Table 7.2.10 Distribution of Input Capacity for Electric Storage Water Heaters 

Input 

Capacity 

(PON) (kW) 

Rated Volume (gallons) 

30-gal (%) 40-gal (%) 50-gal (%) 66-gal (%) 80-gal (%) 119-gal 

(%) 

1.5 1.4 - - - -

1.7 2.2 - - - -

2.0 0.7 - - - -

3.0 0.7 0.4 1.1 - 1.7 2.4 

3.8 25.9 19.9 14.1 9.7 11.0 4.9 

4.5 69.1 79.3 82.1 90.3 83.9 92.7 

5.5 - 0.4 2.7 - 3.4 

Table 7.2.11 shows the percentage of PON values in each oil-fired water heater volume 

category. 

Table 7.2.11 Distribution of Input Capacity for Oil-Fired Water Heaters 

Input Capacity (PON) Rated Volume (gal) 

(kBtu/h) 32-gal (%) 50-gal (%) 

83 9.1 -

90 36.4 -

103 9.1 25.0 

104 27.3 25.0 

105 18.2 50.0 

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 (less than 30-gallons) 

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

households having a large tank (greater than 50-gallon) tank DOE assigned a gas-fired 

instantaneous water heater having 199 kBtu per hour input capacity. To households having a 

medium tank (31- to 49-gallons), DOE assigned some a gas-fired instantaneous water heater 

having 180 kBtu per hour input capacity, and the rest a gas-fired instantaneous water heater 

having 199 kBtu per hour input capacity (see Table 7.2.12). 

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Table 7.2.12 Distribution of Input Capacity for Gas-Fired Instantaneous Water Heaters
 

Range of Input 

Capacity 

(kBtu/h)* 

Assigned Input 

Capacity 

(kBtu/h) 

Fraction Assigned to 

Sample Households 

(%) 

140–169 140 13.7 

170–185 180 38.1 

186–199 199 48.2 

* DOE did not include units having a capacity <140 kBtu/h because such units are considered unsuitable for whole30 house water heating. 

7.2.4.5 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 2005 provides data on heating and cooling degreedays, but not on air temperatures for each household in the sample. To each RECS 2005 

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 

31, 32 annual average outdoor air temperatures. DOE also gathered the heating and cooling 

33 degree-days at a base temperature of 65 o

F for 2005 for those weather stations. The 

2005 heating and cooling degree-days match the period used to determine the degreedays in RECS 2005. 

•	 RECS 2005 reports both heating and cooling degree-days to base temperature 65 oF for 

each housing record. DOE assigned each RECS 2005 household to one of the 282 

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

large state) gave the best fit of RECS 2005 data to weather data. 

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

2005 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 59.3 o

F 

for electric storage water heaters, 55.9 oF for gas-fired storage water heaters and gas-fired 

instantaneous water heaters, and 48.9 o

F for oil-fired storage water heaters. Figure 7.2.4 shows 

the range of average annual outdoor air temperatures among sample households. 

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EE-2006-STD-0129 COMMENT 56.10
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Average Annual Outdoor Temperature (°F) 

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

55.8 

48.9 

59.3 

55.9 

35 

40 

45 

50 

55 

60 

65 

70 

75 

80 

Gas WH Electric WH Oil WH Instantaneous Gas 

WH 

Temperature(°F) 

median 

Average 

Figure 7.2.4	 Range of Annual Outdoor Air Temperature for 

Sample Households by Water Heater Product Class 

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 

RECS 2005, which enables one to identify whether a household has 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 

34, 35 Renewable Energy Laboratory. 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: 

T = T + offset + lag IN air ,avg 

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

calculated the offset term separately for the households that use municipal treatment plants and 

those that use ground well sources. DOE used a method described by the Electrical Power 

36 Research 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 

water that comes from municipal treatment plants.36 The derivation of those values is described 

in appendix 7-D. 

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The analysis identified an average inlet water temperature of 62.8 oF for electric storage 

water heaters, 59.6 oF for gas-fired storage water heaters and gas-fired instantaneous water 

heaters, and 55.1 oF for oil-fired water heaters. Figure 7.2.5 shows the range of inlet water 

temperatures among sample households. 

