NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Laboratory Performance 

Evaluation of Residential 

Integrated Heat Pump Water 

Heaters

B. Sparn, K. Hudon, and D. Christensen

Technical Report

NREL/TP-5500-52635

September 2011
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, Colorado 80401

303-275-3000 • www.nrel.gov

Contract No. DE-AC36-08GO28308

Laboratory Performance 

Evaluation of Residential 

Integrated Heat Pump Water 

Heaters

B. Sparn, K. Hudon, and D. Christensen

Prepared under Task Nos. WTN9.1000, ARRB.2204

Technical Report

NREL/TP-5500-52635

September 2011
NOTICE

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iii 

Contents

List of Figures ............................................................................................................................................ iv

List of Tables ............................................................................................................................................... v

Definitions ................................................................................................................................................... vi

Acknowledgements ................................................................................................................................... vi

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

Test Plan ...................................................................................................................................................... 2

Operating Mode Tests: ...............................................................................................................3

DOE Standard Tests: ..................................................................................................................4

Draw Profile Tests: ....................................................................................................................5

COPhpTests:................................................................................................................................6

Reduced Airflow Tests: .............................................................................................................7

Laboratory Setup ........................................................................................................................................ 7

Test Plenum ...............................................................................................................................8

Air-Side Equipment ...................................................................................................................8

Water-Side Equipment .............................................................................................................10

Test Article Summary ............................................................................................................................... 11

Test Results............................................................................................................................................... 13

Uncertainty Analysis ................................................................................................................13

Unit A HPWH Summary .........................................................................................................14

Overall Impressions and Observations ..............................................................................14

Installation and Usability Considerations ..........................................................................14

Qualitative Test Results and Observations ........................................................................15

System Drawbacks .............................................................................................................20

Unit B HPWH Summary .........................................................................................................21

Overall Impressions and Observations ..............................................................................21

Installation and Usability Considerations ..........................................................................21

Qualitative Test Results and Observations ........................................................................22

System Drawbacks .............................................................................................................27

Unit C HPWH Summary .........................................................................................................29

Overall Impressions and Observations ..............................................................................29

Installation and Usability Considerations ..........................................................................29

Qualitative Test Results and Observations ........................................................................30

Anomalies ..........................................................................................................................35

System Drawbacks .............................................................................................................36

Unit D HPWH Summary .........................................................................................................37

Overall Impressions and Observations ..............................................................................37

Installation and Usability Considerations ..........................................................................37

Qualitative Test Results and Observations ........................................................................38

System Drawbacks .............................................................................................................43

Unit E HPWH Summary ..........................................................................................................44

Overall Impressions and Observations ..............................................................................44

Installation and Usability Considerations ..........................................................................44

Qualitative Test Results and Observations ........................................................................45

System Drawbacks .............................................................................................................50

Data Comparison and Analysis ............................................................................................................... 51

Conclusions............................................................................................................................................... 60
iv

References................................................................................................................................................. 62

Appendix A – Draw Profiles ..................................................................................................................... 63

Appendix B – Laboratory Schematics .................................................................................................... 66

Appendix C – Heating Capacity Curves.................................................................................................. 68

List of Figures

Figure 1. Test Plenum during normal testing (left) and during installation (right). ............................. 8

Figure 2. Duct heaters used to control humidity (top left), heat exchanger to provide cooling (top 

right), evaporation pads (bottom left), and temperature and humidity measurement at inlet to 

plenum (bottom right). ......................................................................................................................... 9

Figure 3. Laminar flow element used to measure airflow rates out of HPWHs.................................... 9

Figure 4. Inlet pressure transducer and thermocouple, outlet thermocouple, and dump solenoid 

valve on Unit B.................................................................................................................................... 10

Figure 5. Thermocouple tree prior to installation.................................................................................. 10

Figure 6. Inlet turbine flow meters (left) and coriolis flow meter for condensate (right)................... 11

Figure 7. Outlet turbine flow meters, proportioning valves, and solenoid valves used to control 

outlet flow rate and draws. ................................................................................................................ 11

Figure 8. Unit A 80 gallon HPWH installed in test plenum (left). A close-up of the compressor and 

refrigerant system in Unit A (upper right) and a side view of the evaporator coils and heat 

exchanger (lower right)...................................................................................................................... 14

Figure 9. Draw Profile 1 for Unit A........................................................................................................... 17

Figure 10. COPhp traces for Unit A. ......................................................................................................... 18

Figure 11. Effect of Humidity on COPhp for Unit A................................................................................. 19

Figure 12. Effect of Humidity on Recovery Time for Unit A.................................................................. 19

Figure 13. Heat Pump COPhp traces for Unit A HPWH with Reduced Airflow..................................... 20

Figure 14. Unit B in the test plenum (left) and the instrumented heat pump components (right).... 21

Figure 15. Draw Profile 1 for Unit B......................................................................................................... 24

Figure 16. COPhp traces for Unit B. ......................................................................................................... 25

Figure 17. Effect of Humidity on COPhp for Unit B................................................................................. 26

Figure 18. Effect of Humidity on Recovery Time for Unit B.................................................................. 26

Figure 19. COPhp traces for Unit B with Reduced Airflow..................................................................... 27

Figure 20. Unit C in the test plenum (left), the compressor and coaxial heat exchanger (top right), 

and the water circulation pump, evaporator, and control wiring (bottom right). ........................ 29

Figure 21. Draw Profile 1 for Unit C......................................................................................................... 32

Figure 22. COPhp traces for Unit C. ......................................................................................................... 33

Figure 23. Effect of Humidity on COPhp for Unit C................................................................................. 34

Figure 24. Effect of Humidity on Recovery Time for Unit C.................................................................. 34

Figure 25. COPhp traces for Unit C with Reduced Airflow..................................................................... 35

Figure 26. Unit D, an 80 gallon HPWH, installed in the test plenum (left). A close up of the air intake 

(top right) and the heat pump (bottom right) are also shown........................................................ 37

Figure 27. Draw Profile 1 for Unit D......................................................................................................... 40

Figure 28. COPhp traces for Unit D........................................................................................................... 41

Figure 29. Effect of Humidity on COPhp for Unit D................................................................................. 42

Figure 30. Effect of Humidity on Recovery Time for Unit D.................................................................. 42

Figure 31. COPhp traces for the Unit D with Reduced Airflow .............................................................. 42

Figure 32. Unit E installed in test plenum (left). Some of the refrigerant system is shown (upper 

right), along with the blower that is designed to connect to a 6” duct (lower right)................... 44

Figure 33. Draw Profile 1 for Unit E......................................................................................................... 47

Figure 34. COPhp traces for Unit E........................................................................................................... 48

Figure 35. Effect of Humidity on COPhp for Unit E................................................................................. 49

Figure 36. Effect of Humidity on Recovery Time for Unit E.................................................................. 49

Figure 37. COPhp Traces Unit E with Reduced Airflow.......................................................................... 49

Figure 38. DOE 24-hour test are compared for all HPWHs................................................................... 52
v 

Figure 39. Comparison of First Hour Rating Estimates for all HPWHs. .............................................. 53

Figure 40. COPhp Comparison for Five HPWHs at 14°C Inlet Wet Bulb Air Temperature.................. 54

Figure 41. Heating Capacity Comparison for Five HPWHs at 14°C Inlet Wet Bulb Air Temperature55

Figure 42. Graphical representation of Draw Profile 1.......................................................................... 63

Figure 43. Graphical representation of Draw Profile 2.......................................................................... 65

Figure 44. Water-side Schematic............................................................................................................. 66

Figure 45. Air-side Schematic.................................................................................................................. 67

Figure 46. Heating Capacity as a Function of Average Tank Temperature for Unit A....................... 68

Figure 47. Heating Capacity as a Function of Average Tank Temperature for Unit B....................... 68

Figure 48. Heating Capacity as a Function of Average Tank Temperature for Unit C....................... 69

Figure 49. Heating Capacity as a Function of Average Tank Temperature for Unit D....................... 69

Figure 50. Heating Capacity as a Function of Average Tank Temperature for Unit E....................... 70

All figures in this report were created by NREL.

List of Tables

Table 1. Tests performed on each HPWH, with the air and water conditions for each test shown. .. 2

Table 2. Summary of the physical characteristics of the HPWH units tested. ................................... 12

Table 3. Summary of manufacturer ratings and measured DOE test estimates for Unit A............... 16

Table 4. Summary of manufacturer ratings and measured DOE test estimates for Unit B............... 23

Table 5. Summary of manufacturer ratings and measured DOE test estimates for Unit C............... 31

Table 6. Summary of manufacturer ratings and measured DOE test estimates for Unit D............... 39

Table 7. Summary of manufacturer ratings and DOE Standard test estimates for Unit E. ............... 47

Table 8. Comparison of operating modes and control logic for all 5 units. ....................................... 51

Table 9: Draw Profile System COPsys Values ......................................................................................... 56

Table 10: Recovery Rate Comparison vs. Electric Resistance Water Heater..................................... 57

Table 11: Cooling Capacity and SHR Comparison................................................................................ 58

Table 12: Coefficients for Normalized COPhp Curve .............................................................................. 58

Table 13: Coefficients for Normalized Heating Capacity Curve........................................................... 59

Table 14: Rated COPhp and Heating Capacity ........................................................................................ 59

Table 15. Draw Profile 1 details. Morning segment includes the four showers and the evening 

segment contains remaining draws. ................................................................................................ 63

Table 16. Draw Profile 2 details. .............................................................................................................. 64

All tables in this report were created by NREL.
vi 

Definitions 

Acronym Definition

AC Alternating Current

COPhp Coefficient of Performance of the Heat Pump

COPsys Coefficient of Performance of the System

DC Direct Current

DOE Department of Energy

DX Direct Expansion

EF Energy Factor

EPA Environmental Protection Agency

FHR First Hour Rating

HPWH Heat Pump Water Heater

NIST National Institute of Standards and Technology

ORNL Oak Ridge National Laboratory

WFO Work for Others

Acknowledgements 

The authors thank Jeff Tomerlin and Greg Perrin for their contributions to this laboratory 

experiment. Without them, this work would not have been possible. The authors also appreciated 

the input from representatives for each test article manufacturer, who provided additional 

information about the geometry, functionality, and operation of their units. 
1

Introduction 

Advanced technologies for residential buildings are sought that provide broadly applicable, 

measurable, and cost-effective energy savings compared to legacy products. One significant 

opportunity for energy savings is domestic water heating, where an emerging technology has 

recently arrived in the U.S. market: the residential integrated heat pump water heater (HPWH). 

The heat pumps in these units are small refrigerant-based direct expansion (DX) systems which 

absorb energy from the surrounding air and transfer it to water in an attached tank. This type of 

system is expected to have a high coefficient of performance (COP), which is the ratio of the 

useful energy transferred to the electrical energy consumed, across wide ranges of operation. 

Electric resistance water heaters have a COP of nearly 1.0, but HPWHs are expected to provide 

annual COP values over 2.0. 

