Field Performance of Heat 

Pump Water Heaters in the 

Northeast

C. Shapiro and S. Puttagunta

Consortium for Advanced Residential Buildings 

August 2013
NOTICE

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iii

Field Performance of Heat Pump Water Heaters in the Northeast

Prepared for:

The National Renewable Energy Laboratory

On behalf of the U.S. Department of Energy’s Building America Program

Office of Energy Efficiency and Renewable Energy

15013 Denver West Parkway

Golden, CO 80401

NREL Contract No. DE-AC36-08GO28308

Prepared by:

C. Shapiro and S. Puttagunta

Steven Winter Associates, Inc.

of the 

Consortium for Advanced Residential Buildings

61 Washington Street

Norwalk, CT 06854

NREL Technical Monitor: Cheryn Metzger 

Prepared under Subcontract No. KNDJ-0-40342-03

August 2013
iv

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v

Contents

List of Figures ............................................................................................................................................ vi

List of Tables ............................................................................................................................................. vii

Definitions................................................................................................................................................. viii

Executive Summary................................................................................................................................... ix

1 Introduction........................................................................................................................................... 1

1.1 Background..........................................................................................................................3

1.2 How Heat Pump Water Heaters Work.................................................................................4

1.3 Space Conditioning Interactions and Installation Considerations.......................................6

1.4 Model Operation and Control Logic....................................................................................6

1.4.1 General Electric Control Logic................................................................................6

1.4.2 A.O. Smith Control Logic........................................................................................7

1.4.3 Stiebel Eltron Control Logic....................................................................................8

2 Recent Studies...................................................................................................................................... 9

2.1 Laboratory Testing: U.S. Department of Energy 10 CFR Part 430 and Related Testing...9

2.2 Laboratory Testing: System Coefficient of Performance ..................................................11

2.3 Field Testing Studies..........................................................................................................12

2.4 Cost-Benefit Studies..........................................................................................................13

3 Technical Approach ........................................................................................................................... 14

3.1 Research Questions............................................................................................................14

3.2 Measurements....................................................................................................................14

3.3 Equipment..........................................................................................................................15

3.4 Analysis..............................................................................................................................16

4 Performance Results.......................................................................................................................... 18

4.1 Impact of Water Use on Efficiency ...................................................................................20

4.2 Impact of Air Temperature on Efficiency..........................................................................22

4.3 Delivered Water Temperature............................................................................................25

4.4 Calculating Standby Losses...............................................................................................28

4.5 Impact of Confined Spaces................................................................................................32

4.6 Impact on Space Conditioning Systems ............................................................................33

5 Energy and Cost Analysis ................................................................................................................. 35

6 Homeowner Surveys .......................................................................................................................... 40

7 Recommendations ............................................................................................................................. 42

8 Conclusion .......................................................................................................................................... 43

References................................................................................................................................................. 46

Appendix A: A History of Heat Pump Water Heaters in the United States ........................................ 51

The Early Years: 1935–1970 ...................................................................................................51

The Second Generation: 1970–1999........................................................................................52

The Third Generation: 1999–2010...........................................................................................52

Current Products: 2010–Present ..............................................................................................53

Outlook for the Future .............................................................................................................54

Appendix B: Alternative Water Heating Calculations ........................................................................... 55

Appendix C: Monitored and Modeled Energy Use and Costs by Site ................................................. 56

Appendix D: Identifying Heating Events................................................................................................. 58

Appendix E: Laboratory Tests of Heat Pump Water Heaters Under 10 CFR Part 430 ...................... 60

Appendix F: HPWH Informational Tri-Fold............................................................................................. 62
vi

List of Figures

Figure 1. Timeline of HPWH development in the United States ............................................................. 3

Figure 2. How HPWHs work ....................................................................................................................... 5

Figure 3. EF for GE GeoSpring by set point temperature. Blue represents laboratory 

measurements. Red indicates the rated EF..................................................................................... 11

Figure 4. Example of a HPWH monitoring system installation ............................................................ 16

Figure 5. Ideal operation of the GE HPWH in hybrid mode (Site 9) ..................................................... 20

Figure 6. Low water use can reduce the overall benefit of a HPWH when assessing cost benefit.. 21

Figure 7. Scatter plots of daily hot water use versus COP color coded by electric resistance 

fraction................................................................................................................................................. 22

Figure 8. Due to the Site 5 basement being too cold, this GE HPWH switched primarily to electric 

resistance mode during the winter months..................................................................................... 23

Figure 9. Ambient temperature at Site 11............................................................................................... 24

Figure 10. This HPWH benefited from the waste heat of the boiler used for space heating ............ 24

Figure 11. Typical winter performance of the GE HPWH in the Site 11 warm mechanical room ..... 24

Figure 12. Scatter plots of COP versus daily hot water use color coded by ambient temperature. 

Periods with electric resistance fractions > 0.04 and Sites 5 and 13 were excluded.................. 25

Figure 13. This Stiebel Eltron HPWH was able to maintain high COPs during high water demand ....

.............................................................................................................................................................. 26

Figure 14. Even with high water demand, the hot water temperature only dropped as low as 118°F..

.............................................................................................................................................................. 26

Figure 15. Normalized histograms of delivered water temperature. Red lines indicate set point 

temperatures. Green lines indicate minimum delivery temperature (110°F). The percentage of 

occurrences with water temperatures below 110°F is listed. ........................................................ 27

Figure 16. Regression results and data points for GE standby losses. Temperature difference is 

between the tank average temperature and ambient air. ............................................................... 31

Figure 17. Performance of Site 13 in confined space ........................................................................... 32

Figure 18. Box plots of maximum potential space conditioning impacts for monitored HPWHs .... 34

Figure 19. Annual operating cost of monitored HPWHs and alternative water heaters.................... 36

Figure 20. Annual source energy of monitored HPWHs and alternative water heaters .................... 37

Figure 21. Annualized energy related cost of monitored HPWHs and alternative water heaters..... 39

Figure 22. First patented HPWH (Source: Wilkes and Reed 1937)....................................................... 51

Figure 23. Results from using daily data for analysis........................................................................... 58

Figure 24. Example of five heating events as defined in this study .................................................... 59

Unless otherwise noted, all figures were created by CARB.
vii

List of Tables

Table 1. Performance Summary of Monitored HPWHs by Model ......................................................... ix

Table 2. Residential Water Heating Energy Consumption and Operating Costs (per annum) ........... 2

Table 3. Key Specifications of Integrated HPWHs Currently Available in the U.S. Market ................. 3

Table 4. Standard DOE Test Procedure Conditions ................................................................................ 9

Table 5. Aggregated Laboratory Studies................................................................................................ 10

Table 6. COP Values of Lab Studies ....................................................................................................... 12

Table 7. System COP Reference Studies and Test Conditions............................................................ 12

Table 8. Installed Monitoring Equipment................................................................................................ 16

Table 9. Summary Statics of Performance by Model. ........................................................................... 18

Table 10. Summary Table of Performance by Site ................................................................................ 19

Table 11. Tank Heat Loss Robust Regression Results With Constant (N = 113, df = 111) ............... 30

Table 12. Tank Heat Loss Robust Regression Results Without Constant (N = 113, df = 112).......... 30

Table 13. Alternative Water Heaters........................................................................................................ 35

Table 14. Fuel Prices and Characteristics for Energy and Cost Analysis .......................................... 36

Table 15. Cost Analysis Assumptions .................................................................................................... 37

Table 16. Installation Costs and Lifetimes of Water Heating Products............................................... 38

Table 17. Installed HPWH Cost Estimates from Study.......................................................................... 38

Table 18. Survey Results of Whether the Homeowner or a Qualified Professional Performed Any 

Preventive Maintenance Procedures................................................................................................ 40

Table 19. Survey Results of Homeowners.............................................................................................. 41

Table 20. Summary of Test Results by Model........................................................................................ 43

Table 21. Key Specifications of Integrated HPWHs Currently Available in the U.S. Market ............. 54

Table 22. Annual Site Energy Consumption by Site ............................................................................. 56

Table 23. Annual Source Energy (MMBtu) Consumption by Site ........................................................ 56

Table 24. Annual Operating Costs ($) by Site ........................................................................................ 57

Table 25. Annualized Energy Related Costs ($) by Site........................................................................ 57

Table 26. Air Generate AirTap.................................................................................................................. 60

Table 27. A.O. Smith Voltex Integrated ................................................................................................... 60

Table 28. Rheem EcoSense ..................................................................................................................... 61

Table 29. GE GeoSpring ........................................................................................................................... 61

Table 30. Stiebel Eltron Accelera 300 ..................................................................................................... 61

Unless otherwise noted, all tables were created by CARB.
viii

Definitions

AERC Annualized energy related cost

Btu British thermal unit

CO2 Carbon dioxide

COP coefficient of performance

df Degrees of freedom

DOE U.S. Department of Energy

EF Energy factor

EPRI Electric Power Research Institute

ERWH Electric resistance water heater

FHR First hour rating

GE General Electric

gpd Gallons per day

gph Gallons per hour

gpm Gallons per minute

kW Kilowatt

kWh Kilowatt-hour

lbm Pound mass

MMBtu Million British thermal units

N number of observances

NREL National Renewable Energy Laboratory

NREMD National Residential Efficiency Measures Database

HPWH Heat pump water heater

ORNL Oak Ridge National Laboratory

PG&E Pacific Gas & Electric Company

quad Quadrillion British thermal units

RE Recovery efficiency

SEE Source energy efficiency

SWA Steven Winter Associates, Inc.

therm 100,000 British thermal units

UA Overall heat loss coefficient

Wh Watt hour
ix

Executive Summary

Heat pump water heaters (HPWHs) are finally entering the mainstream residential water heater 

market. Potential catalysts are increased consumer demand for higher energy efficiency electric 

water heating and a new federal water heating standard that effectively mandates use of HPWHs 

for electric storage water heaters with nominal capacities > 55 gal. When compared to electric 

resistance water heating, the energy and cost savings potential of HPWHs is tremendous. 

Converting all electric resistance water heaters (ERWHs) to HPWHs could save American 

consumers $7.8 billion annually ($182 per household) in water heating operating costs and cut 

annual residential source energy consumption for water heating by 0.70 quads.

Steven Winter Associates, Inc. embarked on one of the first in situ studies of these newly 

released HPWH products through a partnership with two sponsoring electric utility companies, 

National Grid and NSTAR, and one sponsoring energy efficiency service program administrator, 

Cape Light Compact. Recent laboratory studies have measured performance of HPWHs under 

various operating conditions, but publically available field studies have not been as available. 

This evaluation attempts to provide publically available field data on new HPWHs by monitoring 

the performance of three recently released products (General Electric GeoSpring, A.O. Smith 

Voltex, and Stiebel Eltron Accelera 300). Fourteen HPWHs were installed in Massachusetts and 

Rhode Island and monitored for more than one year. Of the 14 units, 10 were General Electric 

models (50-gal units), two were Stiebel Eltron models (80-gal units), and two were A.O. Smith 

models (one 60-gal and one 80-gal unit).

Although this study used a small sample size and all of the water heaters were in unconditioned 

basements in homes in the Northeast, the HPWHs studied show great promise. Excluding one 

site, the monitored units had coefficients of performance (COPs) of 1.5–2.6. The excluded site

had ambient air temperatures < 50°F for much of the year resulting in excessive electric 

resistance backup use. The average COP for each model is provided in Table 1.

Table 1. Performance Summary of Monitored HPWHs by Model

Model Capacity

(gal)

Rated Energy 

Factor

First Hour 

Rating 

(gph)

Measured 

Average

COP

COP Range

General Electric 50 2.35 63.0 1.82 1.5–2.1

A.O. Smith 60/80 2.33 68.0/84.0 2.12 2.1

Stiebel Eltron 80 2.51 78.6 2.32 2.0–2.6

The monitored data show that the primary variables that affect HPWH performance are hot water 

use (daily volume and draw pattern) and ambient temperature. High hot water demand will 

reduce efficiency by increasing auxiliary electric resistance use. Higher ambient temperature will 

increase efficiency by increasing the efficiency of the heat pump and reducing standby losses.

While the General Electric unit shows two distinct operating regions that correspond to large and 

small electric resistance loads, the A.O. Smith and Stiebel Eltron units operate entirely in the low 

electric resistance region. This is most likely a product of the larger tank volumes and the control 

logic that allows the heat pump to reengage, in the case of A.O. Smith, or operate 

simultaneously, in the case of Stiebel Eltron.
x

Despite the slower recovery rate of the heat pump compared to electric resistance elements, all 

three models delivered hot water at temperatures above a minimum acceptable temperature of 

110°F during the vast majority of draws. The hybrid nature of these systems allows them to 

deliver hot water reliably. 

Unfortunately, the standby losses of the systems are higher than traditional ERWHs. Potential 

causes are the additional piping, wrap-around condensing unit, or inadequate insulation.

Installation of HPWHs in confined spaces was also found to reduce efficiency by approximately 

16%, which is consistent with other studies.

The HPWH monitoring results were compared to several alternative natural gas, electric 

resistance, fuel oil, and propane storage tank water heaters. Tankless water heaters were not 

considered. With the exception of condensing storage natural gas water heaters, annual operating 

costs and source energy consumption for the monitored HPWHs were lower than the alternative 

storage tank water heaters considered. The annualized energy-related costs—which are a 

measure of total lifetime costs and include first costs, operating costs, replacement costs, and the 

time value of money—of the monitored HPWHs were slightly lower than ERWHs and 

condensing natural gas water heaters. Annualized energy-related costs for HPWHs are 

considerably lower than those for propane- and fuel-oil fired systems. Natural gas storage water 

heaters, with the exception of condensing storage water heaters, have lower annualized energyrelated costs than all other options. Space conditioning interactions for HPWHs, however, may 

change the relative costs depending on the climate and location of the HPWH. Natural gas water 

heaters, however, are still the lowest cost storage water heater option on an annualized energyrelated cost basis.
1

1 Introduction

A confluence of regulatory and economic factors is rapidly pushing heat pump water heaters 

(HPWHs) into the mainstream residential marketplace. The primary regulatory catalyst is a new 

federal water heating standard that takes effect in 2015 that mandates energy factors (EFs) around 

2.0 for all new electric storage water heaters with capacities > 55 gal (DOE 2010).1 This regulation 

is a major driver of change in residential water heating technologies (Maynard 2011) because it 

effectively requires installation of HPWHs in applications with large hot water loads and where 

electricity will be used for water heating. In addition, for energy-conscious consumers looking to 

decrease energy use, HPWHs are currently the only ENERGY STAR®-qualified electric water 

heating products on the market (EPA 2012).

In addition to a changing regulatory environment in the residential electric water heating market, 

financial factors are also pushing HPWHs into the mainstream. Inflation of residential retail 

electricity prices significantly outpaced general inflation, as measured by the Consumer Price Index, 

between 2002 and 2009.