Average Annual Inlet Water Temperature (°F) 

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

59.5 

52.1 

59.6 

62.8 

40 

45 

50 

55 

60 

65 

70 

75 

80 

85 

Gas WH ElectricWH Oil WH Instantaneous Gas 

WH 

Temperature (°F) 

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 the 

RECS 2005 households based on a 2006 survey commissioned by Plumbing & Mechanical 

magazine.37 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 o

F; and the other 39 percent usually install the water 

heater with a thermostat at 120 oF. Based on this information, DOE estimated that half of the 

latter water heater installations have a set point temperature of 120 oF, 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 2005 

water heater household sample. 

b 140 o

F is the maximum allowed to avoid scalding. 

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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. 

•	 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 

19, 38 crawlspaces, or inside the house. For single-family houses, RECS 2005 reports whether the 

house has a basement, whether the basement is heated or unheated, and the presence or absence 

of a garage, crawlspace, or attic. 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 are in the garage. For two-story 

houses built after 1980 that have a garage and attic, DOE estimated that half the water heaters are 

in the garage and half in the attic. For all other houses with a garage, DOE assumed that the 

water heater was in the garage. For houses built after 1980 that have an attic only, DOE assumed 

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

half the water heaters were in the crawlspace and half were 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 storage water heaters were not installed indoors or in the 

attic. In the absence of a basement, garage, or crawlspace, DOE assumed that the water heater 

was installed in an outdoor closet or a similar location. 

DOE determined ambient air temperatures based on information about typical locations 

of water heaters as follows: 

•	 For heated basement installation, DOE assumed the ambient temperature equaled the 

home’s average house indoor temperature given by RECS 2005. 

•	 For unheated basement installation, DOE calculated the ambient temperature as the mean 

of the average house outdoor and indoor temperatures. 

•	 For a garage, crawlspace, or outside closet installation, DOE assigned an ambient 

38 temperature that was 5 o

F higher than the outdoor air temperature for that location.

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

home’s average house indoor temperature given by RECS 2005. 

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•	 For attic installation, DOE assigned an ambient temperature that was 5 oF higher than the 

outdoor air temperature for the months when the average outdoor air temperature is 

below 80 o

F, and 10 oF higher than the outdoor air temperature for the months when the 

38 average outdoor air temperature is above 80 o

F for that location.

Table 7.2.13 shows the percentages of the various water heater locations as well as the 

ambient air temperature derivation for each installation location. 

Table 7.2.13 Water Heater Location by Product Class 

Location Temperature Derivation GasFired (%) 

Electric 

(%) 

Oil-Fired 

(%) 

Heated Basement Avg. Indoor Temperature 18.1 10.1 20.2 

Unheated Basement (Avg. Indoor Temp + Avg. 

Outdoor Temp) / 2 12.1 8.5 49.5 

Garage, Crawlspace, 

or Outdoor Closet Avg. Outdoor Temp + 5 °F 43.1 39.9 30.3 

Inside House Avg. Indoor Temperature 24.2 39.2 0.0 

Attic Installation Avg. Outdoor Temp + 5 °F 

(winter), + 10 °F (summer) 2.5 2.4 0.0 

Total 100 100 100 

The analysis identified average annual ambient temperatures of 68.2 o

F for electric 

storage water heaters, 65.8 oF for gas-fired storage water heaters and gas-fired instantaneous 

water heaters, and 59.2 o

F for oil-fired water heaters. Figure 7.2.6 shows the range of annual 

ambient air temperature among sample households. 

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Average Annual Ambient Air Temperature (°F) 

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

65.8 

59.2 

68.2 

65.8 

50 

55 

60 

65 

70 

75 

80 

Gas WH ElectricWH Oil WH Instantaneous Gas 

WH 

Temperature (°F) 

median 

Average 

Figure 7.2.6	 Range of Annual Ambient Air Temperature in 

Sample Households by Water Heater Product Class 

7.2.4.6 Overview of Approach for 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. 

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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 Process for Calculating Water Heater Energy Use 

7.2.5 Average Annual Energy Consumption 

This section presents the average annual energy use and the average energy savings for 

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

water heater product class. For its LCC and PBP analyses, DOE used the full distribution of 

energy use values calculated for the sample households. 