The development of the heat pump water heater began in the 1950s, when the Hotpoint 

Company, which later became a division of General Electric (GE), designed and built a HPWH 

intended for mass production (Calm 1984). The technology performed well, but was stricken

with reliability issues. In the end, development efforts ceased because energy prices were low 

and there was little demand for the product. Fueled by rising energy prices in the 1970s, HPWH

products re-emerged, this time backed with improved heat pump technology. Oak Ridge 

National Laboratory (ORNL) tested both add-on and integrated units in 1982. There are two 

basic configurations for add-on HPWHs: either the heat pump package is located remotely from 

the storage tank and connected via hoses or the heat pump has a heat exchanger (the HPWH 

condenser) that is pushed into an existing hot water tank, often through the drain port. Integrated 

HPWHs are factory-integrated with a water tank and backup resistance heating elements. 

ORNL’s field and laboratory testing showed that HPWHs use about half the energy to heat 

domestic hot water when compared to an electric resistance water heater (Levins 1982).

Despite this promising research, few HPWHs were sold once energy prices fell sharply in the 

1980s. It was not until the turn of the century that a resurgence in the technology began again. 

An Australia study in 2001 used a TRNSYS model based on test results from three HPWHs to 

show that the annual COP for an integrated HPWH was 2.3, which translated to annual energy 

savings of 56% (Morrison 2003). During the same timeframe, the development of an integrated 

HPWH through collaboration between Arthur D. Little, Inc. (ADL), EnviroMaster International 

(EMI) and ORNL was also taking place. Extensive durability and field tests were conducted, and 

the design of the product was greatly improved as a result. Field test results for eighteen HPWHs 

placed in residences throughout the U.S. showed an average installed COP of about 2.0 (Baxter 

2005). The technology was market-ready, but a 2004 study identified cost, consumer awareness, 

and contractor technology perceptions as barriers to market acceptance in the United States

(Ashdown 2004). Meanwhile, foreign markets embraced the technology. For example, models 

such as the EcoCute® became prevalent in Japanese markets (Hashimoto 2006). 

Backed by the U.S. Department of Energy (DOE) and the Environmental Protection Agency 

(EPA), integrated HPWH technology was added to the ENERGY STAR® program in 2008 (CEC 

2011). This encouraged key manufacturers to revisit integrated HPWHs, resulting in those now 

available on the U.S. market. 
2

In this report, we present a laboratory evaluation of the five integrated HPWHs available in the 

U.S. market today. The results describe how these products function, demonstrate that efficient 

operation is typical but limitations exist, and provide information so reasonable expectations for 

the products can be determined. Performance testing occurred at the National Renewable Energy 

Laboratory (NREL) Advanced Thermal Conversion Laboratory, which is configured for highaccuracy performance measurement of thermal conversion systems and was used to explore 

HPWH performance across the full range of operating conditions.

Test Plan

The test plan for this project can be broken down into five sections. A summary of this test plan 

can be found in Table 1 and includes the air and water conditions, and operating modes 

associated with each test run during this experiment. The test plan was originally written by 

Ecotope, with a few additional tests added by NREL, and the complete list of tests performed is 

shown in Table 1. Following the summary are detailed descriptions of each of the tests that were 

performed. 

Five integrated heat pump water heaters were tested: the A. O. Smith Voltex® hybrid electric 

heat pump water heater, the GE GeoSpringTM water heater, the Rheem Hybrid Electric water 

heater, the Stiebel Eltron Accelera® 300 heat pump water heater, and the Air Generate AirTapTM 

Integrated heat pump water heater. These will be referred to from this point on as Unit A, Unit B, 

Unit C, Unit D, and Unit E, respectively. The testing took place between October 2010 and May 

2011, and occurred in three rounds. Two HWPH were tested at a time during each round of 

testing. Unit B and Unit C were tested during the first round, Unit A and Unit D during the 

second round and Unit E in the final round. 

Table 1. Tests performed on each HPWH, with the air and water conditions for each test shown.

Test Name

Dry 

bulb 

(°C/°F)

RH

Inlet 

Water 

(°C/°F)

Tank Set 

Point

(°C/°F)

Airflow Operating 

Mode

1. OPERATING MODE TESTS

OM-67 20/67.5 50% 14/58 57/135 100% All Factory 

Modes

OM-95 35/95 40% 14/58 57/135 100% Hybrid Modes

OM-47 8/47 73% 14/58 57/135 100% Hybrid Modes

2. DOE STANDARD RATING POINT TESTS

DOE-1hr 20/67.5 50% 14/58 57/135 100% Factory Default

DOE-130-1hr 20/67.5 50% 14/58 54/130 100% Factory Default

DOE-140-1hr 20/67.5 50% 14/58 60/140 100% Factory Default

DOE-24hr 20/67.5 50% 14/58 57/135 100% Factory Default

3. DRAW PROFILES

DP-1 20/67.5 50% 7/45 49/120 100% Factory Default
3

DP-2 20/67.5 50% 7/45 49/120 100% Factory Default

4. COPhp CURVE DEVELOPMENT – PERFORMANCE MAPPING

COPhp -47 8/47 73% 2/35 57/135 100% Compressor 

Only

COPhp -57 14/57 61% 2/35 57/135 100% Compressor 

Only

COPhp -67 20/67.5 50% 2/35 57/135 100% Compressor 

Only

COPhp -77 25/77 40% 2/35 57/135 100% Compressor 

Only

COPhp -85 29.5/85 42% 2/35 57/135 100% Compressor 

Only

COPhp -95 35/95 40% 2/35 57/135 100% Compressor 

Only

COPhp-95 dry 35/95 20% 2/35 57/135 100% Compressor 

Only

COPhp -105 40.5/105 42% 2/35 57/135 100% Compressor 

Only

COPhp -105 dry 40.5/105 16% 2/35 57/135 100% Compressor 

Only

5. REDUCED AIRFLOW

AF-1/3 20/67.5 50% 2/35 57/135 66% Compressor 

Only

AF-2/3 20/67.5 50% 2/35 57/135 33% Compressor 

Only

NOTE: The factory default mode for all units was Hybrid mode, or equivalent. 

During the second and third rounds of testing, DOE-130-1-hr and DOE-140-1hr were added to 

the test plan. The only test not run during the second round of testing was OM-47. During the 

third round of testing, the OM-95, OM-47, DP-3, COPhp -105 dry, and AF-1/3 tests were 

excluded to shorten the testing schedule by removing low-value tests.

Operating Mode Tests

The operating mode tests were designed to discover the control strategies for each water heater 

in each mode of operation. Each test began with the water heaters full of water at the “Outlet 

Water” set point and the inlet air temperature at 20°C dry bulb. A draw was initiated and 

continued until the compressor turned on. The draw was then stopped and the unit was allowed 

to recover. A second draw was performed for the same air conditions and set point. This second 

draw was allowed to continue until the electric heaters, if possible, for that operating mode came 

on, or until 80% of the tank volume had been drawn. The units were then allowed to recover. 

In addition to the 20°C dry bulb air condition, this procedure was followed in hybrid mode only 

for air at 8°C dry bulb and 35°C dry bulb. This included the Hybrid mode for Unit A, the Hybrid 
4 

and High Demand modes for Unit B, the Energy Saver and Normal modes for Unit C, on for 

Unit D, and Auto Mode for Unit E (Note: Units A, D, and E were not tested at the 8°C dry bulb 

air temperature because the results from this air condition provided little useful information 

during the first round of testing). Each mode of operation was tested at both the hottest water set 

point (60°C for Units A, B and D and ~57°C for Units C and E) and the standard 49°C set point. 

DOE Standard Tests 

Standard test methods for both the DOE 1-hour test and 24-hour test were followed per Federal 

Register 10 CFR Part 430, Subpart B, Appendix E (DOE 1998). The first round of testing 

revealed that the results of these tests can be very sensitive to set point temperature. It was not 

feasible to repeat the 24-hour test for multiple set point temperatures, but the 1-hour test was 

repeated for three set point temperatures for all subsequent test articles. If the maximum set point 

temperature was 60°C (Units A and D), the set point temperatures of 54°C, 57°C, and 60°C were 

used for the first hour rating test. For the units with a maximum set point temperature of 57°C 

(Units B and E), the first hour rating was determined at 52°C, 54°C, and 57°C, despite the fact 

that 52°C is outside the range allowed by the DOE test protocol. This lower set point was tested 

to determine the effect of set point on the rating. 

The calculation for FHR as described in the DOE test protocol was closely followed, but two 

equations leading up to the calculation for energy factor (EF) had to be modified for HPWH 

application. The recovery efficiency calculation in the DOE test protocol assumes that the water 

heater recovers fully in between each draw and calculates the recovery efficiency based on the 

first draw and the subsequent recovery. This was an inaccurate method for the HPWH tested 

because none were able to recover between each draw. The modified ηr equation was provided 

by William Healy at NIST and is expected to be incorporated into the official DOE testing 

specification document in the near future. The modified equation is as follows: 

ߟ ௥ൌ ሺܶ௥ െ ܶ௦ሻߩଵܸ௥௘௖ܥ௉ଵ ൅ ሺܶ௠ௗଵ െ ܶ௠௜ሻߩଶܸ௦௧ܥ௉ଶ

ܳ௥

where Tr is the average outlet temperature, Ts is the average inlet temperature, ρ1 is the water 

density at Tr, Vrec is the volume of water drawn before the first recovery, CP1 is the specific heat 

of water at Tr, Tmd1 is the maximum mean tank temperature after the first recovery, Tmi is the 

maximum mean tank temperature preceding the first draw, ρ2 is the density of water at the 

average between Tmd1 and Tmi, Vst is the volume of the storage tank, CP2 is the specific heat of 

water at the average between Tmd1 and Tmi, and Qr is the total energy used by the water heater 

between cut-out before the first draw and the first cut-out (or recovery) following the first draw. 

The biggest difference between this equation and the standard equation provided in the DOE test 

method protocol is that Vrec is the volume of water drawn before the first recovery. Some units 

did not fully recover until all six draws were complete; others recovered after a couple of draws 

and Vrec varies accordingly. The temperatures in the first term and the energy use in the 

denominator all correspond to the period before the first recovery in this modified equation. The 

standard DOE protocol uses only the volume of the first draw and the associated temperatures 

and energy use. This equation was used to calculate recovery efficiency for all the HPWHs 

tested. 
5 

A modified equation for standby heat loss coefficient was also used since the equation provided 

in the DOE testing specification improperly takes recovery efficiency into account. Following 

recovery after the last water draw, the period before any heating elements turned on to reheat the 

tank was evaluated for standby losses. If reheating did not occur, the entire remainder of the test 

following the final recovery was evaluated for standby heat losses. The standby heat loss 

coefficient was calculated as: 

ܷܣ ൌ ൫ܶ௠௔௫ െ ܶ௙,௦௧௕௬൯ܸ௦௧ܥߩ௉ൗ߬௦௧௕௬

ܶ௧,௦௧௕௬ െ ܶ௔,௦௧௕௬

Where Tmax is the maximum mean tank temperature following the sixth draw, Tf,stby is the mean 

tank temperature at the end of the standby period, Vst is the volume of the storage tank, ρ is the 

density of stored hot water at the average of Tmax and Tf,stby, CP is the specific heat of the stored 

water at the average of Tmax and Tf,stby, τstby is the elapsed time of the standby period, Tt,stby is the 

average tank temperature over the entire standby period and Ta,stby is the average ambient air 

temperature over the entire standby period. This is almost identical to the equation given in the 

DOE specification but the numerator of the equation was also divided by ηr, which is 

inappropriate during periods when heating is not occurring. The equation above was used when 

calculating UA for all the water heaters. 