2 While electricity prices have since stabilized due to a slowdown in 

economic growth and declining natural gas prices (EIA 2012a), the relative increase in electricity 

prices over the past decade has played an important role in HPWH development. Furthermore, 

while HPWHs have higher first costs than traditional electric resistance water heaters (ERWHs), 

many utility companies are offering sizeable rebates for HPWH installations3 in the hopes of 

making HPWHs more attractive in the residential marketplace.

The move to HPWHs from standard ERWHs is not trivial; energy used by electric water heaters is a 

substantial portion of total residential energy consumption, and HPWHs are significantly more 

efficient than traditional ERWHs. Water heating is the second largest contributor (2.11 quads or 

20% of site energy) to residential energy consumption in the United States after space heating (EIA 

2005), and nearly 44% of American households use electricity as their primary water heating fuel 

(EIA 2009). To demonstrate the magnitude of the energy consumed through residential water 

heating, site energy, source energy, and annual operating costs are listed in Table 2.

 

1 The regulation specifies the minimum EF as a function of rated storage volume, EF = 2.057-0.00113V, which 

corresponds to an EF range of 1.92–1.99 for rated tank volumes between 55 and 119 gallons, respectively. 2 The average residential retail price of electricity increased 36% between 2002 and 2009 (EIA 2012b), while the 

increase in the consumer price index was 19% over the same period (BLS 2012). 3 For example, Massachusetts utilities offer up to $1,000 (Mass Save 2012).
2

Table 2. Residential Water Heating Energy Consumption and Operating Costs (per annum)

Fuel

Number of 

Households 

(millions)

Site Energy Source Energy Operating Costs

Per 

Household

U.S. 

Total 

(quads)

Per 

Household 

(MMBtu)

U.S. 

Total 

(quads)

Per 

Household 

($)

U.S. 

Total

(billion 

$)

Electricity

(kWh) 43.3 2,813 0.42 32.3 1.40 330 14.28

Natural Gas

(therms) 59.8 241 1.44 26.3 1.57 265 15.80

Fuel Oil

(gal) 4.3 226 0.14 36.6 0.16 887 3.81

Propane

(gal) 5.8 277 0.15 29.2 0.17 790 4.58

Source: Adapted from EIA (2005). 

Since all new HPWHs have a listed EF of 2.0 or higher, compared with EFs of 0.9–0.95 for 

resistance water heaters,

4 if all electric water heaters were replaced with HPWHs and these water 

heaters performed at their rated efficiencies, American consumers could save $7.8 billion annually

(average of $182 per household) in water heating operating costs and cut annual residential source 

energy consumption for water heating by 0.70 quads. As HPWHs are not appropriate in all 

circumstances, and they may increase space-conditioning loads in some cases, these figures 

represent the upper limit of potential savings based on EF ratings associated with the move from 

ERWHs to HPWHs in the United States.

At the time this report was published, there were five major integrated5 HPWH products in the 

American market. Key specifications for these HPWHs are shown in Table 3. These specifications 

include the two U.S. Department of Energy (DOE) performance metrics—EF and first hour rating 

(FHR).6 The EF represents the efficiency of the electric heating elements and tank losses under a 

specific 24-h test procedure. The FHR represents the amount of hot water that can be supplied by a 

fully heated storage water heater during the first hour of operation. Storage tank volume is often 

used as a proxy for water heating capacity, but FHR is the more appropriate metric. See Section 2.1

for more details regarding the EF and FHR test procedures.

 

4 EF is meant to represent the efficiency of residential water heaters as a fraction. For example, 0.92 EF water heaters 

are expected to be 92% efficient when tested under the rating procedure. See Section 5.1 for more information about the 

EF test. 5 Integrated refers to water heaters in which the heat pump and tank are packaged together as a one-piece, drop-in unit.

There are add-on HPWHs that are plumbed into existing storage water heaters. 6 Recovery efficiency (RE) is also used to quantify the efficiency of natural gas, fuel oil, and propane water heaters. RE

is an approximate metric of the burn efficiency of the heating device. Since electric resistance elements have an 

efficiency of 1, RE is not tested or reported. 
3

Table 3. Key Specifications of Integrated HPWHs Currently Available in the U.S. Market7

Model Capacity 

(gal) EF FHR(gal)

Electric Resistance 

Elements (kW)

GE GeoSpring 50 2.35 63.0 Upper: 4.5

Lower: 4.5

A.O. Smith Voltex 60/80 2.33 68.0/84.0 Upper: 4.5

Lower: 2.0

Stiebel Eltron Accelera 300 80 2.51 78.6 Upper: 1.7

Rheem EcoSense 40/50 2.00 56.0/67.0 Upper: 2.0

Lower: 2.0

AirGenerate AirTap 

Integrated 50/66 2.39/2.40 60.0/75.0 Upper: 4.0

1.1 Background

While regulatory and economic factors may finally be pushing HPWHs into the mainstream 

residential market, HPWHs are not new. The first patented HPWH dates back to 1935, and they 

were first commercialized in the 1950s. For more than 50 years, HPWHs have followed a boombust cycle of product development and subsequent abandonment that closely follows retail 

electricity prices and demonstrates the reliability issues of past products. See Figure 1 for a timeline 

of the development of HPWHs in the United States and Appendix A for more detailed information.

Figure 1. Timeline of HPWH development in the United States

 

7 Any omission of a manufacturer or product is unintentional, and no endorsement of any commercial product or 

manufacturer is implied.
4

As a result of the convergence of regulatory and financial forces that have pushed HPWHs into the 

mainstream residential market and the magnitude of the savings potential represented by this 

market, Steven Winter Associates, Inc. (SWA) embarked on one of the first in situ studies of these 

newly released HPWH products. Recent laboratory studies have quantified the performance of 

HPWHs under various operating conditions, but publically available field studies have not been as 

forthcoming. This evaluation attempts to provide publically available field data on new HPWHs by 

quantifying the field performance of three recently released products (General Electric [GE] 

GeoSpring, A.O. Smith Voltex, and Stiebel Eltron Accelera 300).

Fourteen HPWHs were installed in Massachusetts and Rhode Island and monitored for more than 

one year. The sites were chosen in the residential markets of the sponsoring electric utility 

companies, National Grid and NSTAR, and the sponsoring energy efficiency service program 

administrator, Cape Light Compact. Of the 14 units, 10 were GE models (50-gal units), two were 

Stiebel Eltron models (80-gal units), and two were A.O. Smith models (one 60-gal and one 80-gal 

unit). The ten 50-gal units were intended to be split between the GE and Rheem HPWHs, but the 

Rheem units were not available during the installation phase of this project.

1.2 How Heat Pump Water Heaters Work

HPWHs are primarily designed as replacements for traditional ERWHs and are able to achieve 

higher efficiencies by using the vapor compression heat pump cycle. Generally speaking, heat 

pumps are devices—such as air conditioners and refrigerators—that move thermal energy from one 

location to another. While a refrigerator moves heat from the inside of the appliance into the 

kitchen, the heat pump inside a HPWH moves heat from the surrounding air into the hot water 

storage tank. By moving thermal energy instead of converting electricity to heat, heat pumps are 

more efficient and operate at efficiencies > 200% in most cases.

Most HPWHs, however, are hybrid devices that combine a heat pump, backup electric resistance 

element(s), and a storage tank (Figure 2). While the heat pumps in these hybrid water heaters can 

heat water at high efficiencies, the recovery rate of these mechanisms is significantly slower than 

that of traditional electric resistance heating mechanisms. A typical 4.5-kW electric resistance 

element can reliably heat more than 20 gph of water. The heat pump has a lower heating rate; GE, 

for example, publishes a rate of 8 gph at 68°F air temperature (General Electric 2010). As a result, 

auxiliary electric resistance elements are also installed in HPWHs for reliability and quicker hot 

water recovery. Most HPWHs use the heat pump whenever possible, but built-in controls switch to 

conventional resistance heating when there are large hot water needs. 
5

Figure 2. How HPWHs work

Historically, heat pumps have been differentiated into two categories: integrated (or “drop-in”) and 

remote (or “add-on”). These categories describe the relationship between the heat pump and the 

storage tank. Add-on devices are standalone heat pumps that are combined with traditional electric 

resistance tank water heaters. Integrated devices include the heat pump, storage tank, and resistance 

element(s) in one package and have the advantage of being able to control the operation of the heat 

pump and resistance elements more precisely. Add-on devices, however, can be used as retrofits to 

existing water heaters at a lower cost (Ashdown et al. 2004).

Integrated devices come in three different configurations of heat exchangers. The most common 

configuration in the United States is a wrap-around condenser. In this configuration, the refrigerant 

coils are wrapped around the storage tank, but are not in direct contact with the potable water inside 

the tank. Immersion coils, where the refrigerant coils are directly immersed in the potable water, are 

used in the Air Generate AirTap. Finally, coaxial heat exchangers can be used, as with the Rheem 

EcoSense, where the water is pumped through the heat exchanger to transfer energy from the heat 

pump to the potable water (Sparn et al. 2011).

The most common arrangement of current HPWHs is an integrated water heater with a wrap-around 

condenser and two backup electric elements. Figure 2 illustrates this arrangement and describes the 

typical components and operation of modern HPWHs in the U.S. market. Among the systems 

evaluated in this study, the only model to deviate from this configuration is the Stiebel Eltron unit.

The Stiebel Eltron HPWH has only one smaller (1.7-kW) upper element and always operates in 

hybrid mode.
6

1.3 Space Conditioning Interactions and Installation Considerations

HPWHs move thermal energy from the surrounding air into the storage tank. Therefore, units 

installed in conditioned spaces will directly affect the space conditioning loads of the building. 

During the heating season, HPWHs increase loads on space heating systems. Conversely, HPWHs 

will reduce cooling loads during the cooling season. When HPWHs are installed in unconditioned 

spaces such as attics and garages (possible in warm climates only), space conditioning impacts are 

minimal. When in “quasi-conditioned” spaces—such as uninsulated basements—space conditioning 

impacts of HPWHs are very difficult to assess. As most basements are thermally connected to 

conditioned space, HPWHs will still have an impact on conditioning loads. Finally, by using a 

vapor compression system, HPWHs typically remove moisture from the surrounding air, which can 

be a significant benefit in hot-humid climates and damp basements. 

Since these hybrid HPWHs are new to the mainstream market, installation will not be as 

straightforward for installers as for traditional ERWHs. HPWHs require special attention to airflow 

around the unit and condensate collection. Furthermore, to improve the efficiency of the units, A.O. 

Smith and Stiebel Eltron manufacture models substantially larger than typical ERWHs. Installers 

may not be familiar with these units or heat pump models in general, and it may be difficult to 

install these units in existing homes. Additional licensing considerations are also applicable to 

HPWHs. As with all systems connected to potable water, installers are typically required to be 

licensed plumbers. In some cases state or local regulations may be vague as to whether licensed 

plumbers may install HPWHs without also obtaining additional licensing for mechanical or 

electrical trades (Ashdown et al. 2004).

During installation the Consortium for Advanced Residential Buildings noted specific challenges 

faced by the installer at the various sites. Many of these details are discussed in more depth by

Shapiro et al. (2012). Installation considerations are further discussed in SWA (2012), which SWA 

worked on with the sponsors of this evaluation for use in their rebate programs. In addition, a trifold brochure for consumers (Appendix F) was created for their rebate programs.

1.4 Model Operation and Control Logic

While all three of the monitored HPWHs have similar components, the control logic differs 

substantially between models. The GE and A.O. Smith tanks have two electric resistance (upper and 

lower) elements in addition to the heat pump and operate in several modes, while the Stiebel Eltron 

unit has only one mode of operation and uses a small upper element to supplement the heat pump.

All three units have wrap-around condensers and an integrated storage tank.

1.4.1 General Electric Control Logic

The GE model has two 4.5-kW electric resistance elements, one placed at the top third and one

placed at the bottom third of the unit. The unit can operate under five operating modes, Hybrid, 

eHeat, Standard Electric, High Demand, and Vacation (General Electric 2009). The control logic for 

these modes is described below:

• Hybrid Mode is the default operating mode. While the control logic for the GE unit is 

proprietary, the general operating principle behind the hybrid mode is to balance energy 

savings while providing hot water at rates similar to comparably sized ERWHs. As a result, 

the control logic tries to meet certain targets for availability of hot water. The unit will use 

the heat pump until approximately 75% of the available hot water has been depleted and the 
7

heat pump cannot keep up with the hot water demand. Since the unit does not measure 

average tank temperature or hot water flow rate directly and measures only the temperature 

at the top third of the tank, the control logic uses the current and previous values of the

temperature at the top of the tank as a proxy for available hot water and hot water flow rate. 

The system uses a closed-form model that employs autocorrelation functions to map upper 

tank temperature to hot water availability (Tsai 2012). 

In practice, this control strategy results in the electric elements being used in three 

circumstances. If the ambient air temperature is outside the safe operating range (45°–

120°F), the system will revert to standard resistance mode. If the water in the tank is 

significantly lower than the set point, then upper element will operate. The difference 

between the tank temperature and the set point depends on the circumstances, but is 

generally 25°–30°F. If the system senses that the water use is too high, then the lower 

element will operate. In general, 25–30 gal within a short time period is considered high 

water use. Once the lower electric resistance element engages, the entire tank will be 

reheated like a traditional ERWH (Tsai 2011).

• eHeat Mode uses only the heat pump, unless the ambient temperature is outside the safe 

operating range. This mode is more efficient, but may fail to provide water at the set point

temperature. The temperature dead band at the top of the tank that regulates heat pump 

operation is 1°F (Sparn et al. 2011)

• Standard Electric Mode operates like a traditional ERWH.

• High Demand Mode is similar to the Hybrid Mode, but the control logic changes from 75% 

of the available hot water being depleted to 50% (Tsai 2012). 

• Vacation Mode is similar to eHeat Mode, but with a temperature set point of 50°F.

1.4.2 A.O. Smith Control Logic

The A.O. Smith model has two electric resistance elements, a 4.5-kW upper element and a 2.0-kW 

lower element. The A.O. Smith model has four operating modes, Hybrid, Efficiency, Electric, and 

Vacation (A.O. Smith 2010). The control logic of the A.O. Smith unit, however, is quite different 

than the GE unit:

• Hybrid Mode of the A.O. Smith model uses a simple temperature dead band algorithm. If 

the average tank temperature—which is the weighted average of upper and lower 

thermostats, where the upper thermostat gets 75% weighting (Sparn et al. 2011)—drops 9°F 

below the set point, the heat pump will be turned on to heat the water back to the set point. 

If, however, the heat pump fails to heat the water sufficiently, and the average tank 

temperature drops more than 20°F below the set point, the upper element will replace the 

heat pump as the heating source. The lower element is not used in hybrid mode (A.O. Smith 

2011). Unlike the GE unit, the heat pump may reengage during the reheat cycle after the 

electric resistance elements have been operating.