Table 7.2.14 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 baseline 

water heater. The energy savings include the higher use of electricity at an EF of 0.64 and above. 

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Table 7.2.14 Annual Energy Consumption for Gas-Fired Storage Water Heaters
 

Energy 

Efficiency Level 

(EF) 

Average Energy 

Consumption Average Energy 

Savings (Gas) 

(MMBtu/yr) Gas 

(MMBtu/yr) 

Electricity 

(kWh/yr) 

0.59 (baseline) 16.6 0 -­

0.62 15.7 0 0.9 

0.63 15.2 6 1.4 

0.64 14.7 69 1.9 

0.65 14.3 68 2.3 

0.67 13.5 66 3.1 

0.80 11.3 59 5.3 

Table 7.2.15 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 7.2.15 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) 2,618 -

0.91 2,583 35 

0.92 2,549 69 

0.93 2,529 89 

0.94 2,481 138 

0.95 2,445 173 

2.00 1,404 1,214 

2.20 1,283 1,336 

Table 7.2.16 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. 

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Table 7.2.16 Annual Energy Consumption for Oil-Fired Storage Water Heaters
 

Energy 

Efficiency Level 

(EF) 

Average Energy 

Consumption Average Energy 

Savings (Oil) 

(MMBtu/yr) Fuel Oil 

(MMBtu/yr) 

Electricity 

(kWh/yr) 

0.53 (baseline) 21.6 62 -­

0.54 21.1 61 0.5 

0.56 20.1 58 1.5 

0.58 19.2 55 2.4 

0.60 18.3 53 3.3 

0.62 17.5 50 4.1 

0.66 16.4 47 5.2 

0.68 15.8 45 5.8 

Table 7.2.17 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. The energy savings include the higher use of electricity at an EF of 0.78 and above. 

Table 7.2.17 Annual Energy Consumption for Gas-Fired Instantaneous Water Heaters 

Energy 

Efficiency Level 

(EF) 

Average Energy 

Consumption Average Energy 

Savings (Gas) 

(MMBtu/yr) Gas 

(MMBtu/yr) 

Electricity 

(kWh/yr) 

0.62 (baseline) 16.8 0 -­

0.69 14.9 0 1.9 

0.78 11.6 26 5.2 

0.80 11.3 29 5.5 

0.82 11.0 29 5.8 

0.84 10.8 29 6.1 

0.85 10.7 29 6.2 

0.92 9.8 29 7.0 

0.95 9.5 28 7.3 

7.3 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 

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 

developed a household sample from RECS 2005.1 For each household in the sample, DOE used 

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RECS 2005 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 

provided in the March 2008 AHRI directory.39 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 2005 households used for evaluating the energy use of direct heating 

equipment met all of the following criteria: 

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

primary or secondary source of heat. 

•	 The heating fuel was natural gas or LPG. 

•	 The equipment heated only one housing unit. 

•	 Secondary fireplace equipment had a flue vent. 

•	 The equipment had energy consumption greater than zero. 

The subset of households using direct heating equipment represents 375 housing records, 

representing 9.4 million households (8.4 percent of the total weighted RECS sample). Appendix 

7-A, RECS 2005 Values and Variables, describes the subset of households. 

DOE matched each RECS heating equipment category to the direct heating equipment 

product classes using several assumptions. For gas wall fan and gas wall gravity DHE, DOE 

used a household sample that includes the RECS 2005 categories of floor/wall furnace for both 

primary and secondary equipment. For gas floor DHE, DOE used a household sample that 

includes only the RECS 2005 category for primary floor/wall furnaces.c For gas room DHE, 

DOE used a household sample that includes the RECS 2005 room heater categories for both 

primary and secondary equipment. For gas hearth DHE, DOE used a household sample that 

includes both primary and secondary equipment fireplace categories. 

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

of households included in the analysis. 

c RECS 2005 does not distinguish between wall and floor furnaces. Thus the samples for gas wall fan furnace, gas 

wall gravity furnace, and floor furnace direct heating equipment utilize the same household sample (not including 

secondary equipment which is only part of gas wall fan and gas wall gravity). 