It should also be noted that NREL is not a certified ratings laboratory, therefore, the results from 

the DOE tests are not official. These tests were performed for comparison to the rated values and 

to determine if set point temperature had an impact on first hour rating. The other properties of 

the water heaters calculated as a part of the 24-hour test, ηr and UA, are also valuable for 

characterizing the water heaters for future modeling efforts. 

Draw Profile Tests 

Two draw profiles were used to challenge the water heaters with high volume draws and low 

volume draws over the course of several hours. The draw profiles, suggested by Ecotope, Inc., 

are based on NREL work (Hendron 2010) and a tabular representation of each profile can be 

found in Appendix A. Each test began with a full tank at the standard 49°C (120°F) set point. 

Draw Profile 1 contained a “morning” segment and an “evening” segment. The morning segment 

contains four showers over the course of an hour to test the performance in high demand 

situations. The evening segment of Draw Profile 1 contained a range of flow rates and draw 

durations to simulate the variety of hot water draws that can occur, such as dishwashing and food 

preparation. After the morning draws were completed, the units were allowed to recover fully 

before starting the evening segment. Full recovery was deemed complete when all the heating 

elements, either the electric resistance elements and/or the heat pump, were turned off by the 

water heater’s controller. Draw Profile 2 consisted of many short draws and was allowed to run 

uninterrupted. Depending on tank size and recovery rate, these draw profiles have the potential to 

deplete the hot water reserve, resulting in warm or even cold water being delivered to the users. 

For the results of these tests, “hot” water was defined as anything above 40.5°C (105°F). 
6 

The results from the Draw Profile tests were used to calculate system COP values for the HPWH 

running as a complete system. This means that if the HPWHs used back-up electric resistance 

heat, the COP value would reflect this. For the remainder of this report, the system COP will be 

referred to as COPsys. The method used to calculate COPsys values is defined in the following 

subsection. 

COPhpTests 

The energy efficiency gained from a HPWH is due to its ability to transfer more heat from the air 

into water than the energy consumed by that transfer process. Thus, a series of tests were 

conducted to examine refrigerant system performance alone. To ensure the resistance heaters did 

not turn on, they were electrically disconnected for the units without a heat pump-only mode. 

The results from these tests were used to calculate a COP for the heat pump operating alone. The 

COP for the heat pump operating alone will be referred to as COPhp for the remainder of this 

report. 

Unlike the other tests, the COPhp tests began with a tank full of cold water. Each tank was empty 

to start the day and the tanks were filled with 3°C (or colder) water. Once the water heaters were 

full of cold water, the units were turned on in their most efficient operating mode: Efficiency for 

Unit A, eHeat for Unit B, Energy Saver for Unit C, and Econ for Unit E. Unit D has only one 

operating mode, so this unit was turned on without selecting a mode. The test was deemed 

complete when the tank set point was achieved. 

To run these tests using the heat pump only, Units A and C needed to be modified because the 

compressor is disabled when the water temperature is below 27°C in the case of Unit C and 14°C 

in the case of Unit A. Also, the electric resistance elements were disabled for Units C and D 

since both these units can use their heating elements in conjunction with the heat pump, even in 

their most efficient operating mode. The modifications needed to ensure compressor-only 

operation are discussed in more detail in the summary sections for each HPWH. 

As mentioned previously, the results of these tests were used to calculate the COPhp for the 

operation of the heat pump alone across a wide range of air and water temperatures. The 

coefficient of performance is the measure of useful energy transferred to the water (output) 

divided by the input energy to the system (supplied work). The equation used to calculate COP is 

given below. 

ܥܱܲ ൌ ܳ௧௛௘௥௠௔௟

ܹூ௡௣௨௧

ൌ ݉ · ݌ܥ · ሺ∆ܶሻ

ܹூ௡௣௨௧

In this equation, m is the mass of water in the tank, Cp is the specific heat of water at the average 

water temperature, and ΔT represents the difference in average tank temperature over a given 

time step, which in this case is one minute. Average tank temperature is calculated as the 

arithmetic mean of the six thermocouples mounted inside the tanks. The supplied work consists 

of the overall input energy to the unit, which includes the energy used by the heat pump, fan, 

electronic display, and circulation pump, if applicable. The heating capacity of the compressor, a 
7 

useful quantity for modeling purposes, was also captured during this test. Heating capacity is 

defined as: 

௟௔௠௥௘௛௧ܳ ൌ ݕݐܽܿ݅݌ܽܥ ݅݊݃ݐ݁ܽܪ

ݐ

where Qthermal is the thermal energy from the previous equation and t is the time step. The heating 

capacity for all units can be found in Appendix C. 

The COP of the entire system, COPsys, values were calculated for the draw profiles mentioned 

above. This calculation is different from the COPhp calculations in that m is the mass of the 

volume drawn and is determined using the volumetric flow rate and ΔT is the difference between 

the outlet and inlet water temperatures during draws. The supplied work term also includes the 

energy used by the electric resistance elements. 

NOTE 1: Standby losses are not inherently included in the thermal energy term of the COPsys

calculations because the inlet and outlet temperatures are used to calculate the thermal energy. 

This is in contrast to COPhp, which uses the average temperature within the tank to calculate 

thermal energy, and therefore takes into account standby losses. In order to include the standby 

losses in the thermal energy term of the COPsys calculations, the UA values calculated from the 

DOE test results were used to calculate the thermal loss associated with the tank. This loss was 

then added to the thermal energy term in the COPsys equation. 

NOTE 2: The performance of the heat pump depends on both temperature and humidity, so 

graphs showing the COPhp curves reference the inlet air temperature as a wet bulb temperature 

(see Figure 10, Figure 16, Figure 22, Figure 28, and Figure 34). 

NOTE 3: Any data for COPhp presented in this report inherently includes standby losses. 

However, when COPhp is used in future models, the curves will be modified to remove the 

standby losses using the UA values calculated during the 24-hour DOE test. 

Reduced Airflow Tests 

The setup for these tests was identical to the COPhp tests described above, but the filter area was 

restricted for each water heater. Tape was used to block 1/3 and then 2/3 of the filter’s surface 

area for these tests. These tests were designed to see the impact on performance if the filter was 

never cleaned or if something was obstructing the air intake area. COPhp curves were calculated 

for these tests using the same method as described above for the COPhp tests. 

Laboratory Setup 

This section describes the laboratory setup used to conduct the above test plan. The setup is 

divided into two sections: the air-side of the experiment and the water-side of the experiment. 

Real-time measurements taken on the two sides of the experiment were used to accurately 

control the air and water conditions at the inlet to the experiment and were used to determine the 

performance of each test article. Schematics for both setups can be found in Appendix B. 
8

NREL/PIX 18678 and 18670, Credit: Kate Hudon

Test Plenum

Each test article was enclosed in an insulated air plenum. During the first round of testing, Units 

B and C were enclosed in the same test plenum that was physically partitioned (but not air 

sealed) to ensure that the operational cycle of one heat pump water heater did not affect the 

operation of the other one. Units A and D were tested together during the second round of 

testing. Most of the third round of testing was conducted on Unit D alone but Unit C was 

reinstalled in the plenum to repeat the DOE tests during the third round of testing. 

Figure 1. Test Plenum during normal testing (left) and during installation (right).

An inlet duct was attached to the test plenum, creating a means to control the environment from 

which the heat pump drew air. At all times, the plenum’s inlet airflow was greater than the total 

airflow used by the HPWHs, to allow excess conditioned air to exit the plenum via a bypass air 

duct. As a result, uniform ambient conditions were assured in the proximity of the tanks. When a 

heat pump was operating, its exhaust air was collected in an outlet plenum connected to an outlet 

duct. The outlet plenums were not connected to each other.

Air-Side Equipment

A schematic of the air conditioning equipment that was used during this project can be found in 

Appendix B, Figure 45. The conditions specified in the test plan require that accurately

controlled conditioned air be supplied to and around the test article. Across the range of tests, the 

air within the test plenum needed to be heated, cooled, and/or humidified to achieve the desired 

inlet air conditions. 

Moisture was added to the air via evaporation pads. Moisture addition was controlled by heaters 

located upstream of the evaporation pads. Once the required water content was attained, the air 

was either heated further by additional electric resistance heaters or cooled using a chiller and

heat exchanger system. The temperature and humidity was measured in a duct directly upstream 

of the plenum inlet to ensure accurate inlet air conditions to the test articles.

Inlet duct

Inlet duct

Partition 

Wall
9

Figure 2. Duct heaters used to control humidity (top left), heat exchanger to provide cooling (top 

right), evaporation pads (bottom left), and temperature and humidity measurement at inlet to 

plenum (bottom right).

Inlet airflow rate was measured using ASME standard flow nozzles located downstream of the 

evaporative pads. The inlet pressure was measured using four static pressure taps located in the 

test plenum. These pressure taps were physically averaged together prior to measurement. The 

temperature and humidity of the air exiting each of the test articles was measured in an outlet 

duct located directly downstream of the units. The outlet pressure was measured with static 

pressure taps located in each test article’s outlet plenum. Each outlet duct was routed to a laminar 

flow element (LFE) for accurate measurement of heat pump airflow rate. Boost fans were used to 

overcome the pressure drop of laboratory equipment in the exhaust airstreams, thus preventing 

the test articles from experiencing any performance-degrading backpressure.

Figure 3. Laminar flow element used to measure airflow rates out of HPWHs. NREL/PIX 18683, 18687, 18688, and 19464 Credit: Kate Hudon NREL/PIX 18679, Credit: Kate Hudon
10

Water-Side Equipment

A schematic for the water-side equipment can be found in Appendix B. For each test, a steady 

and well-controlled inlet water temperature was required to emulate a range of water main 

temperatures. A large holding tank was preconditioned prior to each test and maintained at the 

desired temperature using a heater or chiller with a heat exchanger. An icemaker was used to 

rapidly reach colder inlet water temperatures. 

A water dump solenoid valve was actuated prior to the beginning of each draw to flush the inlet 

pipe with water at the desired “Inlet Water” set point temperature. The inlet water temperature 

and pressure was measured immediately upstream of the test article and the outlet water 

temperature and pressure was measured immediately downstream of the test article. Location of 

inlet and outlet thermocouples was dictated by the DOE test specifications found in 10 CFR Part 

430, Subpart B, Appendix E (DOE 1998). The inlet and outlet water pipes were insulated to limit 

heat transfer between the pipes and their surroundings.

Figure 4. Inlet pressure transducer and thermocouple, outlet thermocouple, and dump solenoid 

valve on Unit B.