• Efficiency Mode does not use the electric resistance elements, unless the ambient 

temperature is outside the safe operating range (45°–109°F) of the heat pump (A.O. Smith 

2011).
8

• Electric Mode operates like a traditional ERWH. The upper element will be used first to 

heat the top of the tank, and then the lower element will be used to heat the bottom of the 

tank (A.O. Smith 2010).

• Vacation Mode is identical to the efficiency mode with a set point of 60°F (A.O. Smith 

2010).

1.4.3 Stiebel Eltron Control Logic

The Stiebel Eltron has only one operating mode. The hot water temperature set point is factory set 

at 140°F and is not easily adjustable by the user. The unit has one 1.69-kW electric resistance 

element installed vertically at the top of the tank, and operates under a fixed operation mode. The 

heat pump is turned on when the temperature 16 in. from the top of the internal tank drops more 

than 4°F below the set point. If the heat pump cannot meet the demand, and the temperature at the 

top of the tank drops below 112°F, the upper element is used as a backup heat source. The upper 

element will heat only the top third of the water heater tank (approximately 27 gal). The Stiebel 

Eltron unit is the only unit that allows simultaneous operation of the heat pump and booster 

resistance heater (Megliola 2011).
9

2 Recent Studies

Since the introduction of the most recent line of heat pump water heating products, a growing body 

of literature has been devoted to measuring the performance, energy, and cost savings potential, and 

reliability of these new HPWHs. These reports can be broken down into four basic categories: (1) 

laboratory tests using variations on the approved DOE 10 CFR Part 430 testing procedures (DOE 

1998); (2) laboratory tests using different draw profiles or analysis metrics that attempt to simulate 

real-world operating conditions; (3) field tests of in situ performance of HPWHs under real, random 

operating conditions; and (4) cost-benefit analyses that analyze the cost effectiveness of HPWHs as 

an energy reduction strategy. In order to compare the results of this field testing study to those of 

previous studies, all publically available prior research was aggregated.

2.1 Laboratory Testing: U.S. Department of Energy 10 CFR Part 430 and 

Related Testing

While EF and FHR are measured by qualified laboratories and advertised prominently for every 

HPWH, several laboratories have used variations on these tests to measure how various operating 

variables affect the rated EF and FHR. Furthermore, the tank heat loss coefficient (UA) and RE, 

which can be calculated from the standard test but are not reported, were calculated by many of 

these laboratories. The standard operating conditions for these tests are shown in Table 4.

Table 4. Standard DOE Test Procedure Conditions

Measurement Required Testing Conditions

Ambient Temperature (°F)a 67.5 ± 1.0

Ambient RH (%)b 50 ± 12

Supply Water Temperature (°F) 58 ± 2

Set Point Temperature (°F) 135 ± 5

Flow Rate (gpm) 3.00 ± 0.25 a For HPWHs only; other systems may be 67.5°F ± 2.5°F

b For HPWHs only; other systems have no relative humidity requirements.

The EF represents the efficiency of the electric heating elements and tank losses under a consistent, 

24-h test procedure. This procedure is known as the 24-h simulated use test and is meant to provide 

a consistent metric with which to measure residential water heating products. In this procedure, 64.3

± 1 gal of water are drawn from the tank in six equal draws spaced 1 h apart. The EF is the ratio of 

energy output to energy input during the test procedure adjusted for the change in the stored energy 

of the tank during the test.

The RE is meant to represent the efficiency of the heating system (i.e. the conversion efficiency of a 

fossil fuel system or coefficient of performance [COP] of a heat pump system). The RE is measured 

using the recovery of the system following the first draw of the simulated use test. The RE is the 

ratio of the energy drawn from the tank over the energy use of the water heater, correcting for the 

change in the stored energy of the tank during the test. RE is not reported for HPWHs, although it 

could be directly measured from the rating procedure.

The FHR is meant to represent the amount of hot water that can be delivered by a residential water 

heater during a 1-h period if the tank begins fully heated. During the test, water is drawn at 3.0 ± 

0.25 gpm until the outlet temperature of the water drops 25°F below the maximum delivered 
10

temperature. A new draw is initiated once the heating element(s) have stopped operation. The FHR

is the amount of water removed from the tank during the test, corrected for the elapsed time of the 

test, which can vary depending on the recovery time of the heating elements and storage volume of 

the tank.

Recent studies have measured the performance of HPWHs using the standard DOE rating 

procedures specified under 10 CFR Part 430. In addition to reporting the EF and FHR, many of 

these studies also report the RE and UA, which are both defined in 10 CFR Part 430, but not 

reported. Furthermore, many of these studies perform the standard tests under conditions that are 

slightly different from those specified in 10 CFR Part 430. These different testing conditions allow 

comparisons between the field performance of water heaters and the laboratory testing ratings at 

varying operating conditions. In order to facilitate these comparisons, laboratory ratings from these 

studies (Table 5) are aggregated in the tables in Appendix E for the five major HPWH products 

currently on the market. The tables in Appendix E compare the laboratory values that were 

determined for EF, FHR, UA, and RE. 

Table 5. Aggregated Laboratory Studies

Reference 

Study Source Units Studied

A Norhwest Energy Efficiency Alliance

(Larson and Logsdon 2012) GE

B Bonneville Power Administration

(Larson et al. 2011) Rheem and GE 

C Electric Power Research Institute (EPRI)

(Amarnath and Trueblood 2010) A.O. Smith, Rheem, and GE 

D Pacific Gas and Electric (PG&E)

(PG&E 2010) Rheem and GE

E National Renewable Energy Laboratory (NREL)

(Sparn et al. 2011)

Air Generate, A.O. Smith, Rheem, 

GE, and Stiebel Eltron

F Advanced Energy 

(Fitzpatrick and Murray 2011) GE

G Florida Solar Energy Centera

(Colon 2012) GE

a Only measured system COP8 and not included in 10 CFR Part 430 tables

The results in the tables in Appendix E show EF values and FHRs that are all over the map. The 

variation in tested EF is unsurprising, considering the large error in the tested values implicitly 

allowed in the EF test (Healy et al. 2003) and the fact that many of these laboratories are not 

certified under 10 CFR Part 430 to test water heaters. For example, the EF measurements for the 

laboratory tests summarized in Appendix E are plotted against the set point temperature in Figure 3. 

The values are considerably different from each other and consistently higher than the rated value. 

 

8 COP is a fraction that represents the efficiency of a system. In the case of HPWHs, COP may represent the efficiency 

of the heat pump or the overall system. In this paper, the COP represents the COP of the overall system. COP is in the 

same units as EF, except it does not have to be calculated under the operating conditions specified by the test procedure.
11

The results, however, do show the expected inverse relationship between set point temperature and 

efficiency. The best fit equation shown in Figure 3 can be used to determine the effect of using a 

lower set point temperature than the set point used under the DOE test.

Figure 3. EF for GE GeoSpring by set point temperature. Blue represents laboratory measurements. 

Red indicates the rated EF.

The EF measurements of the GE and Rheem units operating in electric resistance mode offer some 

interesting insights into the performance of these water heaters. Surprisingly, the measured EF in 

electric resistance mode for these units was 0.84 and 0.82, respectively, which is considerably lower

than a comparably sized ERWH. As a result, the thermal insulation of these units must be lower 

than comparable ERWHs. A possible explanation for this difference is the existence of thermal 

shorts introduced by the wrap-around heat exchanger in the GE unit or the piping for the heat pump 

in the Rheem unit. Improvements could be made to reduce the thermal heat loss for these tanks.

2.2 Laboratory Testing: System Coefficient of Performance

COP values that were generated in laboratory studies are displayed in Table 6. The difference 

between COP values can be explained by difference in testing methods used by each lab. Data from 

these tables were compiled through many of the research labs listed in Table 5. Table 7 gives details 

of the methods used and the conditions which were present for each test. The reference study and 

test condition column is a matrix combination of reference study letters (A, B, C, etc.) and 

individual test conditions for each reference study (1, 2, 3, etc.) that is used to categorize each 

individual test. The test description column provides information on the individual conditions and 

test methods.

y = -0.0161x + 4.5852

R2 = 0.9082

2.3

2.35

2.4

2.45

2.5

2.55

2.6

2.65

2.7

2.75

115 120 125 130 135 140 145

Energy Factor

Setpoint Temperature (°F)
12

Table 6. COP Values of Lab Studies

Brand and Model Reference Study and Test Condition

B1 C1 E1 E2 E3 G1

Air Generate AirTap – – 1.68 2.77 – –

A.O. Smith Voltex 2.3 2.2 3.55 3.42 2.62 –

GE GeoSpring 1.7 2.1 1.21 1.81 3.46 2.27

Rheem EconSense 1.1 2.0 1.44 2.61 2.53 –

Stiebel Eltron Accelera 300 – – 3.85 5.37 3.09 –

Table 7. System COP Reference Studies and Test Conditions

Reference Study 

and Test 

Condition

Test Description

B1 Total daily water use of 49 gal, and the average COP is based on 

heating water from 90°F to 120°F

C1

Performed at DOE 24-h EF test conditions with set point at 120°F

Single continuous water draw

Average 3-gpm flow rate

Draw is stopped when hot water outlet decreases by 25°F 

E1

Draw profile to represent high demand situation in morning. Consists 

of 4 showers (64 gal) over the course of 1 h and 8 mins.

Test begins with tank full of 120°F water

E2

Draw profile to represent medium demand situation in evening. 

Consists of range of flow rates and duration to simulate food 

preparation and dishwashing and draws 40 gal in 4 h.

Test begins with tank full of 120°F water

E3

Draw profile to represent low demand situation (consists of many 

short, uninterrupted draws). Draws 20 gal in 5.75 h

Test begins with tank full of 120°F water 

G1 Alternating Building America and ASHRAE draw profiles in 

uncontrolled conditions in Cocoa, Florida.

2.3 Field Testing Studies

Unfortunately, robust and publically available field test results for the new HPWHs have not been 

as readily available as laboratory testing results. PG&E monitored one GE HPWH in Sonora, 

California. The unit was placed in an unconditioned basement with a floor area of approximately 

400 ft2

. The average COP was 1.29 with average operating conditions of 68.6°F ambient 

temperature, 62.0% relative humidity, 70.5°F inlet water temperature, 1.3 gpm flow rate, 18.6 draw 

events per day, and 18.3 gpd (Hu and Davis 2011).

Working with four electric utility companies—Bonnevile Power Administration, Kansas City 

Power & Light, Tennessee Valley Authority, and Southern Company—EPRI has been testing 160 

water heaters across the U.S. The vast majority of these water heaters are GE, A.O. Smith, and 

Rheem HPWHs, but several ERWHs were installed as control units. EPRI has provided preliminary 
13

results from the study, but unfortunately the results are identified only as Model A, Model B, and 

Model C. The COPs of these units were 0.7–2.7, with a large variation in performance. Model B 

was generally the best performer, while Model C was the worst performer. Reliability has been high 

so far for these units, but EPRI saw only minimal peak demand savings (Amarnath and Bush 2012).

2.4 Cost-Benefit Studies

A 2009 PG&E HPWH study (PG&E 2009) concluded that, in terms of source energy9 efficiency

(SEE), a HPWH was more efficient (67% SEE) than a standard natural gas (57% SEE) or electric 

(29% SEE) tank water heater when the heating, ventilation, and air conditioning interaction of the 

HPWH is ignored. If the heating, ventilation, and air conditioning interaction is included, then 

HPWHs are significantly more efficient (104% SEE) than both standard systems in the cooling 

season and less efficient (44% SEE) than natural gas tank water heaters in the heating season. 

According to a 2010 life cycle cost analysis paper (Franco et al. 2010) by Lawrence Berkeley 

National Laboratory, HPWHs have the lowest life cycle cost in roughly half of all single-family 

homes that heat water with electricity. This study assumed that some houses would need venting for 

successful HPWH installation, and as a result, many older homes could not install HPWHs cost 

effectively. Furthermore, there was a greater cost benefit of HPWHs in new, single-family homes.

A 2012 NREL study (Hudon et al. 2012) modeled HPWH performance against gas and electric 

water heaters in six cities in the United States. HPWHs were found to save source energy compared 

to traditional electric water heaters regardless of climate and location of the unit (i.e., whether 

located in conditioned or unconditioned space). Savings over natural gas depended on climate and 

location of the unit. 

 

9 See Section 5 for discussion of source energy.
14

3 Technical Approach 

This evaluation of HPWHs provides valuable information that can be used to advise consumers and 

builders on efficient methods to provide electric water heating. In addition, this information can be 

used as validation by utilities and other program implementers throughout the United States 

attempting to develop incentive programs for this technology. While this study is primarily 

applicable to colder climates, the measured performance of the HPWH units themselves is relevant 

in many climate zones. Even though the sample for this evaluation was relatively small, there were 

some clear results consistent with other ongoing field testing across the country.

3.1 Research Questions

This research effort focused on answering the following questions regarding the efficiency, 

reliability, and performance of each model evaluated: 

• What is the measured efficiency of a HPWH located in unfinished basements of cold climate 

homes?

• What are the critical criteria that affect the installed performance of HPWHs, and how do 

they impact the performance of each HPWH model? 

• What are the standby losses for these HPWHs, and how do they compare to traditional 

ERWHs?

• Does each model evaluated in this study effectively deliver hot water at the set point

temperature?

• Are homeowners satisfied with hot water delivery, efficiency, noise, and other 

characteristics? 

3.2 Measurements

Long-term performance data was collected at 14 sites in Massachusetts and Rhode Island in the 

service districts of National Grid, NSTAR, and Cape Light Compact. Measurements were taken for 

a minimum of 12 months at all sites to establish the efficiency and performance of each unit on an 

annual basis. These measurements included water temperatures, flow rates, and electricity 

consumption. Sensors were sampled at 5-s intervals and output at 15-min intervals in the form of 

averages, minimums, maximums, and/or totals over that time period, depending on the desired 

outputs. An additional data table captures the duration and volume of each hot water draw.

At each of the sites, the following HPWH parameters were measured every 5 s:

• Inlet water temperature (°F)

• Outlet water temperature (°F)

• Ambient air temperature (°F)

• Ambient air relative humidity (%)

• Hot water flow (gal)

• Compressor energy consumption (Wh)
15

• Energy consumption of each resistance heating element (Wh)

• Entire system energy consumption (Wh).

At each 5-s interval, thermal heat delivered by the water heater was calculated based on water flow 

and temperature differential.

The following values were output for each 15-min logging period:

• Average water inlet temperature (°F)

• Average water outlet temperature (°F)

• Minimum water inlet temperature (°F)

• Maximum water outlet temperature (°F)

• Average inlet air temperature (°F)

• Average inlet air relative humidity (%)

• Total domestic hot water use (gal)

• Domestic hot water energy (Btu)

• Total compressor energy consumption (Wh)

• Total upper heating element energy consumption (Wh)

• Total lower heating element energy consumption (Wh)

• Total system energy consumption (Wh)

• Total heat pump energy (Wh)

• Total standby energy consumption (Wh).