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Table 7.3.1 Summary of Direct Heating Equipment Household Sample
 

Product 

Class 

Category 

Selection Criteria Fuel Type No. of 

Records 

No. of US 

Households 

Represented 

(million) 

Primary Equipment 

Gas Wall Fan, 

Gas Wall 

Gravity, Gas 

Floor DHE 

Primary Equipment Type = Floor/Wall 

Pipeless Furnace 

Number of Housing Units Heated = 1 

Natural Gas 88 2.1 

LPG 7 0.2 

Gas Room 

DHE 

Primary Equipment Type = Room Heater 

Number of Housing Units Heated = 1 

Natural Gas 77 1.9 

LPG 35 0.9 

Gas Hearth 

DHE 

Primary Equipment Type = Fireplace 

Fireplace is Vented = Yes 

Number of Housing Units Heated = 1 

Natural Gas 4 0.1 

LPG 6 0.2 

Secondary Equipment 

Gas Wall 

Fan, Gas 

Wall Gravity 

Secondary Equipment Type = Floor/Wall 

Furnace 

Natural Gas 3 0.1 

LPG 3 0.1 

Gas Room 

DHE 

Secondary Equipment Type = Room 

Heater 

Natural Gas 19 0.5 

LPG 22 0.6 

Gas Hearth 

DHE 

Secondary Equipment Type = Fireplace 

Fireplace is Vented = Yes 

Natural Gas 93 2.3 

LPG 18 0.5 

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 sample housing unit based on the household 

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

calculations, which are based on the DOE test procedure.18 Details are presented in appendix 7­

C. 

⎛
 U ⎞

⎟

⎠

⎟ ∗

Q YR ,RECS −8760 ×QP HHL
 =
 ⎜ + 4160 ×Q
 ×η Adj Factor
 ⎜

⎝ P _ A×(QIN − QP ) 

Where:
 

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QYR,RECS = annual fuel consumption for heating the housing unit, from RECS 2005, 

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, 

100,000 A = TP factor, h/kBtu = , 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 (%), 

Adj_Factor = adjustment factor. 

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

gas wall gravity DHE, 450 Btu/h for gas floor DHE, and 350 Btu/h for gas room and hearth 

DHE.40 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 

support document.40 For gas room DHE 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. 

The adjustment factor (Adj_Factor) takes into account future improvements to the 

building shell and increases in home area based on EIA’s 2009 Annual Energy Overview (AEO 

2009) projections for retrofit and new construction in 2013.41 

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 2005. 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 2005 reports the percentage of 

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

load applicable to the direct heating equipment by subtracting the estimated amount of heat 

provided by the primary system. Details are presented in appendix 7-E, Derivation of Heating 

Load for Pool Heaters and Direct Heating Equipment. 

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). In its LCC analysis, however, DOE used a single representative product class for 

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

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

representative product class in each category of direct heating equipment. 

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DOE selected the class having the greatest percentage of shipments in its category to 

serve as the representative product class. Based on shipments data from the 1993 technical 

support document40 for gas wall fan, gas wall gravity, gas floor, gas room, and gas hearth direct 

heating equipment, and the number of available models listed in AHRI March 30, 2009 

Residential Direct Heating Directory,29 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; 

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

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

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 AHRI March 30, 2009 Residential Direct Heating Directory. Gas 

hearth DHE includes vented gas fireplace heaters as well as gas stoves and fireplace inserts. 

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Table 7.3.2 Distribution of Input Capacity for Representative Product Classes of Direct 

Heating Equipment 

Input 

Capacity 

(kBtu/h) 