A thermocouple tree consisting of six thermocouples was placed within each test article to 

measure stratification in the tank. Care was taken to position these thermocouples at the center of 

uniform volumes of water, which was often a challenge due to the non-uniform interior profile of 

some tanks. The thermocouple tree construction was also specified by the DOE test specification 

document (DOE 1998). These measurements were also necessary to calculate the performance of 

the test articles and to help understand their control logic. 

Figure 5. Thermocouple tree prior to installation.

NREL/PIX 18666,Credit: Kate Hudon

NREL/PIX 18668, Credit: Kate Hudon

Dump 

Valve

Pressure 

Transducer

Inlet and Outlet 

Thermocouples
11

The inlet and outlet water flow rates were measured using turbine flow meters. The only method 

to accurately measure condensate flow and condensate density from the heat pump is a coriolis 

flow meter. A coriolis meter was attached to the evaporator drain pan of each test article to 

measure condensate production. The temperature of the condensate flow was also measured. 

Figure 6. Inlet turbine flow meters (left) and coriolis flow meter for condensate (right).

For the tests having a prescribed water draw profile, an electronically-controlled proportional

valve was used. Draw profiles were preprogrammed and the turbine flow meter measurements at 

the test articles’ outlets were monitored during draws to ensure the correct flow rates. Prior to 

testing, a correlation was established between the percentage opening in the value and the 

resulting flow rate for each HWPH. This correlation was used to write draw profile programs 

that were tailored for each HPWH. The water from the outlet line was directed towards a 

laboratory drain. 

Figure 7. Outlet turbine flow meters, proportioning valves, and solenoid valves used to control 

outlet flow rate and draws.

Test Article Summary

Five integrated heat pump water heaters were tested: the A. O. Smith Voltex® hybrid electric 

heat pump water heater, the GE GeoSpringTM water heater, the Rheem Hybrid Electric water 

heater, the Stiebel Eltron Accelera® 300 heat pump water heater, and the Air Generate AirTapTM 

Integrated heat pump water heater. Again, these will be referred to as Unit A, Unit B, Unit C, 

NREL/PIX 18684 and 18686, Credit: Kate Hudon

NREL/PIX 18685, Credit: Kate Hudon
12

Unit D, and Unit E throughout the report. All five units are considered hybrid heat pump water 

heaters because they all have back-up electric resistance elements that can heat the water like a 

traditional electric water heater. While all units are similar in design and operation, each system 

is unique. The characteristics of all units are summarized in Table 2.

Table 2. Summary of the physical characteristics of the HPWH units tested.

Nominal 

Capacity 

(Gallons)

Compressor 

Power (W)

Electric 

Element 

Sizes (kW)

Refrigerant Condenser 

Type

Circulation 

Pump

Water 

Lines

Unit A 80 960 4.5 Upper

2.0 Lower R-134a

WrapAround 

Tank

No Side/

Horizontal

Unit B 50 700 4.5 Upper

4.5 Lower R-134a

WrapAround 

Tank

No Top/

Vertical

Unit C 50 1000 2.0 Upper

2.0 Lower R-410a

Coaxial 

Heat 

Exchanger

Yes Side/

Horizontal

Unit D 80 500 1.7 Upper R-134a

WrapAround 

Tank

No Side/

Horizontal

Unit E 66 800 4.0 Upper R-410a Immersed 

Coils No Side/Horizontal

Units B and C have a rated capacity of 50 gallons, while the Units A and D have a rated capacity 

of 80 gallons and Unit E has a tank with a rated capacity of 66 gallons. Measured tank capacity 

was about 45.5 gallons for Units B and C, 75 gallons for Unit A, 80 gallons for Unit D and 63 

gallons for Unit E. The compressors have slightly different power draws, with Unit D having the 

smallest compressor at 500 W and Unit C having the largest compressor at 1000 W. The 

combination of electric resistance elements also varies for each unit, with Unit B having two 4.5 

kW elements, Unit C having two 2.0 kW elements, and Unit A having a 4.5 kW upper element 

and a 2.0 kW lower element. Units D and E have only a single back up resistance heater, but 

Unit E has a 4.0 kW heater whereas Unit D has a small 1.7 kW heater. Units A, B and D use R134a refrigerant and a wrap-around style of condenser. In contrast, Unit C uses R-410a 

refrigerant and pumps water from the bottom of its tank and through a coaxial heat exchanger 

that serves as its condenser. Unit E also uses R-410a refrigerant but its condenser is an immersed 

coil that sits inside the tank near the bottom. The inlet and outlet water lines are vertical and 

come through the top of the tank for Unit B, whereas all other units have horizontal inlet and 

outlet water lines that are found on the side of each tank. 

The performance characteristics and control logic of each of these units also differ and will be 

discussed in more detail in the following section.
13

Test Results

The results for each HPWH are described in the following sections. The specific test results are 

summarized and overall impressions are given for each test article. A list of suggested 

improvements is also provided where opportunity for improving performance was uncovered. 

The suggestions given are based on our understanding of expectations that will be placed on a 

HPWH in an American home. By incorporating all the best features across the five brands of 

HPWHs, each one individually could be improved. 

Uncertainty Analysis

An uncertainty analysis was performed for all major results presented in this paper. Standard error 

propagation was performed and the associated uncertainty for each measurement is presented 

alongside the major result (Taylor 1997). The error analysis presented takes into account sensor 

accuracy as provided by the manufacturers. The random error component could not be 

characterized since each test was only completed once.
14

Unit A HPWH Summary

Overall Impressions and Observations

Unit A has a large storage tank and control logic that turns on the heat pump after small draws

and is able to deliver hot outlet water for all tests performed. It has an efficient heat pump that 

can quickly heat the tank of water, even with an 80 gallon tank. The logic controlling the electric 

resistance elements in hybrid mode is intended to quickly provide hot water when demand is 

high, but the majority of the heating load is provided by the heat pump. The engineers also chose 

a smaller lower resistance element that only operates in ‘Electric Only’ mode to encourage the

use of the Hybrid mode. The smaller lower element means that the ‘Electric Only’ mode is equal 

to or slower at heating the full tank than Hybrid mode. Overall, this unit succeeds in providing 

hot water for all scenarios tested and can maintain a high level of performance, even at low 

ambient temperatures. 

Installation and Usability Considerations

Unit A’s inlet and outlet water lines are located on the side of the tank. The 80 gallon unit that 

was tested is nearly 7’ tall. This could limit the possible installation locations in a home, 

especially in a retrofit situation. The 60 gallon version of Unit A is 5’7” and may be a better 

choice if space is limited. Also, the 80 gallon unit tested requires an airflow rate of about 500 

CFM. This means that the installation requirement for a location with unrestricted airflow is very

Figure 8. Unit A 80 gallon HPWH installed in test plenum (left). A close-up of the compressor 

and refrigerant system in Unit A (upper right) and a side view of the evaporator coils and heat 

exchanger (lower right).

NREL/PIX 18675, 18673, and 18674, Credit: Kate Hudon
15 

important for Unit A. The air enters the unit from the left (when viewed from the front), is pulled 

across the evaporator coils located in the middle of the heat pump, and exits from the right of the 

unit. The air filter slides into the front of the top cover and is secured with a screw. Currently, 

Unit A is not available through a major retailer, but can be purchased through plumbing 

suppliers. 

The user interface displays information about the current set point, the heating elements currently 

being used, the current mode and ways to change the mode and set point, but the touch screen is 

not as responsive as it should be. Buttons may need to be pressed a few times before it responds. 

The operating mode controls are accessible and clearly labeled, and changing the set point is also 

obvious. There are three operating modes available: Efficiency, Hybrid, and Electric. There is 

also a vacation mode that reduces the temperature set point to approximately 15°C. Changes 

made to operating mode take place almost immediately. When the heat pump is turned on, from 

off or as a result of a change in operating mode, the fan will turn on immediately and the 

compressor will turn on approximately 1 minute after the fan. 

Qualitative Test Results and Observations 

The descriptions below explain the results of the tests run on Unit A HPWH and the observations 

made during testing. 

Operating Mode Tests 

Unit A has two thermistors affixed to the exterior of the tank (under the insulation layer) that are 

used to control the heat pump and electric elements. The thermistors are located at the same 

height as the backup resistance elements. The following equation, which was provided by the 

manufacturer, shows the average tank temperature estimate that is used to control this unit: 

ܶ௧௔௡௞,௠௙௥ ൌ ൫3 כ ܶ௨௣௣௘௥ ൅ ܶ௟௢௪௘௥൯ 4⁄

In this equation, Tupper is the temperature measured by the upper thermistor and Tlower is the 

temperature measured by the lower thermistor. The upper thermistor is located near the upper 

heating element and the lower thermistor is located near the lower heating element. 

This quantity is monitored to dictate when heating is required. None of the heat sources—the 

heat pump, the upper electric element, or the lower element—can operate concurrently with 

another. Also, if the tank temperature is below 14°C upon initial start up, the heat pump will not 

run and the upper heating element will turn on instead. 

Operation Modes and description: 

1. Efficiency Mode: The heat pump turns on when Ttank,mfr drops 5°C below the set point 

temperature and will run until Ttank,mfr is equal to the set point. This mode of operation 

uses the heat pump exclusively unless the air temperature is outside the operating bounds, 

7°C to 43°C, as defined by the manufacturer, or Ttank,mfr falls below 14°C. If either of 

these conditions occur, the upper heating element turns on until Tupper reaches set point. 

The heat pump will then be used to finish the heating cycle, which is concluded when 

Ttank,mfr equals the set point temperature. 
16

2. Hybrid Mode: The heat pump will turn on when a 5°C drop in Ttank.mfr is detected and the 

upper heating element will turn on in place of the heat pump when Ttank.mfr has dropped 

by 10°C. The upper element will turn off when Tupper is at set point and revert to the heat

pump to finish the heating cycle. 

3. Electric Only Mode: Will only use electric resistance heating elements to heat the tank. A 

small drop in Ttank,mfr (2°C) will cause the upper heating element to turn on. The upper 

element will remain on until Tupper is at set point. The lower element will then turn on to 

heat the rest of the tank. Because the lower heating element has a small heating capacity 

(2kW), this mode has no benefit over the Hybrid mode in either recovery time or 

efficiency. This mode should only be used if the heat pump is not performing correctly. 

NOTE: For most air conditions, the compressor provides more heat input to the tank of water 

than the lower resistance element. In the worst case scenario, they provide about equal heat 

input. According to the engineers at Unit A’s manufacturer, they chose the size of the lower 

element for this exact reason. They did not want the electric resistance mode to have a 

performance advantage over the Hybrid mode. This should ensure that Hybrid mode (or 

Efficiency mode) is used exclusively by homeowners. 

DOE Standard Tests

The results of the DOE standard tests show good agreement with the manufacturer’s published 

values for both the first hour rating and the efficiency factor. The 1-hour DOE test was repeated 

with three different set points, 54°C, 57°C and 60°C, to see how the set point would affect the 

first hour rating results. All results were slightly better than the published values, with a 

difference of only 1 gallon between all three measured values. The 24-hour test was only run at a 

set point of 57°C and the result from that test agreed well with published values. Since the 

changes in set point had little effect on the first hour rating, it does not appear that Unit A has 

any built-in controls to artificially improve the standard ratings. 