In addition, the start time, volume, and duration of each hot water draw were recorded.

3.3 Equipment

As shown in Figure 4, a Campbell-Scientific CR-1000 data logger and various sensors (Table 8) 

were used at each site to take measurements. A wireless modem was used for the remote 

downloading of data for evaluation throughout the monitoring period. Thermistors were used for all 

temperature measurements. Cold water inlet and hot water outlet water temperature were measured 

using an Omega tubular immersion sensor with a 4.5-in. probe length and national pipe threads. 

These were installed directly in the water flow by the HPWH installer. To measure domestic hot 

water flow, Omega FTB4607 low flow (0.22–20 gpm), turbine-type flow meters were used. All 

flow meters were installed by the HPWH installer and located on the cold water inlet side.
16

Figure 4. Example of a HPWH monitoring system installation

Table 8. Installed Monitoring Equipment

Measurement Equipment

Record and Output Measurements Campbell-Scientific CR-1000 datalogger 

Inlet and Outlet Water Temperatures Omega ON-910-44006 NPT pipe plug thermistor 

Inlet Air Temperature and Relative 

Humidity Humirel HTM2500 probe

Compressor Energy, Heating Element 

Energy, and System Energy

Continental Control Systems WattNode WNB-3D240-P watt-hour transducer with appropriately sized 

current transformers

Hot Water Flow Omega FTB4607 low flow, turbine-type flow meter

Air temperature and relative humidity in the space surrounding the HPWH were measured by a 

Humirel HTM2500 located to minimize heat transfer from radiation and surrounding equipment. 

All electrical energy consumption measurements used a Continental Control Systems WattNode and 

right-sized current transformers. The WattNodes are true-root mean squared, alternating current, 

Wh transducers with pulse outputs.

3.4 Analysis

The COP has been defined differently in numerous studies (AIL Research 2001, 2002; Murphy and 

Tomlinson 2002; Zogg 2002). For this evaluation the standard definition of COP, which is the net 

heat delivered by the water heater to the domestic water load divided by the total electrical energy 

consumed over a period of time was used

net energy input 3.413Btu/Wh

useful heating energy

× = =

DHW

DHW

W

Q COP , (1)

where
17

COP = coefficient of performance (dimensionless),

QDHW = useful heat energy (Btu), and

WDHW = energy consumed by the HPWH (Wh).

The water heating energy QDHW was calculated by the datalogger every 5 s using measured data. 

These energy values were summed and logged at 15-min intervals.

( ) QDHW = CpV Tout −Tin ρ  , (2)

where

Tout = outlet water temperature (°F),

Tin = inlet water temperature (°F),

V = hot water volumetric flow rate (ft3

/h),

Cp = specific heat of water (Btu/lbm∙°F), and

ρ = density of water (lbm/ft3

).
18

4 Performance Results

Measured performance as well as the rated capacity, EF, and FHR, for each of the monitored 

models are shown in Table 9. All systems were set to hybrid mode, but there were differences in set 

point temperature. The electric resistance percentage represents the fraction of electricity consumed 

by the electric resistance elements (rather than heat pump, controls, or peripherals). While this is a 

small sample and many variables affect water heater performance, the values do provide insight into 

some differences between the units.

Table 9. Summary Statics of Performance by Model.

Model Capacity

(gal)

Rated

EFa FHRb

Measured 

Average

COP

COP 

Range

% Electric 

Resistance

GE 50 2.35 63.0 1.82c

/1.61 1.0–2.1 32.7%c/44.0%

A.O. Smith 60/80 2.33 68.0/84.0 2.12 2.1–2.1 5.6%

Stiebel Eltron 80 2.51 78.6 2.32 2.0–2.6 5.5% a COP and EF are similar, except the EF is the ratio of heat output to energy input as measured during a specific 24-h 

test procedure. b FHR is measured in gph. c Average COP calculation for the GE units does not include Site 5 (cold air = high electric resistance use)

Average COPs over the entire monitoring period were lower than the rated EF for all three models. 

The A.O. Smith and Stiebel Eltron models were only slightly lower than the rated EF, while the GE

model showed a more pronounced difference between measured COP and rated EF. These results 

are influenced heavily by storage volumes, set point temperature, and the ability to meet high 

demand over short periods of time. While the A.O. Smith and Stiebel Eltron units used the heat 

pump to provide the vast majority of the load (approximately 95% of the total electricity was 

consumed by the heat pump), the electric resistance elements in the GE units consumed almost one

third of the measured electricity (excluding Site 5, where low ambient temperatures forced the unit 

into resistance mode most of the time). The A.O. Smith and Stiebel Eltron models benefit from 

larger storage tanks, and the Stiebel Eltron model benefits from a factory set set point temperature 

of 140°F, which increases the availability of hot water. Increased hot water availability can increase 

COP by preventing electric resistance use.

More detailed summary statistics for each site are shown in Table 10. These statistics include 

operational conditions as well as efficiency values. While the two A.O. Smith units had remarkably 

similar COPs, the COPs of the Stiebel Eltron and GE units varied significantly between sites. The 

difference between the COPs of the Stiebel Eltron sites is largely attributable to the large difference 

between the average daily hot water draws at the two sites. The residents at Site 2 used an average 

of 73 gpd with a COP of 2.6, while the residents at Site 10 used an average of 41 gpd with a COP of 

2.0. Larger hot water draw volumes will dilute the impact of tank thermal losses and elevate the 

COP of the unit—as long as larger draws do not increase the use of resistance heating. As a result, 

Site 2 has a COP slightly higher than the rated EF. The differences between the measured COPs at 

the sites with GE units were significantly larger than those of the other units. With a smaller tank, it 

seems the GE model requires more electric resistance heating to meet the demands.
19

Table 10. Summary Table of Performance by Site

Site HPWH

Model

Adults + 

Children

Water 

Heater 

Set Point 

Temp 

(°F)

Days 

Monitored

Avg. Daily 

Hot Water 

Use 

(gal)

Average

Cold 

Water 

Temp. 

(°F)

Avg. Hot 

Water 

Temp. 

(°F)

% of Electric 

Consumption 

From Electric 

Resistanceb

Avg. 

Air 

Temp. 

(°F)a

Avg. 

RH 

(%)a

Avg. 

Wet 

Bulb 

(°F)a

Total 

COP

1 A.O. Smith-80 2 + 1 120 454 44 54 119 0% 59 47% 49 2.1

2 Stiebel Eltron 5 + 0 140 438 73 57 136 8% 71 45% 58 2.6

3 GE 2 + 1 125 469 60 53 121 48% 64 38% 51 1.8

4 A.O. Smith-60 3 + 0 120 445 45 53 119 11% 63 56% 54 2.1

5 GE 2 + 0 129 460 64 52 127 78% 53 62% 46 1.0

6 GE 2 + 0 122 475 35 53 118 5% 62 55% 53 2.1

7 GE 2 + 0 125 450 23 58 123 11% 66 49% 55 1.8

8 GE 2 + 1 125 430 33 55 122 15% 66 44% 54 2.1

9 GE 2 + 2 120 468 41 55 122 22% 62 48% 52 2.0

10 Stiebel Eltron 2 + 0 140 424 41 57 138 2% 68 55% 58 2.0

11 GE 2 + 3 140 459 72 58 136 58% 76 34% 58 1.5

12 GE 2 + 1 130 492 42 56 128 29% 71 46% 58 1.9

13 GE 2 + 0 130 388 32 59 126 15% 70 57% 60 1.4

14 GE 2 + 0 120 433 32 53 119 15% 62 52% 52 1.9 a Average of daily averages

b Electric resistance percentage = % of total electric kWh that was used by resistance, NOT thermal energy fraction provided by electric resistance.
20

Analysis of the data collected during the year of monitoring uncovered key variables that affect 

HPWH performance and the differences between the operations of the different units. Across all 

models, ambient temperature, the volume of hot water draws, and the pattern of the hot water draws 

were the most important variables that affected water heater performance. These variables can have 

a different impact on different HPWH models, particularly when looking at the effects of hot water 

use and electric resistance element operation.

4.1 Impact of Water Use on Efficiency

As previously discussed, the smaller volume GE units used the electric resistance elements to 

provide a much larger percentage of the needed energy than the other models. Figure 5 shows 7

days of operation for the GE unit at Site 9. These days are representative of typical operation across 

many sites. During day 1 and days 3–6, the water draws from the tank were distributed throughout 

the day and/or relatively small. As a result, the heat pump was able to meet recovery needs for those 

days. During days 2 and 7, the water draws were more concentrated, and the electric resistance 

element was needed to provide additional hot water.

Figure 5. Ideal operation of the GE HPWH in hybrid mode (Site 9)10

While Figure 5 shows how large and concentrated draws can reduce the efficiency of the unit, low 

water use can also result in low COPs, as shown in Figure 6. In this figure, although the heat pump 

was used to meet the entire hot water demand, the low load and relatively high standby losses 

resulted in low daily COPs.

 

10 For this and similar figures, the vertical axes show discrete values for 15-min intervals.

0

200

400

600

800

1,000

1,200

0

5

10

15

20

25

30

11/21 11/22 11/23 11/24 11/25 11/26 11/27 11/28

HW Used (Gallons)

Energy Used (Wh)

HW Use Heat Pump Electric Resistance

44 gals

2.1 COP

57 gals

1.4 COP

76 gals

2.6 COP

92 gals

2.4 COP

40 gals

2.4 COP

53 gals

2.4 COP

80 gals

1.5 COP
21

Figure 6. Low water use can reduce the overall benefit of a HPWH when assessing cost benefit

Scatter plots of the daily hot water use11 versus the daily COP are shown in Figure 7. Each daily 

observation is also color coded by the electric resistance fraction, where blue is zero electric 

resistance use, and red is when the electric elements are used to reheat the entirety of the recovery 

load. The GE unit has the most observations, and therefore the operational patterns can be most 

directly observed in the scatter plot. For the GE unit, the data could be interpreted as lying between 

two exponential curves, where the upper curve has no electric resistance use and the lower curve is 

entirely electric resistance operation. The scatter plot of the GE unit operation also shows that the 

electric resistance fraction has a strong correlation with the hot water use. Days of lower hot water 

demand are far less likely to have instances of electric resistance heating (e.g., less than 30 gpd). On 

the other hand, days with high hot water demand are far more likely to have instances of electric 

resistance heating.

Although the scatter plots for the Stiebel Eltron and A.O. Smith units have less apparent trends 

because there were fewer monitored sites, the scatter plots appear to agree with the trend observed 

for the GE units, with one primary distinction. While the GE unit shows two distinct operating 

regions that correspond to large and small electric resistance loads, the A.O. Smith and Stiebel 

Eltron units operate entirely in the low electric resistance region. This is most likely a product of the 

 

11 Daily values are misleading because the net energy content of the water in the tank can change over the day. As a 

result, heating events with zero net tank energy change are identified from the 15-min raw data. All heating events are 

normalized to daily values and displayed graphically. See Appendix D for more information.

0

40

80

120

160

200

240

0

2

4

6

8

10

12

8/15 8/16 8/17 8/18 8/19 8/20 8/21 8/22

HW Used (Gallons)

Energy Used (Wh)

HW Use Heat Pump Electric Resistance

36 gals

1.7 COP

20 gals

0.9 COP

39 gals

1.9 COP

47 gals

1.4 COP

11 gals

1.3 COP

20 gals

1.5 COP

35 gals

1.4 COP
22

larger tank volumes and the control logic that allows the heat pump to reengage, in the case of A.O. 

Smith, or operate simultaneously, in the case of Stiebel Eltron.

4.2 Impact of Air Temperature on Efficiency

Although hot water demand is a primary driver of efficiency, other factors are still playing a 

prominent role, as shown in the large scatter in the data in Figure 7. The efficiency of the heat pump 

is primarily a function of the ambient temperature in the space. To give a sense of how ambient 

temperature affects overall efficiency, two sites with cold and hot operating environments are 

compared. At Site 5, the HPWH was installed in a cold basement with ambient air temperatures 

dropping below 50°F from December through April. These temperatures are close to the cut-off 

temperature of the heat pump in the GE unit. As a result, the HPWH at Site 5 operated like a 

traditional ERWH, as shown in Figure 8, but with an added parasitic of the heat pump cycling on 

and off at the cutoff temperature of the unit. 

Figure 7. Scatter plots of daily hot water use versus COP color coded by electric resistance fraction
23

Figure 8. Due to the Site 5 basement being too cold, this GE HPWH switched primarily to electric 

resistance mode during the winter months.

At the other end of the spectrum, Site 11 experienced higher COPs during the winter months 

because the HPWH was placed in a boiler room (Figure 10) with ambient temperatures around 80°F 

(Figure 9). At the beginning of January, the unit was able to supply hot water in large quantities 

with COPs greater than the rated EF of 2.35, as shown in Figure 11. There were 2 days over this 

week of data in which high water use concentrations resulted in electric resistance operation to meet 

demand. Furthermore, there are periods with draw quantities lower than the EF test conditions of 

64.3 gal with COPs higher than the rated EF. As we saw in the previous section, lower draw 

quantities can decrease overall COP because standby losses account for a larger percentage of the 

recovery load. The high ambient temperatures are boosting the efficiency of the heat pump and 

increasing the corresponding overall COP of the unit during these periods.

0

200

400

600

800

1,000

1,200

1,400

0

5

10

15

20

25

30

35

1/22 1/23 1/24 1/25 1/26 1/27

HW Used (Gallons)

Energy Used (Wh)

HW Use Heat Pump Electric Resistance

72 gals

0.7 COP

75 gals

0.7 COP

80 gals

0.7 COP

58 gals

0.7 COP

96 gals

0.8 COP
24

Figure 9. Ambient temperature at Site 11

Figure 10. This HPWH benefited from 

the waste heat of the boiler used for 

space heating

Figure 11. Typical winter performance of the GE HPWH in the Site 11 warm mechanical room

To demonstrate the relationship between ambient temperature and efficiency, daily scatter plots 

similar to those in Figure 7, but color coded by average ambient temperature, are shown in Figure 

12. Periods with electric resistance fractions > 0.04 were excluded, as well as data from Sites 5 and 

13, which had lower heat pump COPs than the other units. The scatter plots show a strong 

correlation between ambient air temperature and efficiency. Everything else being equal, higher 

ambient temperatures improve efficiency. At a 50-gpd recovery load, increasing the ambient 

temperature from 50°F to 80°F can increase the COP of the GE and A.O. Smith units from around 

2.0 to nearly 3.0.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

45

50

55

60

65

70

75

80

85

90

Relative Humidity

Temperature [°F]

HPWH Inlet Air Temp (°F) HPWH Inlet Air RH (%)

0

200

400

600

800

1,000

1,200

1,400

0

10

20

30

40

50

60

70

1/1 1/2 1/3 1/4 1/5 1/6 1/7 1/8

Energy Used (Wh)

HW Used (Gallons)

HW Use Heat Pump Electric Resistance

56 gals

3.1 COP

135 gals

1.3 COP

71 gals

2.6 COP

195 gals

1.2 COP

78 gals

2.6 COP

71 gals

2.7 COP

87 gals

2.8 COP
25

Figure 12. Scatter plots of COP versus daily hot water use color coded by ambient temperature.