Percentage of Models 

Gas 

Wall 

Fan 

Gas Wall 

Gravity Gas Floor Gas Room 

Gas 

Hearth 

27.3 - - - - 1.2% 

27.5 - - - - 1.2% 

28 - - - - 6.2% 

29 - - - - 2.5% 

29.6 - - - - 1.2% 

30 - 10.0% - - 32.1% 

30.8 - - - - 1.2% 

31 - - - - 2.5% 

31.5 - - - - 2.5% 

32 - 20.0% - - 3.7% 

33 - 10.0% - - 2.5% 

34 - - - - 2.5% 

35 - 60.0% - 75.0% 2.5% 

35.5 - - - - 1.2% 

36 - - - - 2.5% 

37 - - - - 4.9% 

37.5 - - - - 1.2% 

38 - - - 25.0% 9.9% 

39.5 - - - - 1.2% 

40 - - - - 13.6% 

42 - - - - 1.2% 

45 - - 28.6% - 2.5% 

50 13.6% - 14.3% - -

55 50.0% - - - -

60 4.5% - 14.3% - -

62 - - 14.3% - -

62.5 13.6% - - - -

65 18.2% - 14.3% - -

75 - - 14.3% - -

Average 

(kBtu/h) 57.4 33.7 57.4 35.7 33.7 

7.3.4.2 Efficiency of Existing Equipment 

To represent the energy efficiency of existing direct heating equipment in the sample 

homes, DOE used the current Federal minimum energy conservation standards for such 

equipment that took effect in 1990.42 This choice was based on product ages from RECS 2005 

and the belief that energy efficiency changed little during the 1980s and 1990s. DOE assumed 

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that existing direct heating equipment is equipped with a continuous pilot light. Table 7.3.3 

shows the energy efficiency levels assigned 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 More Than 37,000 Btu/h 57% 

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

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

7.3.5 Calculation 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 for single-stage direct heating equipment using the 

following formula, which is taken from the current revision of DOE’s test procedure.43 

FuelUse = BOH × (Q IN P − Q ) + 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. Calculation of fuel 

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. The current DOE test 

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

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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;
 

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 and Energy Savings 

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

level are shown in Tables 7.3.4 through 7.3.8 for each representative product class. For the LCC 

and PBP analyses, DOE used the full distribution of energy use values calculated for the sample 

households. 

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

Efficiency 

Level 

(AFUE) 

Average Energy 

Consumption Average Energy 

Savings (Gas) 

(MMBtu/yr) Gas 

(MMBtu/yr) 

Electricity 

(kWh/yr) 

74 (baseline) 29.4 38 -­

75 27.9 46 1.5 

76 27.5 45 1.9 

77 27.1 45 2.2 

80 26.0 66 3.3 

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Table 7.3.5 Average Energy Consumption for Gas Wall Gravity Direct Heating 

Equipment 

Efficiency 

Level 

(AFUE) 

Average Energy 

Consumption Average Energy 

Savings (Gas) 

(MMBtu/yr) Gas 

(MMBtu/yr) 

Electricity 

(kWh/yr) 

64 (baseline) 29.4 0 -­

66 28.5 0 0.8 

68 27.7 0 1.6 

71 26.6 0 2.7 

72 25.4 17 3.9 

Table 7.3.6 Average Energy Consumption for Gas Floor Direct Heating Equipment
 

Efficiency 

Level 

(AFUE) 

Average Energy 

Consumption Average Energy 

Savings (Gas) 

(MMBtu/yr) Gas 

(MMBtu/yr) 

Electricity 

(kWh/yr) 

57 (baseline) 30.8 0 -­

58 30.3 0 0.5 

Table 7.3.7 Average Energy Consumption for Gas Room Direct Heating Equipment
 

Efficiency 

Level 

(AFUE) 

Average Energy 

Consumption Average Energy 

Savings (Gas) 

(MMBtu/yr) Gas 

(MMBtu/yr) 

Electricity 

(kWh/yr) 

64 (baseline) 27.3 0 -­

65 26.9 0 0.4 

66 26.5 0 0.8 

67 26.1 0 1.2 

68 25.8 0 1.5 

83 20.2 80 7.1 

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Table 7.3.8 Annual Energy Consumption for Gas Hearth Direct Heating Equipment
 

Efficiency 

Level 

(AFUE) 

Average Energy 

Consumption Average Energy 

Savings 

(MMBtu/yr) Gas 

(MMBtu/yr) 

Electricity 

(kWh/yr) 

64 (baseline) 15.01 0 -­

67 13.64 15 1.4 

72 12.58 37 2.4 

93 9.71 45 5.3 

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 

purchase and use pool heaters, DOE used a combined household sample from RECS 1993, 

1, 11, 12, 13 RECS 1997, RECS 2001, and RECS 2005. DOE did this in order to increase the sample 

size and have a more accurate account of the average pool heater energy use. 