Standby losses for this unit are reasonable and neither the compressor nor electric elements were 

used to maintain tank temperature during the standby portion of the 24-hour test, 

Table 3. Summary of manufacturer ratings and measured DOE test estimates for Unit A.

Operating 

Mode

Set 

point 

(°C)

Mfr. First 

Hour Rating 

(liters)

NREL - First 

Hour Rating 

(liters)

Mfr. Energy 

Factor 

Rating

NREL -

Energy 

Factor

UA 

(kJ/hr-°C)

Recovery 

Efficiency

Hybrid 54 318 337 ± 4 2.3 2.24 ± 0.02 7.4 ± 0.2 2.46 ± 0.05

Hybrid 57 318 337 ± 4

Hybrid 60 318 335 ± 4

Draw Profile Tests

Draw Profile 1 was tested in two parts. During the “morning” segment of Draw Profile 1, four 

showers are simulated over the course of a little more than an hour. Unit A was able to maintain 

an outlet temperature of 49°C during all four showers, with the only decrease in outlet 

temperature occurring at the very end of the fourth shower. This is well above the criteria for 
17

“hot” water (40.5°). The compressor turned on after the first draw and remained on for the rest of 

the morning segment until Ttank,mfr was equal to the set point temperature. The evening draws 

were also completed with no measurable dip in outlet temperature. The compressor came on 

about halfway through the evening portion and remained on until set point was achieved after the 

profile finished. Neither segment of Draw Profile 1 triggered the operation of the upper 

resistance element.

Figure 9. Draw Profile 1 for Unit A

Draw Profile 2 consisted of many small draws over a 6-hour period. Unit A performed well for 

this low-demand situation. The compressor turned on to reheat the tank about four hours into the 

6-hour profile. 

COPhp Tests

The results of the COPhp tests are plotted below in Figure 10. As expected, COPhp increases as 

the inlet air temperatures increases and the COPhp decreases as the water temperature in the tank 

increases. All tests were run in Efficiency Mode to ensure that only the compressor was used to 

the heat the tank. Even at the lowest air temperatures, Unit A did not use its resistance elements. 

While COPhp was lower at the colder air temperatures, there was not a dramatic decrease in 

COPhp at the lowest air temperatures that would suggest there was icing on the evaporator coils. 
18

Figure 10. COPhp traces for Unit A. 

The two warmest COPhp tests were repeated with the same inlet dry bulb temperature for high 

and low relative humidity. Figure 11 shows this comparison for the inlet dry bulb condition at 

35°C with the relative humidity at 40% and 19%. The corresponding wet bulb temperatures for 

these conditions are 24°C and 17°C, respectively. 

The results show that the performance improvements were not significant for the COPhp test run 

at the higher inlet wet bulb temperature. For the lower tank temperatures, a small increase in 

COPhp can be seen. However, as the average tank temperatures increases, the performance 

difference becomes insignificant. The tank recovery time is shown in Figure 12. This shows that 

the lower humidity reduced the recovery rate from 10.5°C/hr to 10.4°C/hr. These results show 

that humidity has a small, but not significant, impact on the performance of this unit.
19

Figure 11. Effect of Humidity on COPhp for Unit A Figure 12. Effect of Humidity on Recovery Time 

for Unit A

NOTE: To prevent situations where icing is more likely, the compressor will not run if the water 

in the tank is below 14°C. In order to run the COPhp tests beginning with 3°C water temperatures, 

the thermistors fastened to the outside of the tank were moved so that they would sense the air 

temperature, rather than the water temperature. At the lowest air temperature tested, 8°C, the 

thermistors were wrapped in insulation and heated slightly to keep the compressor running. 

When tank set point was reached, the HWPH had to be manually shut off since these thermistors 

were also used to measure tank temperature and turn off power when set point is achieved. The 

thermocouple tree installed inside the tank for testing purposes was used to monitor when set 

point was achieved. 

Reduced Airflow Tests

The results of the reduced airflow tests are shown in Figure 13. The results of the onethird airflow blockage test show an insignificant decrease in performance (<1% reduction 

in COPhp). The results of the two-thirds airflow blockage test show a slight decrease in 

performance, ranging from ~6% to ~2 % over the course of the heating cycle. 
20

Figure 13. Heat Pump COPhp traces for Unit A HPWH with Reduced Airflow

System Drawbacks

Below is a list of system drawbacks that could be improved to enhance the capabilities of Unit 

A:

1. The 80 gallon unit is very tall. The large tank capacity has many benefits from a 

performance perspective, but its physical size may limit the homes in which it can be 

installed. The 60 gallon unit is smaller and so should fit in more homes, but there will 

likely be differences in hot water availability under high demand. 

2. Higher airflow required. Unit A requires around 500 cfm of continuous airflow when 

the heat pump is operating. The installed location of this unit will need to accommodate 

this large airflow. Also, 500 cfm of cold air will likely be noticeable to the homeowners 

and could require ducting in some climates to prevent discomfort in the house during the 

winter months. 

3. Control panel touch screen is not sensitive enough. The touch screen on the control 

panel is not very responsive. We found that we often had to touch a button multiple times 

before the command was received. However, the layout of the control panel is simple and 

the icons used are intuitive. 
21

NREL/PIX 18671, 18664, and 18665, Credit: Kate Hudon

Unit B HPWH Summary

Overall Impressions and Observations

Unit B will use its heat pump to heat water the majority of the time, but two large electric 

resistance backup elements will turn on when faster recovery is needed. The compressor has a 

smaller capacity than most of the other units tested but it consistently operates across a wide 

range of conditions. The electric resistance elements were the largest of the five units in terms of 

combined power output, which allows Unit B to heat a full tank of water quickly when demand 

is high. The ‘eHeat’ mode uses only the heat pump for maximum efficiency while the ‘Hybrid’ 

and ‘High Demand’ modes use a combination of heat pump and resistance elements to meet 

demand, which can provide the user with faster recovery times in high demand situations. Unit B

can be operated with minimal energy demand for energy-conscious users but also provides a 

higher energy-use mode to meet high water heating demands. 

Installation and Usability Considerations

Installation of Unit B in our lab space was straightforward, without issue. The inlet and outlet 

water lines come directly out of the top of the unit, like most traditional gas or electric water 

heaters. The air filter lifts up from the top of the unit, and can easily be accessed for cleaning. 

The top cover can be removed without disturbing the water lines or power cable, providing 

Figure 14. Unit B in the test plenum (left) and the instrumented heat pump 

components (right).
22

access to the heat pump components for maintenance or repair. The unit is presently available for 

purchase at Sears and Lowe’s.

The control interface on this unit consists of a LCD screen with a key pad and buttons used to 

change settings. The user must navigate through multiple menus to change the set point 

temperature or select an operating mode. Unit B can be operated in four modes: eHeat, Hybrid, 

High-Demand, and Electric Only. If modes are switched while the compressor or heating 

elements are running, the change takes place immediately. Initial start-up in eHeat or Hybrid 

mode results in the fan turning on immediately followed by the compressor, 2-3 minutes later.

The air enters the heat pump from the neck and sides and is pulled across the evaporator coils to 

the exit at the back of the unit using two variable-speed, direct-current (DC) fans. To measure the 

speed of the variable speed fans, an optical tachometer was installed on one of the fans. We also 

measured the speed of the second fan to ensure that they operate identically, which confirmed the 

information provided by the engineers from the manufacturer. The variable speed fans ramp up 

to compensate in times of reduced air flow and their speed also varied in response to changes in 

inlet air conditions. 

Qualitative Test Results and Observations

The descriptions below explain the results of the tests run on Unit B and the observations made 

during testing.

Operating Mode Tests

Unit B does not allow either of the two electric resistance elements to operate simultaneously 

with the other, nor with the heat pump. Below is a description of the control logic that was 

determined during the operating mode tests. According to the manufacturer, a thermistor located 

near the upper heating element is used as input to the unit’s controller. 

Operation Modes and Description:

1. eHeat Mode: The heat pump will turn on once a small temperature drop (0.5°C) is 

detected by the thermistor. This mode uses the heat pump exclusively unless the air 

temperature is outside the operating bounds, defined by the manufacturer as 7°C to 49°C. 

The heating elements will turn on if icing on the evaporator coils is imminent. This mode 

is very efficient, but tank recovery is slow due to the smaller heating capacity of the 

compressor. 

2. Hybrid Mode: Similar to eHeat mode, the heat pump will turn on once a small drop in 

temperature is measured by the thermistor. A more significant drop in temperature (10°C) 

will cause a heating element to turn on. The lower heating element turns on first for 

moderately large draws and the upper element turns on first for very large draws. If the 

lower element turns on first, it will heat the tank to the set point temperature without ever 

using the upper element. If the upper element turns on first, it will remain on until the 

thermistor at the top of tank reads a temperature 3°C below set point temperature. The 

lower element will then turn on to heat the rest of the tank to the set point temperature. 

During our test, once an electric element has turned on, the remainder of the heating 

cycle will be accomplished with electric heat. However, according to the manufacturer, a 
23

sequence of smaller draws can trigger the use of the upper element but then revert to the 

heat pump after the top of the tank is at set point. This behavior was not seen during our 

tests. 

3. High Demand Mode: This mode is very similar to Hybrid mode, but the electric elements 

will turn on sooner than in Hybrid mode, when the temperature at the thermistor drops 

3°C. 

4. Electric Only Mode: The upper element will turn on after a minimal drop in temperature 

(~0.5°C) at the thermistor and will heat until the top of the tank is 3°C below the set 

point. The lower element will then turn on to heat the rest of the tank. The lower element 

will turn off when the thermistor near the top of the tank reads the set point temperature.

This mode offers a quick recovery to the set point temperature, but is the least efficient of 

the modes due to its sole reliance on electric resistance elements.

DOE Standard Tests

The results of the DOE standard tests show good agreement with the manufacturer’s published 

values for both the first hour rating and the efficiency factor. Any difference between NREL’s 

results and the manufacturer’s rating could be attributed to running the tests at a temperature set 

point of 60°C, instead of the 57°C set point that is typically used to determine these published 

values. The DOE standard test procedure allows the set point temperature to be between 54°F 

and 60°C. Standby losses for this unit were minimal; however recovery time using the heat pump 

alone was slow due to the small capacity of its compressor.

Table 4. Summary of manufacturer ratings and measured DOE test estimates for Unit B.