Periods with electric resistance fractions > 0.04 and Sites 5 and 13 were excluded.

4.3 Delivered Water Temperature

While the efficiency of water heating is important from a performance standpoint, the temperature 

of delivered water from the HPWH is also important for customer satisfaction. Surprisingly, 

graphing several days of high water use at a site with a Stiebel Eltron HPWH revealed that this unit 

was able to meet very large hot water demands at surprisingly high COPs, as shown in Figure 13. 

While the electric resistance element was needed to supply additional heating, the unit performed 

with COPs > 2.6 for the 4 days of high hot water use. The Stiebel Eltron unit is distinguished from 

the other two models by its ability to simultaneously use the electric resistance and heat pump 

heating elements to provide heating. Furthermore, the water heater was able to deliver water 

temperatures in excess of 115°F during all 4 days of high hot water demand, as shown in Figure 14.

The higher COPs for the Stiebel Eltron unit are probably a result of the large tank size and the 

ability of the unit to only heat water for the load directly when the heat pump cannot meet the load 

rather than heating the entire tank. 
26

Figure 13. This Stiebel Eltron HPWH was able to maintain high COPs during high water demand

Figure 14. Even with high water demand, the hot water temperature only dropped as low as 118°F

In order to determine the ability of the HPWHs to deliver water at an acceptable delivery 

temperature, normalized histograms of maximum delivered water temperature for 15-min periods 

with draws > 1 gal were plotted for each site in Figure 15. The red and green vertical lines represent 

the set point temperature and minimum acceptable delivery temperature of 110°F (Hendron and

Engebrecht 2010), respectively. The percentage of occurrences below the minimum acceptable 

delivery temperature is listed in the upper left corner of each plot. The normalized histograms show 

that all three units perform well at delivering hot water above 110°F at a variety of set point

temperatures. However, many of the units did not maintain set point for all draw occurrences, and 

temperatures dropped more than 10°F below set point for a significant fraction of the 15-min 

periods. 

0

100

200

300

400

500

600

0

5

10

15

20

25

30

11/22 11/23 11/24 11/25 11/26 11/27 11/28 11/29

HW Used (Gallons)

Energy Used (Wh)

HW Use Heat Pump Electric Resistance

66 gals

2.6 COP

120 gals

3.1 COP

275 gals

2.6 COP

196 gals

2.7 COP

240 gals

3.0 COP

178 gals

2.6 COP

48 gals

1.7 COP

105

115

125

135

145

155

0

5

10

15

20

25

11/22 11/23 11/24 11/25 11/26 11/27 11/28 11/29

Hot Water Temp (°F)

HW Used (Gallons)

HW Use Delivered HW Temp

setpoint
27

Figure 15. Normalized histograms of delivered water temperature. Red lines indicate set point temperatures. Green lines indicate 

minimum delivery temperature (110°F). The percentage of occurrences with water temperatures below 110°F is listed.

85 105 125 145

0

0.5

1

Site 1

A.O. Smith

Probability Density

85 105 125 145

0

0.5

1

Site 2

Stiebel Eltron

85 105 125 145

0

0.5

1

Site 3

General Electric

85 105 125 145

0

0.5

1

Site 4

A.O. Smith

85 105 125 145

0

0.5

1

Site 5

General Electric

Probability Density

85 105 125 145

0

0.5

1

Site 6

General Electric

85 105 125 145

0

0.5

1

Site 7

General Electric

85 105 125 145

0

0.5

1

Site 8

General Electric

85 105 125 145

0

0.5

1

Site 9

General Electric

85 105 125 145

0

0.5

1

Site 10

Stiebel Eltron

Delivered Water 

Temperature (°F)

Probability Density

85 105 125 145

0

0.5

1

Site 11

General Electric

Delivered Water 

Temperature (°F)

85 105 125 145

0

0.5

1

Site 12

General Electric

Delivered Water 

Temperature (°F)

85 105 125 145

0

0.5

1

Site 13

General Electric

Delivered Water 

Temperature (°F)

85 105 125 145

0

0.5

1

Site 14

General Electric

Delivered Water 

Temperature (°F)

Histogram

Setpoint Temp.

Min Delivery Temp.

0.13% 0.16% 1.44% 0.32%

1.65% 1.63% 0.85% 1.92% 1.7%

0% 0.84% 0.51% 0.32% 0.86%
28

Unfortunately, the aforementioned method of determining whether the HPWHs are able to maintain 

the delivered water at a minimum acceptable temperature has flaws related to the 15-min recording 

interval used in this study. Because the average delivered water temperature will include water 

inside the pipe before the draw, the maximum delivered water temperature for the interval must be 

used. As a result, this analysis is only looking at the delivery temperature at the beginning of the 

draw or at the beginning of a 15-min interval. If the water heater runs out of hot water, the user may 

end the draw and the run-out may not be recorded using this method. However, the very small 

percentage of run-outs recorded at the 110°F minimum acceptable delivery criteria means that 

delivery temperature is probably not dropping significantly below usable temperatures.

4.4 Calculating Standby Losses

Tank losses to the ambient are an important contributor to tank inefficiency; indeed, for ERWHs, 

tank losses represent the entirety of water heat site energy inefficiencies. Unfortunately, tank losses 

from HPWHs are hard to quantify without precise laboratory equipment. The data from this study, 

in conjunction with laboratory HPWHs tests conducted by Sparn et al. (2011), can be used to 

estimate the tank UA. This analysis was possible for the GE unit only, because the Stiebel Eltron 

and A.O. Smith units did not provide enough data points to perform this analysis.

The following analysis differs from the methods outlined in the DOE test procedure (DOE 1998) 

and NREL’s field testing protocol (NREL 2012c). These methods measure the change in the 

average tank temperature during a period of no draws. NREL specifies that this period should begin 

10 min after the heat pump has shut off because the condensing coil will still be giving off heat 

directly after the reheat period. 

Since internal tank temperatures were not measured in this study, a different procedure was used to 

measure standby losses. The basic procedure used was to identify idle periods during which the tank 

begins fully heated, experiences no water draws, and is finally reheated. The tank losses during that 

period are simply the ratio of the thermal energy input to the product of the time elapsed since the 

last reheat and the temperature difference between the tank and the ambient air.

( ) tank amb

standby

t T T

Q UA

∆ − = , (3)

where

UA = overall heat transfer coefficient of the tank (Btu/h-°F),

Qstandby = thermal energy to recover tank to set point (Btu),

Ttank = average tank temperature (°F),

Tamb = ambient temperature (°F), and

Δt = time elapsed since the last reheat (hr).

The average tank temperature was assumed be 2.5°F lower than the temperature set point, which is 

equivalent to assuming an average tank temperature dead band of 5°F for recovery from an idle 

period. For example, if the tank set point were 135°F, the tank would be assumed to start the period 

at 135°F, drop to 130°F due to standby losses, and finally return to 135°F through the recovery 

process. The average temperature would be 132.5°F , which is 2.5°F lower than the set point 

temperature. This dead band temperature is consistent with graphs of the EF testing of the GE unit 

by PG&E (2010) and other studies.
29

To measure the electric power necessary to recover from standby losses, idle periods followed by 

heat pump operation with no hot water demand were identified from the 15-min data. These periods

satisfied the following conditions:

• No draw over the period

• Period begins and end with the termination of heat pump operation to ensure similar 

stratification

• The overall period must be longer than 3 h.

For ERWHs the electricity used is directly converted to thermal energy; for HPWHs, however, the 

electricity input does not directly correspond to the thermal energy input into the tank. As a result, 

laboratory performance maps discussed in Sparn et al. (2011) were used to convert electricity 

energy used by the heat pump to the thermal energy delivered to the storage tank. The equation for 

the efficiency of the GE heat pump is

( ) COPhp COPhp rated C C Twb C Twb C Ttank C Ttank C6TwbTtank

2

5

2

, 1 2 3 = + + + 4 + + , (4)

where

COPhp = efficiency of the heat pump (Btu/Btu),

COPhp,rated = rated COPhp at 57°F wet bulb and 120°F tank temperature (2.76),

Ttank = average tank temperature (°F),

Twb = ambient wet bulb temperature (°F),

C1 = 1.192E+00,

C2 = 4.247E-02,

C3 = –3.795E-04,

C4 = –1.110E-02,

C5 = –9.400E-07, and

C6 = –2.657E-04.

The tank lost during the standby period is

( ) Qstandby = C Eres + EhpCOPhp , (5)

where

Qstandby = thermal energy to recover tank to set point (Btu),

COPhp = efficiency of the heat pump (kWh/kWh),

Eres = electricity used by the resistance elements (kWh), 

Ehp = electricity used by the heat pump (kWh), and 

C = conversion factor for kWh to Btu (3.412 Btu/kWh).

In order to correct for the error inherent in the 15-min datalogging interval, the estimated time of 

electricity cutoff was determined by examining the previous data logging period. If the electricity 

draw for the last interval i before cutoff was smaller than the previous interval i-1, then the runtime 

fraction for the cutoff interval was set as the ratio of the electricity draws, as shown in Equation (6). 
30











≥

< =

−

−

−

1

1

1

1 for

for

i i

i i

i

i

E E

E E

E

E

f (6)

Robust regression12 analysis was used to estimate the UA

( ) standby Ttank Tamb Q = UA∆t − . (7)

Regressions using Equation (7) were performed with and without a constant (i.e., nonzero 

intercept). While the idle periods were chosen such that the water heater tank could be assumed to 

have no net energy change from the beginning of the period to the end of the period, the inclusion 

of the constant in one of the regressions is used to test whether this assumption is valid. Regression 

results with and without a constant are shown in Table 11 and Table 12, respectively, and Figure 16

shows the two regression lines plotted against the observed data. While the intercept estimate is 

significant at the 95% confidence level and the tank loss coefficient is similar to that of other studies 

(Table 29), there are reasons to argue that the model without the constant is preferable.

Table 11. Tank Heat Loss Robust Regression Results With Constant (N = 113, df = 111)

Coefficient Estimate 95% Confidence Interval t Statistic p Value

Intercept 52.821 (4.2826,101.3604) 2.1564 0.033211

UA 3.5008 (2.7034, 4.2982) 8.6994 3.4555E-14

Table 12. Tank Heat Loss Robust Regression Results Without Constant (N = 113, df = 112)

Coefficient Estimate 95% Confidence Interval t Statistic p Value

UA 4.369 (4.3169,4.4211) 166.15 6.907E-136

 

12 Robust regression is similar to a simple linear regression in this case, except points with high leverage are given less 

weight in the regression analysis (Chatterjee and Price 1991). Two points (see Figure 16), which appeared to be outliers, 

did not qualify for exclusion under the leverage criteria of 2/N (Chatterjee and Price 1991)—where N is the number of 

observations—but did have a Cook’s distance greater than 4/N (Bollen and Jackman 1985), which suggests that these 

have a large impact on the regression results. Robust regression allows these points to have lower weights.
31

Figure 16. Regression results and data points for GE standby losses. Temperature difference is 

between the tank average temperature and ambient air. 

Since the estimate of the UA is the desired result from the regression, the model with the more 

confined confidence interval for the slope coefficient is preferred. By including the constant in the 

model, the error of the heat loss coefficient estimate at the 95% confidence level is ± 22.8%, 

meaning that calculations using this coefficient have an acceptably higher error. The corresponding 

error for the model without the constant is only ± 1.2%, which is far more acceptable for energy 

balance calculations. One could also argue that a higher confidence level should be used in this 

analysis because the model represents a physical system whose response is well known and should 

follow a well-described deterministic phenomenon.

The UA for the model without the constant is 4.312 Btu/h-°F, which is significantly higher than the 

consensus UA of 3.7 Btu/h-°F found by the laboratory results shown in Table 29. Several factors 

may have contributed to this discrepancy. The laboratory setups would have included significantly 

shorter pipe runs than the field sites; this would have reduced the effects of thermosiphoning and 

thermal shorts on the heat transfer coefficients measured in the lab. Furthermore, the EF test 

measures the UA of the tank over a period where there may be a change in net energy of the tank. 

While the equation governing this calculation in 10 CFR Part 430 accounts for this change in 

energy (DOE 1998), the difference in measurement methodology between the DOE test and this 

study could play an important part in the difference in results. By starting and ending the analysis 

period just after a reheat cycle, our method includes the net change in energy of the refrigeration 

loop, while the EF test does not. As a result, the cause of the higher UA of this study compared to 

laboratory tests is hard to determine. The field installations could have a higher UA than the 

laboratory installations, but the difference could be attributable to differences in the setup of the 

calculation.

40 45 50 55 60 65 70

100

200

300

400

500

600

700

800

Temperature Difference (°F)

Thermal Losses (BTU/hr)

Observances

Without Constant

With Constant
32

4.5 Impact of Confined Spaces

Because an HPWH removes heat from the ambient air, a sufficient volume of available air is needed 

to ensure proper performance of the unit. Several laboratory studies have attempted to quantify the 

impact of installing HPWHs in spaces without proper air volume. All of these tests were performed 

on the GE GeoSpring. Advanced Energy compared two HPWHs subjected to the EF test, one 

placed in a 220.5-ft3 uninsulated space and another subjected to the default DOE test operating 

conditions. GE recommends a volume of 750 ft3 (General Electric 2010). The efficiency of the unit 

in the confined space was 10% lower than that of the control unit (Fitzpatrick and Murray 2011). 

The Florida Solar Energy Center also tested an HPWH in a confined space (92.5 ft3

) and saw a 

reduction in efficiency of 13% (Colon 2012). NREL’s laboratory tests included blocking the airflow 

of the unit with tape. Blocking the airflow by 33% and 67% had only a minor impact on the 

performance of the unit, most of which was attributable to the increased fan energy use (Sparn et al. 

2011). This suggests that the volume of air available to the unit is more important than constriction 

of airflow.

Site 13 in this study was installed in a confined space with approximately 400 ft3 of available air. 

The heating event shown in Figure 17 has operating conditions similar to that of an EF test 

conducted at a set point temperature of 130°F. The duration of the heating event and the amount of 

water drawn during the heating event are approximately half that of the EF test; the resulting draw 

rate is within 5% of the EF test draw rate of 2.68 gph. With a COP of 2.02, the resulting 

performance drop was approximately 15.8% below an approximate expected EF of 2.4 at a set point 

temperature of 130°F. The performance reduction is similar to those found by Fitzpatrick and 

Murray (2011) and Colon (2012). 