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. 

7.4.1 Household Sample 

The subset of records used to select a RECS 1993–2005 household sample for pool 

heaters met all of the following criteria (see appendix 7-A 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 combined RECS 1993–2005 pool heater sample is weighted such that it represents 

the same number of households as in the RECS 2005. 

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Table 7.4.1 Criteria for Selecting Household Sample for Pool Heaters (RECS 1993–2005)
 

RECS Year Fuel Type Algorithm No. of 

Records 

No. of US 

Households 

Represented 

(million) 

RECS 1993 

Natural Gas 

Have Swimming Pool? = Yes 

Is it heated? = Yes 

Fuel Type = Natural gas 

33 0.5 

Propane 

Have Swimming Pool? = Yes 

Is it heated? = Yes 

Fuel Type = Propane 

9 0.1 

RECS 1997 

Natural Gas 

Have Swimming Pool? = Yes 

Is it heated? = Yes 

Fuel Type = Natural gas 

29 0.7 

Propane 

Have Swimming Pool? = Yes 

Is it heated? = Yes 

Fuel Type = Propane 

5 0.1 

RECS 2001 

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 

RECS 2005 

Natural Gas 

Have Swimming Pool? = Yes 

Is it heated? = Yes 

Fuel Type = Natural gas 

42 1.2 

Propane 

Have Swimming Pool? = Yes 

Is it heated? = Yes 

Fuel Type = Propane 

11 0.3 

Combined 

RECS 1993­

2005 

Natural Gas 

Have Swimming Pool? = Yes 

Is it heated? = Yes 

Fuel Type = Natural gas 

134 1.2 

Propane 

Have Swimming Pool? = Yes 

Is it heated? = Yes 

Fuel Type = Propane 

36 0.3 

7.4.2 Input Capacity and Heating Load 

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

for all covered models listed in the May 2008 FTC pool heater directory.44 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. 

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PHHL = (Q RECS p − Q × POH ) × Et 

Where: 

PHHL = pool heater annual heating load; 

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

was taken from RECS 1993–2005. 

For units that have a pilot light, DOE assigned a pilot light input rate of 1,000 Btu per 

40, 45 hour. DOE assigned pilot lights to 26.5 percent of the households that have pool heaters 

based on the percentage of such models in the 2008 FTC directory. DOE assumed the remaining 

73.5 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. In the absence of adequate data on actual operating 

hours, DOE used a distribution around this average to assign values for pool heater operating 

hours to the sample households. The distribution ranges from 235 hours per year to 8,760 hours 

per year, where the upper limit represents a year-around operation. The value of 235 hours per 

year is the minimum value of the triangular distribution used to calculate pool operating hours. 

DOE assigned the sample households an existing pool heater having thermal efficiency 

based on the survey year. Thus, for example almost all pool heaters associated with RECS 2005 

households were installed after 1989 (based on a maximum lifetime for pool heaters of 16 years), 

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

previous survey the thermal efficiency is assumed to be 1-percent less (i.e., households in RECS 

2001 are assigned a pool heater having thermal efficiency 77-percent). 

7.4.3 Calculation 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: 

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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. 

E = BOH × PE − (POH − BOH ) × PE AE standby 

Where: 

EAE = average annual auxiliary electrical energy for pool heater 

(including stand-by electrical consumption), kWh/yr; 

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

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

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. For its LCC and PBP analyses, DOE used the full 

distribution of energy use values calculated for the sample households. 

Table 7.4.2 Annual Energy Consumption for Gas-Fired Pool Heaters 

Efficiency 

Level 

Average Energy 

Consumption Average Energy 

Savings (Gas) 

(MMBtu/yr) 

(Thermal 

Efficiency in 

Percent 

Gas 

(MMBtu/yr) 

Electricity 

(kWh/yr) 

78 (baseline) 38.2 2 -­

79 37.8 2 0.5 

81 36.9 2 1.4 

82 36.4 2 1.8 

83 34.9 9 3.4 

84 34.4 9 3.8 

86 33.6 9 4.6 

90 32.1 9 6.1 

95 30.5 9 7.8 

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