Operating 

Mode

Set point 

(°C)

Mfr. First 

Hour Rating 

(liters)

NREL - First 

Hour Rating 

(liters)

Mfr. Energy 

Factor 

Rating

NREL -

Energy 

Factor

UA 

(kJ/hr-°C)

Recovery 

Efficiency

Hybrid 60 239 217± 3 2.35 2.37 ± 0.02 7.1 ± 0.1 2.78 ± 0.06

Draw Profile Tests

Draw Profile 1 was implemented in two segments: the morning segment and the evening 

segment. The results of this test are shown in Figure 15. During the first part, which simulated 

four consecutive morning showers, Unit B was able to maintain ‘hot’ water (>40.5°C) for the 

first two showers but the outlet temperature dropped below that temperature during the third 

shower. While supplying the second set of draws, which simulated evening use, the test article 

was able to maintain ‘hot’ outlet temperature during all but one draw. The evening draw profile 

consumed a lower total volume of hot water over a longer period of time, allowing the water 

heater to keep up with demand better than in the morning draw profile. During both portions of 

Draw Profile 1, the compressor came on first, and when demand was too great, electric resistance 

elements took over for the remainder of the heating cycle. The upper element came on first, until 

the top of the tank was slightly below the set point temperature, then the lower element turned on 

to bring the entire tank to the set point temperature. The resulting efficiency was much lower 

than would be accomplished by running the heat pump alone.
24

Figure 15. Draw Profile 1 for Unit B

Draw Profile 2 consisted of many short draws spaced over a 6-hour period. The results of this 

test show that Unit B performed well under this low-demand condition since only the compressor 

was needed to maintain set point temperature. 

COPhp Tests

Results of the COPhp tests are plotted below in Figure 16. As expected, COPhp increases as the 

inlet air temperatures increase and the COPhp decreases as the water temperature in the tank 

increases. All tests were run in the eHeat operating mode to achieve compressor-only operation. 

At the lowest inlet air temperatures, operation switched from the heat pump to the upper element 

while the average tank temperature was still colder. Under normal operation, the heat pump 

would not turn back during the heating cycle, so power was manually cycled. This eventually 

allowed the heat pump to operate continuously to complete the heating cycle. Icing may have 

been the reason that the upper element was called for. 
25

Figure 16. COPhp traces for Unit B. 

The two warmest COPhp tests were repeated with the same inlet dry bulb temperature for high 

and low relative humidity. It was expected that the inlet conditions with the higher humidity (i.e. 

higher wet bulb temperatures) would perform better than the lower humidity conditions. Figure 

17 shows this comparison for the inlet dry bulb condition at 35°C with the relative humidity at 

40% and 19%. The corresponding wet bulb temperatures for these conditions are 24°C and 17°C, 

respectively. The results show that the performance improvements were not significant for the 

COPhp test run at the higher inlet wet bulb temperature. On average, the increase in COPhp was 

~1%. The tank recovery time for two humidity levels is shown in Figure 18. The lower humidity 

slowed the recovery rate from 9.6°C/hr to 9.4°C/hr. These results show that humidity has a 

small, but not insignificant, impact on the performance of this unit.
26

Figure 17. Effect of Humidity on COPhp for Unit B Figure 18. Effect of Humidity on Recovery 

Time for Unit B

Reduced Airflow Tests

Blocking the airflow path by one-third resulted in a minor system performance drop, and an 

additional small performance reduction resulted from the two-thirds airflow blockage. However, 

in neither case was performance impacted significantly; this can be attributed to the variable 

speed fans. These results are shows in Figure 19. For the case of the one-third airflow blockage, 

the fan speed was seen to be greater than the nominal case at the higher water temperatures only.

The case of 1/3 blockage, the fan power reached a maximum of 23W, the same maximum of the 

unblocked case, but it remained at this maximum value for an hour and half longer before the fan 

speed decreased as the tank temperature approached set point. In contrast, for the two-thirds 

airflow reduction, the fans immediately went to their maximum speed, corresponding to 28W,

and remained there for the entire test in an apparent effort to counteract the airflow restriction.
27

Figure 19. COPhp traces for Unit B with Reduced Airflow

System Drawbacks

Below is a list of system drawbacks that could be improved upon to enhance the capabilities of 

Unit B:

1. Slow recovery time. The recovery time for this unit is very long if the heat pump alone is 

used. This could be an issue for homeowners who demand equivalent performance and 

improved efficiency when switching to HPWH technology. A smaller upper electric 

resistance element that could be used in conjunction with the compressor would help 

improve recovery time while still taking advantage of the efficiency of the heat pump. A 

larger tank would not improve recovery time but would provide a larger buffer. Even in 

situation of high demand, a large tank could consistently supply hot water without 

needing to revert to the electric elements.

2. Control algorithm did not switch back to the heat pump if electric resistance 

elements are called for. In Hybrid and High-Demand modes, if the electric resistance 

elements are used because of a high-demand situation, the compressor did not turn on 

again until the reheat cycle is complete. According to the manufacturer, the heat pump 

should turn on after the upper element in times of moderate to high demand, but that 

behavior was not reproduced in the laboratory. However, the manufacturer regularly sees 

this behavior in the numerous field installations they monitor. The control logic that 

allows the heat pump to turn back on may need to be revised so that it is easier to 

achieve, as opposed to the logic that uses the upper element and then the lower element 

when demand is high. 
28

3. User interface is not user-friendly. A simple task like changing the operating mode 

requires navigating through multiple screens. A more straightforward display may 

encourage people to maximize the efficiency of the unit by changing the temperature 

setting and operating mode to meet their changing needs.
29

NREL/PIX 18667, 18672, and 18669Credit: Kate Hudon

Unit C HPWH Summary

Overall Impressions and Observations

Unit C is the most complicated of all the units tested, in both physical design and control logic. 

To transfer heat from the heat pump to the water, Unit C pumps water from the bottom of the 

tank, through a coaxial heat exchanger/condenser at the top of the unit, and back into the top of 

the tank. This requires a pump that consumes 70 W any time the heat pump is running. The 

pump also thoroughly mixes the tank, which can lead to a decrease in outlet temperature since 

the relatively cold inlet water is quickly mixed with the hot water in the tank. Unit C did not 

operate its compressor over the range of operating temperatures stated in its manual; this 

behavior is described in more detail below. However, when in the optimal air conditions, the 

high capacity compressor can quickly heat a full tank of water. This unit also has two smaller (2 

kW each) electric resistance elements, one of which can operate concurrently with the heat 

pump. This allows for faster and more efficient tank recovery than if a larger electric element 

was used alone. Unit C performed very well in a narrow range of conditions but suffered 

performance impacts outside that range. 

Installation and Usability Considerations

The inlet and outlet water lines are located on the side of the tank. The air filter resides on top of 

the unit and is easily accessible for the homeowner. Access to the heat pump components for 

maintenance and repair can be achieved by disassembling the heat pump casing. 

Figure 20. Unit C in the test plenum (left), the compressor and coaxial heat exchanger (top right), 

and the water circulation pump, evaporator, and control wiring (bottom right).

Compressor

Coaxial 

Condenser

Pump

Control 

Wiring

Evaporator 
30

The air enters this unit from the top and is pushed across evaporator coils that encircle about 

two-thirds of the area around the heat pump. A minimum distance between the air inlet and the 

ceiling is specified by the manufacturer, and the location of the water heater must be considered 

to ensure installation requirements are met. This unit can currently be purchased at Home Depot. 

The user interface is straightforward and user-friendly. This interface is used to make changes to 

the set point temperature and to select an operating mode. This unit can be operated in three 

modes: Energy Saver, Normal, and Electric Only. The interface provides a set point gradient 

from Hot to Normal to Vacation, without providing actual temperature of each set point. The 

homeowner must reference the manual to learn what setting corresponds to what temperature. If 

the compressor is turned on, there is a 20-minute delay before the compressor and fan turn on. 

This occurs both during initial start-up and if the user switches to EnergySaver mode from 

Electric-Only mode. In Electric-Only mode, the resistance heaters turn on immediately.

Qualitative Test Results and Observations

The descriptions below explain the results of the tests run on Unit C HPWH and provide 

observations made during testing.

Operating Mode Tests

The heat pump and upper heating element in this unit can operate concurrently. Both heating 

elements can also operate at the same time. A thermistor located near the lower heating element 

is used to trigger the operation of the heat pump and/or the electric resistance elements. A second 

thermistor located near the upper heating element determines when the heat pump or heating 

elements should be turned off. If the tank is filled with water below 27°C at initial start up, the 

heat pump will not run and the upper resistance element will be used until the temperature 

exceeds 27°C. The heat pump will then turn on, if allowed by the operating mode. This 

information was provided by the manufacturer and confirmed during testing.

Operation Modes and Description:

1. Energy Saver Mode: The heat pump turns on when the thermistor located near the lower 

heating element reads a temperature around 22°C. The heat pump operates exclusively if 

the water temperature set point is 52°C or below, unless the air temperature is outside of 

the manufacturer-stated operating bounds (4°C and 49°C). When the set point is at its 

highest (58°C), the heat pump and upper element are used primarily. The use of the heat 

pump alone versus the use of the heat pump and upper element appears to be tied to the 

set point temperature, rather than draw size. Also, other than times when the air 

temperature was outside of acceptable bounds, there does not appear to be a scenario in 

Energy Saver mode that cause the two electric elements to operate alone, without the heat 

pump. 

• Even though the user manual states that the heat pump will operate in air 

temperatures between 4°C and 49°C, we did not see continuous operation of the heat 

pump for dry bulb temperatures below 14°C or above 35°C. For air temperatures 

below 14°C, ice built up on the evaporator coils, as indicated by three surface 

mounted thermcouples that were installed on the coils at the inlet, middle, and exit of 

the evaporator for the purposes of testing. This caused the heat pump to cycle on and 

off three times before switching to electric resistance heat for the remainder of the 
31

heating cycle. Above 35°C, the heat pump cycled on once before switching to the 

heating elements.

• It should also be noted that the upper element turns on at the very end of the heating 

cycle for the higher set points. (See Anomalies section below for further discussion.)

2. Normal Mode: This mode is very similar to the Energy Saver mode. For a 49°C tank set 

point, the heat pump will turn on alone, except at the very end of the re-heat cycle when 

the upper heating element will also be used. For the highest set point (58°C), the heat 

pump and upper electric element turn on. The temperature trigger for the heat pump is the 

same as in Energy Saver mode. There is no apparent advantage to this operating mode 

when compared to the Energy Saver mode.

3. Electric-Only Mode: Both heating elements will turn on when the thermistor temperature 

drops to 20°C but the lower element turns on first, after a small drop in temperature 

(~0.5°C) is detected by the lower thermistor. The upper element will turn off when the 

top of the tank has reached set point and the lower element will remain on to finish 

heating the lower half of the tank.

DOE Standard Tests

None of the results from the DOE standard tests for Unit C match well with the manufacturer’s 

published values. Initially, the 1-hour and 24-hour tests were run at a tank set point of 58°C, 

which is consistent with the recommended set point for these tests. The Energy Factor result was 

less than the rated value, but not alarmingly so. However, the First Hour Rating result was 40% 

lower than the rated value, which was different enough to merit a retest. The tests were repeated 

at different set points. The 24-hour test was repeated at the 54°C set point, the lowest allowable 

set point for the DOE ratings, based on advice from the manufacturer’s engineers. The 1-hour 

test was repeated at three set points: 58°C, 54°C and 52°C. The results from both rounds of 

DOE tests are shown in Table 5 below. The FHR at the 54°C set point increased relative to the 

58°C set point, but still remained 20% below the rated value. The lower set point did not 

improve the EF result and in fact, at the 54°C set point, the EF decreased to 1.2, which is 40% 

below the rated value of 2. The results for the heat loss coefficient and recovery efficiency also 

declined during the second 24-hour test. Standby losses for Unit C were high, indicating that 

additional insulation may be beneficial.