Figure 17. Performance of Site 13 in confined space

15:00 18:00 21:00 00:00

0

1

2

3

4

5

6

Hot Water Flow Rate (gpm)

Site13; COP = 2.02; T = 69.6; RH = 55%; Tmains = 57.7; Tset = 130; 

15:00 18:00 21:00 00:00

0

7.5

15

22.5

30

Cummulative

 Volume (gal)

15:00 18:00 21:00 00:00

0

200

400

600

800

1000

1200

Electric Energy Used (Wh)

Flow Rate

Heat Pump

Lower Element

Upper Element
33

4.6 Impact on Space Conditioning Systems

Since an HPWH moves heat from the air surrounding the water heater into the storage tank, an 

HPWH will have an impact on the space conditioning loads of the building in which it is located. 

When an HPWH is located in conditioned space, there is a very direct effect on space conditioning 

loads. In the summer, the HPWH will reduce the cooling load of the building; conversely, the 

heating load will be increased in the winter. The combined impact of the HPWH energy 

consumption and its impact on the space conditioning systems rely heavily on climate, home 

configuration, HPWH location, and the space conditioning systems used. For more information on 

these impacts, see the modeling results described by Hudon et al. (2012). A less complex analysis 

was also presentedby Shapiro et al. (2012).

For HPWHs installed in unconditioned or “semi-conditioned” spaces such as basements, the space 

conditioning impacts are harder to determine. These spaces act as buffer spaces, so the heat 

transferred from the space into the storage tank is not necessarily transferred from the conditioned 

spaces. Air infiltration into the buffer space, ground coupling, solar gains, and heat transfer from 

mechanical equipment can all affect heat transfer between the buffer space and conditioned space. 

The transient effects of these heat transfer processes means that the HPWH space conditioning 

impacts are potentially reduced.

Since these transient heat transfer interactions cannot be directly addressed for the HPWHs installed 

in unconditioned basements in this study, the space conditioning impacts will be addressed as the 

maximum potential impact. The impact on the space conditioning equipment is defined as (NREL 

2012c)

Qnet,space = Qinput − Qdhw , (8)

where

Qnet,space = maximum potential thermal impact on space (Btu),

Qinput = electric energy used by the water heater (Btu), and

Qdhw = thermal energy removed from storage tank (Btu).

The maximum potential thermal impact of the space conditioning equipment on the space is 

negative when thermal energy is being removed from the space and positive when the energy is 

being added to the space. In the case of standard water heaters, Qnet,space will be positive, but for 

HPWHs the net energy to the space is negative.

While the HPWHs will remove energy from the space, the impact on the space conditioning loads 

depends on the season. For the purposes of this analysis, the cooling season for these sites was

considered to be June through September, and all other periods are considered the heating season. 

Box plots of the space conditioning impacts on an hourly load basis and a seasonal basis are shown 

in Figure 18, which shows negative values reducing the space conditioning loads and positive 

values increasing the space conditioning loads. Box plots display the range of data for each of the 

categories. In the plots, the boxes represent the inner two quartiles—also called the inner quartile 

range—where the lower quartile is the bottom of the box and the upper quartile is the top of the 

box. The red line in the middle of the box represents the median. At the top and bottom of the boxes 

are whiskers that represent the smallest and largest observations within 1.5 inner quartile range. 

Outliers are represented by a red plus, and the mean is represented by a black plus. 
34

Figure 18. Box plots of maximum potential space conditioning impacts for monitored HPWHs

As shown in the box plots, the magnitude of the mean hourly heating impact is slightly higher than 

the magnitude of the mean hourly cooling impact. Similarly, the magnitude of total heating seasonal 

impact is nearly twice the magnitude of the total cooling seasonal. These box plots show that the 

homes in this study, which are all in northern climates, had maximum potential seasonal cooling 

load reductions of approximately 1.5 MMBtu and heating load increases of approximately 3 

MMBtu.

-1000

-500

0

500

1000

1500

Cooling Heating

Mean Hourly Change in Load (Btu/hr)

-2

0

2

4

6

8

Cooling Heating

Seasonal Change in Load (MMBtu)
35

5 Energy and Cost Analysis

Energy and cost savings potentials of HPWHs were investigated against eight alternative storage 

tank water heaters: electric resistance; standard natural gas, fuel oil, and propane; premium natural 

gas, fuel oil, and propane; and condensing natural gas. Tankless water heaters were not considered 

because a more complicated modeling procedure to account for the transient response of these water 

heaters would be necessary. The assumed EFs, REs, and pilot light energy are listed in Table 13, 

where all standard and premium fossil fuel-based systems are assumed to have the same operating 

characteristics, except for the energy source used to provide the heating energy.

Table 13. Alternative Water Heaters

Water Heater Fuel Types EF RE

Pilot Light 

Energy 

(Btu/h)a

Electric Resistance Electricity 0.92 100% 0

Standard 

Fossil Fuel

Natural Gas, Fuel 

Oil, and Propane 0.59 78% 400

Premium 

Fossil Fuel

Natural Gas, Fuel 

Oil, and Propane 0.67 82% 400

Condensing Fossil 

Fuel Natural Gas 0.83b 95%b 0

a WATSMPL default (EPRI 2000)

b Efficiencies from laboratory testing by PG&E (2008)

The alternative water heaters were simulated using the equations underlying the WATSMPL 

simplified water heater software (EPRI 2000). For each of the alternative water heaters, the daily 

operating conditions at each site—set point temperature, mains temperature, volume of water 

drawn, and ambient temperature—were simulated using the equations in Appendix B. The 

monitored, in the case of the HPWHs, and simulated energy consumption, in the case of the 

alternative water heaters, of each water heater was normalized to yearly energy consumption. The 

normalization procedure accounts for the fact that the monitoring period at each site was different 

and that there was some overlap in the days that were monitored.

While energy use is usually measured in site energy, which is the energy used at the home and is 

typically measured at a utility meter in units of kilowatt-hours (electricity), therms (natural gas), or 

gallons (fuel oil or propane), a better metric for measured energy use is source energy, which is the 

sum of energy used at the home and the energy lost to extraction, conversion, or transmission. Site 

energy easily is converted to source energy using site-to-source ratios (Deru and Torcellini 2007), 

which are defined to the fuels studied in Table 14. As a result, energy use will be reported in source 

energy and cost to consumers. The site energy consumption for each water heater-site combination 

was converted to source energy and cost using the values listed in Table 14. 
36

Table 14. Fuel Prices and Characteristics for Energy and Cost Analysis

Energy Source Site-toSource Ratioa Costb Energy Contentc

Electricity 3.365 $0.1172/kWh 3,412 Btu/kWh

Natural Gas 1.092 $1.10/therm 100,000 Btu/therm

Fuel Oil 1.158 $3.93/gal 140,000 Btu/gal

Propane 1.151 $2.85/gal 91,600 Btu/gal a Deru and Torcellini (2007)

b Prices obtained from EIA (2012b, 2012c, 2012d). Electricity and natural gas prices represent average national prices 

for 2011. Fuel Oil and propane prices are the average national prices for October 2011 through March 2012. c Thumann and Mehta (1991)

To compare the installed HPWHs and the simulated alternative water heaters, box plots of annual 

source energy consumption and annual operating costs are shown in Figure 19 and Figure 20. 

Figure 19 shows that HPWHs have operating costs significantly lower than electric resistance, oil, 

and propane, and operating costs comparable to natural gas systems. Furthermore, HPWHs have the 

lowest mean and median operating costs of all water heating systems except condensing natural gas. 

In terms of source energy consumption, Figure 20 shows that HPWHs have the second lowest 

source energy consumption of all water heating products (a natural gas condensing tank unit is the 

lowest). Unlike ERWHs, which have the highest source energy consumption of all products by far, 

HPWHs are able to counteract the source energy penalty of using electricity to provide water 

heating. The outlier for the HPWHs in both plots is Site 5, which was operating in electric 

resistance mode throughout the winter months, and therefore has operating costs similar to the 

ERWH category. 

Figure 19. Annual operating cost of monitored HPWHs and alternative water heaters

$100

$200

$300

$400

$500

$600

$700

$800

$900

Heat

Pump

Electric

Resistance

Gas

Standard

Gas

Premium

Gas

Condensing

Oil

Standard

Oil

Premium

Propane

Standard

Propane

Premium

Annual Operating Cost ($/yr)
37

Figure 20. Annual source energy of monitored HPWHs and alternative water heaters

While HPWHs have the second lowest operating costs and source energy use of the water heating 

products considered in this study, installation costs will have an impact on the overall financial 

outcome of HPWHs versus alternative water heaters. The cost analysis was performed as described 

by Polly et al. (2011). The assumptions and results are listed in Table 15. The costs were assumed to 

be wrapped into a 30-year mortgage at a 4.42% interest rate. These assumptions are based on 

wrapping the cost of the water heaters into the cost of buying a new house or mortgage refinance. 

Table 16 lists the installation costs and typical lifetimes of the various water heating products 

considered. Using these values, the annualized energy related cost (AERC) of each of the water 

heating units was calculated. AERC represents the equivalent annual cost of the complex cash flow 

and represents an equivalent annual cost of operating the water heater in present dollars.

Table 15. Cost Analysis Assumptions

Cost Metric Assumption

Analysis Period 30 yearsa

Inflation Rate 3%a

Real Discount Rate 3%a

Real Fuel Escalation Rate 0%a

Mortgage Rate 4.42%b

Mortgage Period 30 yearsb

Marginal Income Tax Rate 28%a

a BEopt defaults (NREL 2012b)

b Average rate for 2011 (Freddie Mac 2012)

10

20

30

40

50

60

70

Heat

Pump

Electric

Resistance

Gas

Standard

Gas

Premium

Gas

Condensing

Oil

Standard

Oil

Premium

Propane

Standard

Propane

Premium

Annual Source Energy (MMBtu/yr)
38

Table 16. Installation Costs and Lifetimes of Water Heating Products

Water Heater Installation Costa Typical Lifetime 

(years)a

Heat Pump (80 gallons) $3,300 10

Heat Pump (50-60 gallons) $2,100 10

Electric Resistance $590 13

Standard Natural Gas $700 13

Premium Natural Gas $880 13

Condensing Natural Gas $4,500b 15c

Standard Fuel Oil $820 13

Premium Fuel Oil $960 13

Standard Propane $890 13

Premium Propane $1,400 13

a From National Residential Efficiency Measures Database (NREL 2012a).

b From American Water Heaters (2009) c Estimated lifetime based on National Residential Efficiency Measures Database (NREMD) values for gas storage and 

condensing tankless heaters.

The installation costs for the cost analysis were derived from the NREMD, but HPWH installation 

costs reported for this study, as shown in Table 17, were generally in line with the costs reported in 

the NREMD. The NREMD reports shorter lifetimes for the HPWHs than the other water heater 

technologies, which is probably caused a desire to be conservative and rate the lifetime equal to the 

product warranty period. The reliability of these units is as yet undetermined, however, so the 

lifetime of HPWHs may be similar to other water heaters.

Table 17. Installed HPWH Cost Estimates from Study

Small Tank 

(50–60 gal)

Large Tank 

(80 gal)

Unit $1,399 $2,403

Extra Labor $69 $69

Condensate Pump $154 $154

Electric and Plumbing Permit $100 $100

Breaker – $54

Tempering Valve – $142

Labor $200–$400 $400–$600

Total $1,922–$2,122 $3,318–$3,518

A boxplot showing the spread of the AERCs for the nine water heating technologies is shown in 

Figure 21. The HPWHs have lower AERCs than the electric resistance, fuel oil, and propane water 

heaters, although the AERC reduction over ERWHs is smaller than the annual operating cost 

savings due to the increased first cost of HPWHs. As expected, the noncondensing natural gas water 

heaters are still the lowest cost option on a total life cycle basis. Space conditioning interactions 

may change the relative costs of HPWHs in relation to other water heaters, depending on the 

climate. These interactions could not be measured in this study. Condensing tankless water heaters, 

which were not investigated in this analysis, will have lower operating costs than even condensing 
39

storage water heaters, but the analysis method employed here does not account for the transient 

behavior of tankless water heaters. Other issues are related to tankless units, so a direct comparison 

may not be appropriate.

Figure 21. Annualized energy related cost of monitored HPWHs and alternative water heaters

$200 

$300 

$400 

$500 

$600 

$700 

$800 

$900 

$1000

Heat

Pump

Electric

Resistance

Gas

Standard

Gas

Premium

Gas

Condensing

Oil

Standard

Oil

Premium

Propane

Standard

Propane

Premium

Annuallized Energy Related Cost ($/yr)
40

6 Homeowner Surveys

After complete monitoring of the HPWHs, residents at the 14 sites were surveyed about their 

satisfaction with the HPWHs. All of the homeowners were satisfied with the HPWHs. The majority 

(70%) noticed cooling and/or dehumidification. Some noted that noise was an issue (18%). Some 

homeowners experienced running out of water (36%), though these were isolated incidences and 

weren’t a significant concern to the homeowners. The vast majority (73%) noticed lower utility 

bills. One “No” response to noticing utility bill savings was because the homeowner switched from 

oil and did not know how to do the cost comparison. Another “No” response was that the 

homeowner was not sure if the savings were a result of HPWH or lower electric rate (the savings 

were indeed due to the HPWH). Detailed survey results are shown in Table 18 and Table 19.

Table 18. Survey Results of Whether the Homeowner or a Qualified Professional 

Performed Any Preventive Maintenance Procedures

GE AO SE All

Y N Y N Y N Y N

Testing the temperature 

& pressure-relief valve? 0 8 0 1 0 2 0 11

Flushing and/or draining 

the tank? 0 8 0 1 0 2 0 11

Periodically inspecting 

and clearing the 

condensate strainer and 

drain lines?

5 3 1 0 0 2 6 5

Visually inspecting the 

surrounding floor area, or 

the drain pan for signs of 

water leakage?

3 5 1 0 2 0 6 5

Cleaning the air filter? 4 3 1 0 0 0 5 3

Cleaning the evaporator? 0 8 0 1 0 2 0 11

Checking the condition of 

the sacrificial anode rod? 0 8 0 1 0 2 0 11

Checking and descaling 

the heating elements? 0 8 0 1 0 2 0 11

Would you encourage a 

friend or family member 

to buy the same water 

heater?

8 0 1 0 1 1 10 1

The ideal response is in green text and non-ideal response is in red text. Questions in gray were not expected to be 

addressed because these units have not been installed long enough to warranty this level of maintenance.
41

Table 19. Survey Results of Homeowners

GE AO SE All

Y N Y N Y N Y N

Have you ever run out of 

hot water? 4 4 0 1 0 2 4 7

Has the water been hot 

enough? 8 0 1 0 2 0 11 0

Have you noticed a 

difference in your energy 

bills since the water heater 

was installed?

6 2 1 0 1 1 8 3

Have you noticed the 

water heater cooling 

and/or dehumidifying the 

space?

6 2 0 1 1 0 7 3

Has the water heater’s 

operating noise been a 

problem?

1 7 0 1 1 1 2 9

Has this water heater 

changed how you use hot 

water?

2 6 0 1 0 2 2 9

Are you satisfied with 

your HPWH’s 

performance?

8 0 1 0 2 0 11 0

Have you read your 

HPWH’s manual? 6 2 1 0 1 1 8 3

Do you know how to 

change the settings on 

your HPWH?

7 1 1 0 0 0 8 1

Have you ever changed 

the settings of the water 

heater?

5 3 1 0 0 2 6 5

Do you know what 

operating mode your 

HPWH is set to?

5 2 1 0 0 0 6 2

Has the water heater 

required servicing? 1 7 0 1 2 0 3 8

The ideal response is in green text, non-ideal response is in red, and questions without an ideal response are in blue. 
42

7 Recommendations

Based on the results of this report, the Consortium for Advanced Residential Buildings provides 

several recommendations for successful installation of HPWHs. The efficiency of HPWHs is 

profoundly affected by the way hot water is used. When large quantities of hot water are used in 

clusters, HPWHs will revert to electric resistance mode, reducing the efficiency of the unit. A 

homeowner can reduce this effect by purchasing a larger HPWH, increasing the set point

temperature, or changing behavior. By increasing the size and temperature of a HPWH, more hot 

water can be delivered at a given time before the resistance elements are needed. Spreading the 

water load over a longer period of time may also provide similar benefits and reduce standby losses. 

The installation location of a HPWH is also an important factor to consider when installing an

HPWH. HPWHs operate at higher efficiencies when they are subjected to higher ambient 

temperatures. If possible, HPWHs should not be installed in cold locations, such as garages in 

northern climates. Furthermore, HPWHs require additional space to operate at peak efficiency. In 

this and other studies, an efficiency reduction of > 10% was observed when HPWHs are installed in 

confined spaces. Finally, HPWHs remove heat from the surrounding air, and can therefore affect the 

space conditioning loads of a building. In cold climates, this heat removal is typically undesired, 

and installing a HPWH in a buffer space such as an unconditioned basement can temper the space 

conditioning affects. In hot climates, the removal of heat can be beneficial, and the total building 

energy consumption can be reduced by placing HPWHs in the conditioned space. 

Although HPWHs are a promising technology that may finally be here to stay, there are some 

installation hurdles that installers must address. Successful installation requires careful 

consideration of clearance and weight. By adding an additional heating mechanism, these HPWHs 

are markedly larger than ERWHs with identical storage volumes. Because HPWHs remove 

humidity from the air, drain pans and condensate pumps may be necessary to protect the HPWH 

and the floor from excess moisture. Finally, the inclusion of a heat pump means that filters must be 

cleaned regularly and noise could be a problem for some installations. Many of these details are 

discussed in more depth by Shapiro et al. (2012), SWA (2012), which SWA worked on with the 

sponsors of this evaluation for use in their rebate programs, and a tri-fold brochure for consumers 

(Appendix F) that was created for their rebate programs.
43

8 Conclusion

Though the study used a small sample set, the overall performance of these 14 HPWHs has been 

enlightening and shows promise for this technology. To date, there has only been one instance in 

which a compressor for a HPWH unit had to be replaced. It is unclear as to the cause of this failure. 

No other major issues have been identified regarding durability and reliability of these units, but 

this is something that will need to be followed up as these systems age. This evaluation was 

successful in answering the following research questions:

What is the measured efficiency of a HPWH located in unfinished basements of cold climate 

homes?

Even when installed in unfinished basements of cold climate homes, the measured efficiency of 

these units was much higher than that of conventional electric water heaters. With the exception of 

one unit that was placed in a basement with very low ambient temperatures, the monitored units had 

COPs > 1.5, which represents more than a 50% reduction in energy consumption compared to an

ERWH. The highest COP was 2.6. 

Table 20. Summary of Test Results by Model

Model Units 

Monitored

Capacity

(gal) Rated EF

Measured 

Average

COP

COP 

Range

GE 10 50 2.35 1.82 1.5–2.1

A.O. Smith 2 60/80 2.33 2.12 2.1

Stiebel Eltron 2 80 2.51 2.32 2.0–2.6

Annual operating costs and source energy consumption for the monitored HPWHs were lower than 

the alternative storage tank water heaters considered. Annualized energy related costs for HPWHs 

were slightly lower than ERWHs and considerably lower than propane and fuel oil fired systems. 

Natural gas storage water heaters, however, are still the lowest-cost storage water heater on an 

annualized energy related cost basis. Tankless water heaters were not considered here because of 

the more complex modeling requirements.

What are the critical criteria that affect the installed performance of HPWHs, and how do 

they impact the performance of each HPWH model? 

The key variables that affect performance were studied based on the data from this study. Domestic 

hot water use (daily volume and draw pattern) is the primary driver of efficiency, but ambient 

temperature plays a considerable role in expected efficiency. Lower water demand will reduce the 

overall efficiency of the water heater by increasing the fraction of the recovery load that is standby 

losses. High water demand will also reduce efficiency by requiring more electric resistance use. 

Higher ambient temperatures will increase the efficiency of the heat pump and reduce standby 

losses.

While the GE unit shows two distinct operating regions that correspond to large and small electric 

resistance loads, the A.O. Smith and Stiebel Eltron units operate entirely in the low electric 
44

resistance region. This is most likely a product of the larger tank volumes and the control logic that 

allows the heat pump to reengage, in the case of A.O. Smith, or operate simultaneously, in the case 

of Stiebel Eltron. The Stiebel Eltron unit’s higher factory-set set point temperature may also play a 

role.

Does each model evaluated in this study effectively deliver hot water at the set point

temperature?

Despite the slower recovery rate of heat pump compared to electric resistance elements, the HPWHs 

were surprisingly able to meet very large hot water demands. The Stiebel Eltron unit in particular 

was able to meet large loads around 200 gpd of hot water at high COPs. The outlet water 

temperature during these periods never dropped below the minimum accepted water temperature of 

110°F.

What are the standby losses for these HPWHs, and how do they compare to traditional 

ERWHs?

While HPWHs are significantly more efficient than ERWHs, the standby losses of these water 

heaters are considerably higher than ERWHs. Laboratory measurements of the EF of these units in 

resistance mode show rated values about 10% lower than a comparably sized ERWH. Laboratory 

testing and analysis in this study also show higher UA values than for ERWHs. UAs calculated in 

this study were about 25% higher than laboratory tests, which may reflect the real-life performance 

of these units better than the laboratory installations. A possible explanation for this difference is 

the existence of thermal shorts introduced by the wrap-around heat exchanger. Improvements in 

tank insulation would need to be made to reduce the thermal heat loss for these tanks.

Although the efficiency numbers of these units is impressive, this study did not completely address 

the interactions between the HPWHs and the space conditioning systems. Because the HPWHs 

were placed in unconditioned basements, which act as buffer spaces, the total impact on the space 

conditioning system cannot be measured without more information. However, the maximum 

potential impact of the HPWH on the conditioning loads of the houses was approximately 3 MMBtu 

of increased heat load and 1.5 MMBtu of decreased cooling load. In other climates, the overall 

effect may be quite different.

Are homeowners satisfied with hot water delivery, efficiency, noise, and other characteristics?

Ten of the 11 survey respondents said that they would recommend an HPWH to a friend or family 

member. The one dissenting homeowner had an issue with the noise of the HPWH because a home 

office was in the room adjoining the basement mechanical room.

Overall, this study provides considerable data about the performance of new HPWHs in 

unconditioned basements in the Northeast. The efficiencies were remarkable in these installations, 

and these units show considerable promise. However, more research is still necessary in several 

areas. Understanding the affects of HPWH installation in unconditioned basements is vital for 

quantifying HPWH impacts on the total building energy use. Calculating the interaction between 

the HPWH, conditioned space, buffer space, ground, and ambient is not a trivial task. 
45

Furthermore, more information is needed to predict the effect of draw profiles on the efficiency of 

these HPWHs. While the concentration of draws clearly affects the switching between the heat 

pump and the electric resistance elements, a direct connection between which draw profiles trigger 

resistance operating and which do not has not been drawn. Understanding this connection is 

particularly vital for developing test procedures that can accurately predict the field performance of 

HPWHs.

Although HPWHs are a promising technology that may finally be here to stay, there are some 

installation hurdles that installers must address. Successful installation requires careful 

consideration of clearance and weight, drain pans and condensate pumps filer maintenance and 

noise. Many of these details are discussed in more depth by Shapiro et al. (2012), Selection and 

Quality Installation Guide for HPWHs, by SWA (2012), which SWA worked on with the sponsors 

of this evaluation for use in their rebate programs, and in a tri-fold brochure for consumers 

(Appendix F) that was created for their rebate programs.
46

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51

Appendix A: A History of Heat Pump Water Heaters in 

the United States

HPWHs are not new, despite their recent reintroduction into the mainstream residential 

marketplace. Since the first commercialization of HPWHs in the 1950s, HPWHs have followed a 

boom-bust cycle of product development and subsequent abandonment that closely follows retail 

electricity prices and demonstrates the reliability issues of past products. While HPWHs failed to 

gain significant market traction in the past, the current confluence of regulatory, economic, and 

market forces may finally be pushing HPWHs into the mainstream.

The Early Years: 1935–1970

The invention of HPWHs dates back at least to 1935, when the first HPWH was patented in the 

United States (Wilkes and Reed 1937).13 The HPWH described in the patent (Figure 22) looks very 

similar to the current batch of HPWHs, with a supplemental electric resistance element, an 

integrated storage tank, a motor-driven compressor, and an immersed coil condenser. A unique 

feature of this HPWH, however, was the use of an upper and lower chamber to prevent the electric 

resistance element from heating the entire storage volume.

Figure 22. First patented HPWH (Source: Wilkes and Reed 1937)

 

13 The search term heat pump water heater was used in Google’s patent search, which has digitized every patent 

application in the United States since 1790.
52

Development of the first mass-market, commercialized HPWH, however, did not begin until the 

1950s. A prototype developed by the Hotpoint Company—now a division of GE—performed well 

when monitored in collaboration with the Tampa Electric Company (Calm 1984). Throughout the 

1940s and 1950s electricity prices declined considerably14 (EIA 1996), which undoubtedly reduced 

the economic and commercial viability of the product. As a result, development was discontinued, 

and HPWHs would not return to the mainstream market for more than 20 years (Calm 1984). 

The Second Generation: 1970–1999

Interest in HPWHs subsided in the 1960s, during a period of low and falling electricity prices. By 

the 1970s, however, the emerging energy crisis and a spike in retail electricity rekindled interest in 

HPWHs as a mainstream, commercialized product. Retail electricity prices began to rise as fossil 

fuel prices spiked, utility companies overbuilt capacity based on inaccurate demand forecasts, and 

environmental regulation increased construction costs for capital improvements (EIA 1996). 

During the mid to late 1970s, the National Rural Electric Cooperative Association and DOE

provided funding to develop a prototype HPWH. Through this funding, Energy Utilization Systems 

manufactured 100 HPWHs—85 integrated units and 15 add-on units—which were tested by 20 

electric utilities. While the add-on units were unsuccessful, the integrated units showed significant 

promise. Average COPs were around 2.0, operating savings approached 50%, the lifespan of the 

units matched similar natural gas and ERWHs, and consumer satisfaction was high (Calm 1984).

By the 1980s, the successful pilot program had driven HPWHs into the mainstream marketplace. 

Popular Science trumpeted HPWHs as a way to achieve “solar savings—without the cost” (Powell 

1980), and HPWHs were soon eligible for state tax credits. Electric utilities, such as the Tennessee 

Valley Authority, pushed adoption through zero-interest loans and incentives (Changing Times 

1982). By 1984, at least 17 different commercialized HPWHs were in the marketplace under a 

multitude of trade names (Calm 1984).

While these new units were met with enthusiasm during the early 1980s, with sales sitting at more 

than 10,000 units per year, the HPWH market soon collapsed because “these early machines 

suffered from high purchase prices, high maintenance costs, excessive noise, poor longevity, and 

limited installation options.” By 1995, only two manufacturers remained and sales sat at 

approximately 2,000 units per year (Bodzin 1997). High first costs have been cited as a key reason 

for the collapse of the market (Ashdown et al. 2004), and declining electricity prices (EIA 1996, 

2012e) certainly played a role in the diminishing economic viability of these HPWHs.

The Third Generation: 1999–2010

Facing a collapse of the HPWH market, a collaboration effort between Arthur D. Little, Inc., ECR 

International, and Oak Ridge National Laboratory (ORNL) began development of a new integrated 

HPWH in the late 1990s to address the aforementioned issues with the previous generation of 

products. At that time, the only HPWHs on the market were add-on units to existing ERWHs. The 

first prototype was developed in 1999, and within a year the final prototype was produced that had 

an EF of 2.47. The final product, which was branded as the WatterSaver, was tested for durability 

 

14 All references to electricity price changes are relative to inflation (i.e., electricity price fluctuations are stated in real 

terms).
53

and field performance in subsequent years (Ashdown et al. 2004; Baxter and Linkous 2004;

Tomlinson 2002). 

During field-testing of units across the United States, the average COP was 2.0, which is more than 

double the efficiency of a comparable ERWH (Ashdown et al. 2004). In-house durability testing at 

ORNL uncovered no major durability issues (Baxter and Linkous 2004). Outside studies, however, 

had more mixed results from these products. A study in California found an average COP of 1.27 

for sites in California (Zogg et al. 2004). SWA performed field testing of 20 WatterSaver HPWHs 

in 2002 in the Northeast for Connecticut Light & Power, which is a division of Northeast Utilities. 

With an average COP of 1.67, the efficiency of the unit was slightly lower for these field sites, and 

operational and reliability issues with the units were uncovered (SWA 2004).

During the course of SWA’s monitoring, customer satisfaction was fairly high and many 

participants noted the dehumidification benefits. However, the study also identified some consistent 

drawbacks with the daily operation of the systems. Many customers complained about excessively 

hot water, and monitoring showed water temperatures near the tops of the tanks often exceeding 

150°F. These high temperatures were partly due to excessive tank stratification—water 

temperatures near the top could be 50°F higher than temperatures near the bottom—and in many 

systems the high-temperature safety switches shut down the water heaters completely (these were 

designed to shut down the system when temperatures reached 170°F). These issues were 

communicated to the manufacturer (SWA 2004). Ultimately, the WatterSaver was removed from 

the market because of the identified problems with installed performance and a nonexistent service 

infrastructure (Environmental Building News 2005).

Current Products: 2010–Present

A variety of economic and regulatory factors are pushing HPWHs back into the mainstream 

marketplace, as discussed in Section 1. There are currently five major HPWH products in the water 

heater market. Key specifications for these HPWHs are shown in Table 21. These specifications 

include the two DOE performance metrics—EF and FHR.