Table 5. Summary of manufacturer ratings and measured DOE test estimates for Unit C.

Operating 

Mode

Set 

point 

(°C)

Mfr. First 

Hour Rating 

(liters)

NREL - First 

Hour Rating 

(liters)

Mfr. Energy 

Factor 

Rating

NREL -

Energy 

Factor

UA 

(kJ/hr-°C)

Recovery 

Efficiency

EnergySaver 58 254 149 ± 4 2.0 1.68 ± 0.02 9.6 ± 0.2 2.04 ± 0.04

EnergySaver 54 254 199 ± 7 2.0 1.19 ± 0.01 9.9 ± 0.2 1.55 ± 0.03

EnergySaver 52 254 137 ± 3
32

Draw Profile Tests

Draw Profile 1 was tested in two segments, the morning segment and evening segment. During 

the morning segment, which simulated four consecutive morning showers, Unit C was able to 

maintain acceptably hot outlet water (>40.5°C) for the first shower but the outlet temperature 

started to drop by the beginning of the second shower. The outlet water temperature dropped 

quickly because the circulation pump turns on with the compressor and mixes the cold water 

from the bottom of the tank with the hot water at the top of the tank. During this portion of the 

draw profile, the heat pump cycled on and off three times, alternating between the heat pump and

both electric resistance elements, before the elements remained on for the remainder of the 

heating cycle. 

Figure 21. Draw Profile 1 for Unit C

During the second part of Draw Profile 1, which simulated evening use, Unit C was able to 

maintain hot outlet temperature during all draws. The compressor turned on twice during this 

profile, and the upper element assisted the compressor during a portion of the test. The combined 

use of the heat pump and electric resistance elements during Draw Profile 1 resulted in lower 

energy efficiency compared to running the heat pump alone.

Draw Profile 2 consisted of many small draws over a 6-hour period. Unit C performed well 

under this low-demand situation since only the compressor was needed to maintain set point 

temperature. 

COPhp Tests

The results of the COPhp tests are plotted below in Figure 22. As expected, COPhp increases as 

the inlet air temperatures increases and the COPhp decreases as the water temperature in the tank 
33

increases. For inlet dry bulb temperatures ranging from 25°C to 35°C, the heat pump heated the 

water quickly and efficiently. For air temperatures below 25°C (16°C wet bulb), icing occurred 

on the evaporator coils during a portion of the heating cycle, resulting in compressor cycling. For 

the COPhp tests at these cooler inlet air conditions, the power was reset after the compressor 

cycled three times to force heat pump operation until the set point was reached. For the case of 

20°C inlet air, the compressor cycled at the lower water temperatures but remained on once the 

average tank temperature reached ~24°C. Below 20°C dry bulb (14°C wet bulb), the heat pump 

was never able to achieve continuous operation.

For COPhp tests at air temperatures at and above 38°C, the compressor cycled on and off once 

before switching to both electric elements but did not turn on again. This was due to a fault that 

occurs when the temperature difference across the compressor is less than 20°C. This fault 

prevented the compressor from running at both high and low humidity for dry bulb temperatures 

above 35°C. It is unclear what this control feature is trying to prevent.

Figure 22. COPhp traces for Unit C. 

The COPhp test run at the highest dry bulb temperature was repeated with a lower relative 

humidity to determine the impact of humidity on performance. Figure 23 shows this comparison 

for the inlet dry bulb condition at 35°C with the relative humidity at 40% and 19%. The 

corresponding wet bulb temperatures for these conditions are 24°C and 17°C, respectively.

The COPhp dropped by ~20% at the lower humidity case, showing that the performance of Unit 

C is sensitive to the humidity of the ambient air around it. The tank recovery time is shown in

Figure 24. This shows that the higher humidity increased recovery rate from 14.1°C/hr to 

17.8°C/hr relative to the lower humidity case at the same dry bulb temperature. 
34

Figure 23. Effect of Humidity on COPhp for Unit C Figure 24. Effect of Humidity on Recovery 

Time for Unit C

NOTE 1: Because of control logic restrictions, when initially turned on, the compressor does not 

turn on for water temperatures lower than 27°C. In order to perform the COPhp tests with 3°C

starting water temperatures, we added a circuit containing a potentiometer in parallel with the 

lower thermistor. Using a manual switch, we could provide the control board the actual water 

temperature or a false, warmer temperature. This successfully allowed the compressor to run for 

the coldest water temperatures. However, icing occurs more often on the evaporator when the 

water was cold (<27°C), which explains why that control logic is built in. 

NOTE 2: Unit C regularly uses its electric resistance elements when in its most efficient mode. 

Manual switches were installed on both elements that could be used to disable their use. These 

switches were only used during the COPhp tests to ensure that only the compressor was running. 

NOTE 3: To determine why the compressor was not operating under various conditions, we 

looked at the Fault Indicator Light, which is a red LED located on the circuit board (behind the 

control panel). This light will blink a prescribed number of times to indicate the reason for a 

compressor fault. The LED is not visible unless a hole is drilled in the plastic housing. The 

manufacturer provided us with flash code definitions to help diagnose problems. 

Reduced Airflow Tests

The results of the reduced airflow tests are shown in Figure 25. The results of the one-third 

reduced airflow test show a slight, but insignificant, decrease in performance. The results of the 

two-third reduced airflow test show a more significant performance reduction, ranging from 5% 

to 20% over the course of the heating cycle. 
35

Figure 25. COPhp traces for Unit C with Reduced Airflow

Anomalies

The heat pump design appears to be optimized for speed at the expense of the operating range. 

The compressor is oversized, which results in fast recovery times, but also contributes to a more 

limited ambient temperature operating range. For air temperatures below ~15°C, icing starts to 

accumulate on the evaporator coils and the compressor will cycle on three times before switching 

over to the electric resistance elements to heat the water. In addition, when air temperatures are 

at or above ~38°C, the heat pump will cycle on once and then turn off in favor of the electric 

resistance elements. As mentioned previously, this is due to control logic that shuts off the heat 

pump if the temperature difference across the compressor is less than 20°C. (The reason for this 

restriction is still unknown and may or may not be a result of the oversized compressor.) These 

restrictions greatly limit the operating environments where this HPWH will be an efficient water 

heating option. In particular, garages and other unconditioned (or passively-conditioned) spaces 

may be poor locations for this unit. 

Another result of the heat pump being designed for speed is that the upper resistance element 

must come on in place of the heat pump to finish the heating cycle. The refrigerant used in Unit 

C’s heat pump, R-410a, operates at a higher pressure than R-134a. At the higher refrigerant 

temperature, this pressure starts to approach the maximum capacity of the copper tubing in the 

heat pump. Design changes could prevent the need for the electric resistance elements at the 

higher tank temperatures, such as reducing the diameter of the copper tubing, but these changes 

may slow the recovery time for the system. 
36

System Drawbacks

Below is a list of system drawbacks that could be improved upon to enhance the capabilities of 

Unit C:

1. Inadequate tank insulation. It is apparent from the DOE 24-hour test that the water tank 

insulation is insufficient. The UA value is high and improving the insulation in this 

design may significantly reduce standby losses.

2. Condenser design is ineffective. The circulation pump that brings water to the coaxial 

heat exchanger at the top of the unit was found to be a detriment to the outlet 

temperature. For example, when performing water draws, the circulation pump mixes the 

cold water at the bottom of the tank with the hot water at the top of the tank, limiting 

availability of water at the desired (a.k.a. set point) temperature. It is assumed that the 

pump is needed to achieve sufficient (forced) convection in the coaxial condenser and 

prevent the water from boiling.

3. Inefficient circulation pump. The circulation pump draws ~70W and produces enough 

heat to burn someone who touches it. This wastes energy and is a safety risk if the outside 

cover that protects the circulation pump is ever removed. If the pump could be moved to 

inside the heat pump compartment, this would reduce the safety risk and allow the heat 

pump to reabsorb the heat generated by the circulation pump.

4. Control logic consists of long delay before compressor starts up. The compressor in 

this unit has a starting delay of 20 minutes, presumably for internal diagnostics. This does 

not seem to be necessary. The long delay also might make homeowners think that 

something is wrong with the heat pump and switch its operating mode to all resistance 

heat to make sure that water will be heated. 

5. Control logic restricts upper range of operating conditions. The heat pump will not 

run at air temperatures above ~38°C. Since this range also represents the range in which 

the heat pump could be most effective, the control logic should be reconsidered so the 

heat pump can operate at higher air temperatures. It is also unclear why this restriction 

was implemented by the manufacturer from either a safety or performance perspective.

6. Excessive icing occurs on the evaporator coils at moderate air temperatures. 

Frequent icing occurs at air temperatures less than 20°C, making this unit an unfavorable 

choice for unheated or passively heated installation locations in cooler climates. This 

appears to be a result of using a larger compressor to speed recovery time. 

7. The upper resistance element is needed to achieve set points above 49°C. As 

mentioned above, the design of the heat pump necessitates the use of the upper element at 

higher tank temperatures (and thus higher refrigerant temperatures) to avoid reaching the 

maximum pressure of the copper tubing.
37

NREL/PIX 18676, 19460, and 19461Credit: Kate Hudon

Unit D HPWH Summary

Overall Impressions and Observations

Unit D is a simple heat pump water heater with only one operating mode and no control panel. 

Where other companies have created many options and interactive displays, Unit D’s 

manufacturer took the opposite path. There is only one operating mode and no way to turn the 

unit off. The controls for changing set point temperature are difficult to access, making it 

unlikely that users will ever adjust the default setting. There is a single, small electric resistance 

heater located near the hot water outlet, designed to provide a small amount of boost heating but 

the resistance heater alone could not be used to heat the entire tank. The large tank can provide 

plenty of hot water, but the heat pump and back up element are small and so recovery time is 

slow. However, this unit has a high EF rating and so can provide very efficient water heating. 

Installation and Usability Considerations

Unit D’s inlet and outlet water lines are located on the side of the tank. The 80 gallon unit tested 

was just over 6’ tall. The air enters the unit from the front, is pulled across the evaporator coils 

located in the middle of the heat pump, and exits out the back of the unit. There is no air filter in 

this unit. Currently, Unit D is not available through a major retailer, but can be purchased 

through plumbing suppliers. 

Figure 26. Unit D, an 80 gallon HPWH, installed in the test plenum (left). A close up of 

the air intake (top right) and the heat pump (bottom right) are also shown.
38

There are no operating modes for Unit D and it is intended to be hard-wired when installed, 

meaning that the user cannot manually turn the unit on or off. There are knobs located inside on 

the control board that can be used to change the temperature set point for the heat pump and 

electric resistance heater separately. However, the top of the heat pump case must be removed to 

access these knobs and there are no labels on the knobs to indicate the temperature range 

available or to identify which heating source they control. For laboratory testing, Unit D was 

wired to a switch for power and when turned on, the fan and compressor turned on immediately.