15

The EF represents the efficiency of the electric heating elements and tank losses under a consistent, 

24-h test procedure. The FHR represents the amount of hot water that can be supplied by a fully 

heated storage water heater during an hour of operation. Storage tank volume is often used as a 

proxy for water heating capacity, but FHR is the preferred metric. See Section 2.1 for more details 

regarding the EF and FHR test procedures.

 

15 RE is also used to quantify the efficiency of natural gas, fuel oil, and propane water heaters. RE is an approximate 

metric of the burn efficiency of the heating device. Since electric resistance elements have an efficiency of 1, RE is not 

tested or reported. 
54

Table 21. Key Specifications of Integrated HPWHs Currently Available in the U.S. Market16

Model Capacity 

(gal) EF FHR (gal)

Electric Resistance 

Elements 

(kW)

GE GeoSpring 50 2.35 63.0 Upper: 4.5

Lower: 4.5

A.O. Smith Voltex 60/80 2.33 68.0/84.0 Upper: 4.5

Lower: 2.0

Stiebel Eltron Accelera 300 80 2.51 78.6 Upper: 1.7

Lower: None

Rheem EcoSense 40/50 2.00 56.0/67.0 Upper: 2.0

Lower: 2.0

AirGenerate AirTap 

Integrated 50/66 2.39/2.40 60.0/75.0 Upper: 4.0

Outlook for the Future

While HPWH development in the United States focused on traditional refrigerants, Japanese 

manufacturers, in conjunction with the Tokyo Electric Power Company, focused on developing a 

HPWH based on the carbon dioxide (CO2) cycle. At the same time the WatterSaver was being 

developed, a Japanese consortium started development of the CO2 HPWH, which had a COP of 3.4 

when subjected to an ambient air temperature of 46.4°F and average tank temperature of149°F. 

Marketed under the EcoCute brand, this CO2 HPWH was released in 2001. The EcoCute is a split 

system, meaning that it does not affect space conditioning loads. Furthermore, the units can be 

connected to a smart grid to try to move reheating periods to off-peak electricity periods. COPs 

have increased to > 4.0 in more recent models, and sales of the EcoCute have increased 

dramatically since its introduction (Hashimoto 2006). 

Development of CO2 HPWHs in the United States is just beginning. While market, technological, 

regulatory, and structural hurdles still remain for this technology, there has been active engagement 

in pushing CO2 HPWHs in recent years. A collaboration between ORNL and GE has developed a 

prototype integrated HPWH based on the CO2 cycle for the residential U.S. market. While this 

prototype is still in its early phases of development, future products may take advantage of CO2 as a 

more environmentally benign refrigerant (Abdelaziz et al. 2012).

 

16 Any omission of a manufacturer or product is unintentional, and no endorsement of any commercial product or 

manufacturer is implied.
55

Appendix B: Alternative Water Heating Calculations

Alternative water heating products were evaluated using the equations underlying the WATSMPL 

simulation program (EPRI 2000, 1995). For electric tanks the assumed RE is 1 and pilot load is 0. 

The net water heating load is computed using the hot water load, inlet temperature, and outlet 

temperature

( ) Qnet = Vc p Tout −Tin γ , (9)

where

Qnet = net water heating load (Btu),

V = hot water use (gallons),

cp = specific heat of water (Btu/lb°F),

γ = specific weight of water (lb/gal),

Tout = outlet temperature (°F), and

Tin = inlet temperature (°F).

The energy lost from the tank during the EF test Qtank,EF (Btu) is





 



 = × −1 tank,EF EF

RE Q Qwater , (10)

where

Qwater = thermal energy drawn during EF test (41,063 Btu/day),

RE = recovery efficiency, and

EF = energy factor.

The energy lost from the tank during operation minus the lost energy from the pilot load Qtank (Btu) 

is

pilot

out amb RE Q T T Q Q  − ×



 





−

− = ×

135 67.5 tank tank,EF , (11)

where

Qpilot = pilot load (Btu), and

Tamb = ambient temperature (°F).

The tank combustion losses Qcombustionloss (Btu) is

( ) 



 



 = + × −1 1

combustion loss tank RE Q Qnet Q . (12)

The gross water heating load Qnet (Btu) is

Qgross = Qnet +Qtank +Qcombustion loss + Qpilot . (13)
56

Appendix C: Monitored and Modeled Energy Use and Costs by Site

Using the equations listed in Appendix B and the operating characteristics discussed in Section 5, 

the following site energy, source energy, yearly operating costs are listed in the following tables. 

HPWH numbers are annualized based on monitored data, and all other systems are modeled using 

the operating conditions measured for each HPWH site.

Table 22. Annual Site Energy Consumption by Site

Site HPWH

(kWh)

ERWH

(kWh)

Standard

Gas

(therms)

Premium

Gas

(therms)

Condensing

Gas

(therms)

Standard

Oil

(gal)

Premium

Oil

(gal)

Standard

Propane

(gal)

Premium

Propane

(gal)

1 1,155 3,045 173 149 117 124 106 189 163

2 1,969 5,899 304 271 221 217 193 331 295

3 1,838 4,152 222 195 157 158 139 242 213

4 1,186 2,981 169 145 115 120 104 184 159

5 3,612 4,739 258 225 180 184 161 281 246

6 892 2,502 149 126 98 106 90 162 137

7 714 1,756 116 95 71 83 68 126 103

8 909 2,403 144 122 94 103 87 157 133

9 1,169 2,665 154 132 103 110 94 168 144

10 1,475 3,434 198 169 133 141 121 216 185

11 3,080 5,927 303 271 222 216 193 330 296

12 1,380 3,114 176 151 120 125 108 192 165

13 1,317 2,393 145 122 94 104 87 158 133

14 922 2,235 136 114 88 97 82 148 125

Table 23. Annual Source Energy (MMBtu) Consumption by Site

Site HPWH ERWH Standard

Gas

Premium

Gas

Condensing

Gas

Standard

Oil

Premium

Oil

Standard

Propane

Premium

Propane

1 13.3 35.0 18.9 16.3 13.5 20.1 17.2 19.9 17.1

2 22.6 67.7 33.1 29.6 25.4 35.2 31.3 34.9 31.2

3 21.1 47.7 24.2 21.3 18.1 25.7 22.6 25.5 22.4

4 13.6 34.2 18.4 15.9 13.2 19.5 16.8 19.4 16.7

5 41.5 54.4 28.1 24.6 20.8 29.8 26.1 29.6 25.9

6 10.2 28.7 16.3 13.8 11.3 17.2 14.6 17.1 14.5

7 8.2 20.2 12.6 10.3 8.2 13.4 11.0 13.3 10.9

8 10.4 27.6 15.7 13.3 10.8 16.7 14.1 16.6 14.0

9 13.4 30.6 16.8 14.4 11.9 17.8 15.3 17.7 15.2

10 16.9 39.4 21.6 18.5 15.3 22.9 19.6 22.8 19.5

11 35.4 68.1 33.1 29.6 25.5 35.1 31.3 34.8 31.2

12 15.8 35.8 19.2 16.5 13.8 20.3 17.5 20.2 17.4

13 15.1 27.5 15.8 13.3 10.8 16.8 14.1 16.7 14.0

14 10.6 25.7 14.8 12.5 10.1 15.7 13.2 15.6 13.1
57

Table 24. Annual Operating Costs ($) by Site

Site HPWH ERWH Standard

Gas

Premium

Gas

Condensing

Gas

Standard

Oil

Premium

Oil

Standard

Propane

Premium

Propane

1 135 357 191 164 129 486 418 539 463

2 231 691 334 298 243 852 760 944 842

3 215 487 244 214 173 622 547 690 607

4 139 349 185 160 126 473 408 525 452

5 423 555 283 248 198 723 632 801 700

6 105 293 164 139 108 418 353 463 392

7 84 206 127 104 78 325 266 360 295

8 107 282 158 134 104 404 341 448 378

9 137 312 170 145 114 433 370 480 410

10 173 403 218 186 146 556 476 616 527

11 361 695 333 298 244 850 760 942 842

12 162 365 193 167 132 493 425 546 471

13 154 280 159 134 103 407 342 451 379

14 108 262 150 126 97 382 320 423 355

Table 25. Annualized Energy Related Costs ($) by Site

Site HPWH ERWH Standard

Gas

Premium

Gas

Condensing

Gas

Standard

Oil

Premium

Oil

Standard

Propane

Premium

Propane

1 471 417 262 253 437 577 560 622 561

2 567 751 405 387 551 943 902 1,028 940

3 429 547 315 304 481 713 690 773 704

4 353 410 257 249 434 564 550 608 550

5 637 616 355 337 506 814 774 885 798

6 318 353 235 228 415 508 496 547 490

7 298 266 199 194 386 416 408 444 392

8 320 342 230 223 411 495 484 532 476

9 351 372 241 234 421 523 512 563 508

10 509 463 289 276 454 647 618 700 625

11 575 755 404 387 551 940 902 1,025 940

12 376 425 264 256 439 584 568 630 569

13 368 341 231 224 411 498 485 535 477

14 322 322 221 215 404 472 463 507 453
58

Appendix D: Identifying Heating Events

Choosing an appropriate analysis period to investigate the trends in the collected data is vital for 

untangling the various input variables that can affect performance. Looking at the data on a yearly 

site-by-site basis is inappropriate because the sample size for this project is not large enough and 

seasonal swings in operating conditions cannot be considered. Conversely, looking at the 15-min 

data is also inappropriate because knowing the internal state of the tank, which was not monitored 

in this study, would be necessary to understand the transient response of the water heater to the 

operating conditions. Using daily data would be more appropriate because the system can be 

assumed to be in steady state; i.e.. the system operating variables do not change considerably, but 

there is a larger sample size to investigate.

Daily data, however, are not quite perfect because the response rate of the heat pump system is 

considerably slower than other heating mechanisms. As a result, a large draw at the end of one day 

can be recovered entirely during the following day, as shown in Figure 23, where the red ellipse 

highlights an incidence of such a draw. As a result of this issue, the COP trends in relation to the 

draw volume are reversed. One would expect the first day to have a lower COP than the second day 

because the draw volume is smaller, which means that the standby losses account for a larger 

percentage of the recovery load. The trend from using the daily data, however, is reversed, which 

confuses the analysis.

Figure 23. Results from using daily data for analysis

0

40

80

120

160

200

240

0

2

4

6

8

10

12

8/20 8/21 8/22

HW Used (Gallons)

Energy Used (Wh)

HW Use

Heat Pump

Electric Resistance

20 gals

1.5 COP

35 gals

1.4 COP
59

A solution to this problem is to find periods of time where the water heating tank starts fully heated 

and ends fully heated. The criteria for finding such periods, which are defined as heating events in 

this study, are: (1) the heating event must be ≥ 4 h in length; and (2) the heating event must begin 

and end after a reheat cycle that has a duration ≥ 1 h during which no draws occurred. There are 

some tolerances to allow for noise in the measurements. 

To demonstrate how this works in practice, consider Figure 24. Five heating events are displayed in 

this graph, four of which have no electric resistance use. Each heating event begins and ends at the 

end of a reheat cycle. As shown in the figure above, the COPs are related to the draw volumes as 

expected. Furthermore, the heating events have the advantage of operating in near steady state, just 

as daily values, but without the limitations of daily values. 

Figure 24. Example of five heating events as defined in this study

Fri 02:30 Fri 12:45 Fri 20:00 Sat 02:45 Sat 15:45 Sun 17:15

0

10

20

30

HW Used (Gallons)

Fri 02:30 Fri 12:45 Fri 20:00 Sat 02:45 Sat 15:45 Sun 17:15

0

500

1000

1500

Energy Used (Wh)

HW Use Heat Pump Electric Resistance

60 gals

2.6 COP

27 gals

2.2 COP

53 gals

2.4 COP

44 gals

2.4 COP

70 gals

1.1 COP
60

Appendix E: Laboratory Tests of Heat Pump Water Heaters

Under 10 CFR Part 430

Table 26 through Table 30 display information that corresponds to laboratory study test results and 

manufacturer ratings for various HPWHs. All laboratory study test were performed at DOE 

laboratory rating standards. Deviations from DOE specifications are denoted by footnotes.

Table 26. Air Generate AirTap

Laboratory Study Results Rated Reference Study A B C D17 E F

EF – – – – 2.25 – 2.39/2.40

FHR (gal) – – – – 65.0, 64.7S129, 74.0 S126 – 60.0/75.0

UA (Btu/h-°F) – – – – 3.30 – –

RE – – – – 2.64 – –

Table 27. A.O. Smith Voltex Integrated

Laboratory Study Results Rated Reference Study A B C D17 E F

EF – 2.29 2.50 S120 – 2.24 S129 – 2.33/2.33

FHR (gal) – 87.0 – – 89.0 S129, 89.0, 88.5S140 – 68.0/84.0

UA (Btu/h-°F) – 3.9 – – 3.9 S129 – –

RE – – – – 2.46 S129 – –

 

17 Ambient conditions not maintained to DOE specifications for lab study S120 120°F set point S126 126°F set point S129 129°F set point S136 136°F set point S140 140°F set point

† Thermostat set point reduced to between normal and hot (~129°F) from hot (~133°F) A50 Ambient air temperature at 50°F N Normal mode ES Energy saver mode HP Heat pump only mode H Hybrid mode EH eHeat mode HD High Demand Mode R Resistance mode

* Standard DOE procedure was not followed
61

Table 28. Rheem EcoSense

Laboratory Study Results Rated Reference Study A B C D17 E F

EF – 1.69 2.40S120 1.33N,1.61ES,

1.98†ES ,0.82R

1.68S136,

1.19S129 – 2.00/2.00

FHR (gal) – 37.5 – 60.0ES,59.0ES,

61.0N

39.4S136, 52.6S129,

36.2S126 – 56.0/67.0

UA (Btu/h-°F) – 5.1 – – 5.1S136, 5.2S129 – –

RE – – – – 2.04S136,

1.55S129 – –

Table 29. GE GeoSpring

Laboratory Study Results Rated Reference Study A B C D17 E F

EF 2.42,

1.92A50 2.41 2.70S120 2.44H,0.84R 2.37S140

2.38,2.36EH,

1.08HD,0.83R,

2.61S120,

1.83EH,S120,

1.63HD,S120,

0.92 R,S120

2.35

FHR (gal) 66.0 57.0 –

59.0,

34.0EH,

62.0R

57.3S140 – 63.0

UA (Btu/h-°F) 3.6 3.8 – – 3.7S140 – –

RE – – – – 2.78S140 – –

Table 30. Stiebel Eltron Accelera 300

Laboratory Study Results Rated Reference Study A B C D17 E F

EF – – – – 2.48S129 – 2.51

FHR (gal) – – – – 77.1S129,74.0,75.3S140 – 78.6

UA (Btu/h-°F) – – – – 4.6S129 – –

RE – – – – 3.08S129 – –
62

Appendix F: HPWH Informational Tri-Fold
63
64
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