Since the default set point for the heat pump is 60°C and it is unlikely that the set point will be 

changed due to the inaccessibility of the controls, some tests were done with this set point to 

capture the more likely operating conditions. 

Qualitative Test Results and Observations

The descriptions below explain the results of the tests run on Unit D and the observations made 

during testing.

Operating Mode Tests

Unit D has a single thermistor as a control input, located at the top of the tank. There is only one 

operating mode for this unit. The heat pump can operate at the same time as the electric 

resistance element, but the electric element cannot run on its own. There are two separate set 

points for the heat pump and resistance element. There are knobs to adjust these set points inside 

the heat pump casing, on the control board. Based on our tests, the compressor set point can vary 

between 54°C and 60°C and the resistance heater set point temperature has a range from 29°C 

and 57°C. There are no markings to indicate the current set point for either heat source.

Operation Mode description:

1. On: The heat pump will turn on when the thermistor registers a small drop in temperature 

(2°C) from the heat pump set point. The heating element will turn on when the top of the 

tank drops 5°C from the element set point. When the draw is large enough to trigger the 

heating element, it will remain on until the thermistor reaches the heating element set 

point. The default element set point is 55°C, which is below the default heat pump set 

point of 60°C. The rest of the tank will be heated with the heat pump until the thermistor 

reading reaches the heat pump set point. According to the user manual, the operating 

range for this water heater is between air temperatures of 6°C and 42°C.

DOE Standard Tests

The results of the DOE standard tests show good agreement with the manufacturer’s published 

values for both the first hour rating and the efficiency factor. The 1-hour DOE test was repeated 

with three different set points, 54°F, 57°C and 60°C, to see how set point would affect the first 

hour rating results. As expected, the result for the 60°C set point is the closest to the published 

rating, but there was only a difference of 12 liters in first hour rating across the three set points. 

The 24-hour test was only run at a set point of 60°C and the result from that test agreed with 

published values. Since the changes in set point had little effect on the first hour rating, it does 

not appear that Unit D has any additional controls to improve their standard ratings. 
39

Table 6. Summary of manufacturer ratings and measured DOE test estimates for Unit D.

Operating 

Mode

Set 

point 

(°C)

Mfr. First 

Hour Rating 

(liters)

NREL - First 

Hour Rating 

(liters)

Mfr. Energy 

Factor Rating

NREL -

Energy 

Factor

UA

(kJ/hr-°C)

Recovery 

Efficiency

On 54 298 292 ± 5 2.51 2.48 ± 0.02 8.7 ± 0.2 3.08 ± 0.07

On 57 298 280 ± 5

On 60 298 285 ± 5

Draw Profile Tests

Draw Profile 1 was tested in two parts. Unlike the other units where the draw profile tests were 

done with a 49°C set point, the set point for Unit D cannot be set below 54°C and it is unlikely 

that users will ever adjust the default set point of 60°C, so the draw profiles were run at the 

default set point temperature. During the morning portion of Draw Profile 1, four showers are 

simulated over the course of a little more than an hour. The outlet temperature through the first 

three remained close to 60°C but started to decrease during the fourth shower, with the outlet 

temperature falling to a minimum of 53°C. The heat pump turned on during the third shower and 

the electric resistance element turned on about an hour after the fourth shower was complete. The 

resistance element remained on for a half an hour, after which, the heat pump finished heating 

the tank to set point. 

The evening draws produced outlet water at 60°C for the first half of the draws, but the outlet 

temperature started to drop by the end of the evening segment. The final draw delivered water at 

54°C. The heat pump came on a little past the halfway point of the evening draw profile and 

remained on until set point was reached again. 
40

Figure 27. Draw Profile 1 for Unit D

Draw Profile 2 consisted of many small draws over a 6-hour period. Unit D performed well for 

this low-demand situation. The compressor turned on to reheat the tank about three and a half 

hours into the 6-hour profile. 

COPhp Tests

The results of the COPhp tests are plotted below in Figure 28. As expected, COPhp increases as the 

inlet air temperatures increases and the COPhp decreases as the water temperature in the tank 

increases. The electric resistance heater was disabled during these tests to ensure that it would not 

turn on. Even though COPhp decreased for lower air temperatures, Unit D maintained a 

consistently high level of performance for all air conditions tested. 
41

Figure 28. COPhp traces for Unit D. 

The two warmest COPhp tests were repeated with the same inlet dry bulb temperature for high 

and low relative humidity. Figure 29 shows this comparison for the inlet dry bulb condition at 

35°C with the relative humidity at 40% and 19%. The corresponding wet bulb temperatures for 

these conditions are 24°C and 17°C, respectively. 

The results in Figure 29 show that the performance increases were small for the COPhp test run at 

the higher inlet wet bulb temperature. For the lower tank temperatures, a difference in COPhp of

approximately 5% can be seen. As the tank temperature increases, the performance difference 

drops to about 2%. The tank recovery time is shown in Figure 30. A reduction in recovery rate 

from 8.1°C/hr to 7.7°C/hr was observed in the lower humidity case. These results show that 

humidity has a small impact on the performance of this unit.
42

Figure 29. Effect of Humidity on COPhp for Unit D Figure 30. Effect of Humidity on Recovery 

Time for Unit D

Reduced Airflow Tests

The results of the reduced airflow tests are shown in Figure 31. The results of the one-third 

airflow blockage test showed an insignificant decrease in performance. The results of the twothirds airflow blockage test showed a slight decrease in performance, ranging from ~3% to ~4 % 

over the course of the heating cycle. 

Figure 31. COPhp traces for the Unit D with Reduced Airflow
43

System Drawbacks

Below is a list of system drawbacks that could be improved upon to enhance the capabilities of 

Unit D:

1. The 80 gallon unit is tall. The large tank capacity has many benefits from a performance 

perspective, but its physical size may limit the homes it can be installed in. Unit D is a 

little over 6’ tall and it could be limiting depending on where the water heater is installed 

in the home. 

2. Tank recovery does not start fast enough. While the large tank provides ample hot 

water, reheating the tank is very slow. Rather than having the heat pump turn on after a 

small or moderate volume draw, Unit D waits to turn on both the heat pump and electric 

resistance heater until almost all the hot water in the tank has been drawn. This will result 

in very long recovery times during which, no hot water will be available to the 

homeowners. 

3. There is only one operating mode. Unit D has only one operating mode. Users may see 

this as a disadvantage since they cannot change modes in anticipation of higher hot water 

demand (having guests, for instance). The advantage to having only one operating mode 

is that users will be unable to switch to using all resistance heat by accident, but the lack 

of options will likely be seen as a disadvantage from the consumer’s perspective. 

4. Means of controlling set point temperatures are inaccessible. The knobs used to 

control heat pump and resistance heater set point temperatures are only accessible by 

taking off the top of the water heater casing. Even then, the knobs are not very obvious 

and there are no markings to indicate current set point and available range of set points. 

The default values will likely never be changed, even though the heat pump set point is 

60°C, which is higher than the recommended set point temperature. 
44

NREL/PIX 18919, 19462, and 19463Credit: Kate Hudon

Unit E HPWH Summary

Overall Impressions and Observations

Unit E uses immersed condensing coils to reduce recovery time and provides an option to duct 

the outlet air. The heat pump operates over a wide ambient temperature range and has a defrost 

feature that allows the heat pump to operate in very cold air temperatures without reverting to the 

electric resistance element. However, the user interface is difficult to use and the air filter is 

nearly inaccessible. Unit E also provides an option to duct the outlet air, which adds a degree of 

flexibility. Taking advantage of the outlet duct option would add to installation costs, but sending 

the cold outlet air out of the house or to the existing HVAC ducts in the house could add value to 

the water heater installation and eliminate some of the negative impact on the home’s heating 

system in colder climates. A preproduction model of the Unit E HPWH was provided by the 

manufacturer for these tests and as such, some features and functionality will be modified in the 

full production model. 

Installation and Usability Considerations

Unit E’s inlet and outlet water lines are located on the side of the tank. The 66 gallon unit that 

was tested is nearly 6’ tall. There also is a 50 gallon version of Unit E that is a little less than 5’ 

tall. The 66 gallon unit tested requires an airflow rate of about 300 CFM. The air enters the unit 

from the right (when viewed from the front), is pulled across the evaporator coils located in the 

middle of the heat pump, and exits out the top of the unit on the left side. This unit is designed to 

Figure 32. Unit E installed in test plenum (left). Some of the refrigerant system is shown 

(upper right), along with the blower that is designed to connect to a 6” duct (lower right).
45

allow the outlet air to be ducted so the unit has a blower instead of a typical axial fan. Ducting 

the outlet air would be useful in both hot and colder climates. If installed in a home in a hot 

climate that has a central air conditioning system, a duct could be used to route the cold outlet air 

from the HPWH to the existing duct system in the house. Additionally, in colder climates, the 

duct could be used to send the outlet air outside, so as not to add to the heating load during the 

winter time. This would add to the cost and complexity of installation but it could improve the 

HPWH’s impact on the HVAC system. The air filter is only accessible if the heat pump cover is 

removed. If the outlet was ducted, it would be impossible to remove the heat pump cover without 

disassembling the duct. Even if the ducting option was not in use, the heat pump cover is not 

easily removed and power must be disconnected, making it very unlikely that any users will ever 

clean the air filter. Currently, Unit E is not available through a major retailer.

The user interface displays temperature settings, as well as current air and water temperature 

readings, and can be used to change a number of options. There are three operating modes: Econ 

(heat pump only), Auto (combination heat pump and electric resistance heat), and Heater (all 

electric resistance heating). There are indicators on the user interface to show when the unit is 

actively heating and what heat source(s) are being used. However, the usability of the user 

interface is poor, making it difficult to perform simple tasks like changing the set point 

temperature. One unique control feature is that the user can set time periods that restrict when the 

heat pump will operate in Econ mode. This may be a nice feature for advanced users, but there is 

no default time period, so if Econ mode is selected and operating periods have not been defined, 

the heat pump will never turn on. This feature may discourage users from using the most 

efficient mode. 

This unit contains two 4 kW electric resistance heaters but only one of them will be used during 

normal operation. The second resistance heater is wired for power separately from the main 

power supply, so it will never turn on in typical installations. According to the manufacturer, the 

second resistance heater should only be used if the primary electric heater fails. The backup 

resistance heater sits just below the midpoint of the tank and the primary resistance heater sits 

above that, a third of the height of the tank from the top.

Qualitative Test Results and Observations

The descriptions below explain the results of the tests run on Unit E and the observations made 

during testing.

Operating Mode Tests

Unit E has two thermistors that are monitored to control the heat pump and electric element. The 

lower thermistor is located just above the unused back up resistance heater, at roughly the midpoint of the tank. The upper thermistor sits above the usable resistance heater and is halfway 

between the top of the tank and the lower thermistor. The lower thermistor is used to control the 

heat pump and the upper thermistor controls the resistance element. The resistance element and 

heat pump can operate simultaneously.

Operation Modes and description:

1. Econ Mode: The heat pump will turn on when the lower thermistor measures a 

temperature 10-15°C below the set point. The heat pump will turn off when the lower 
46

thermistor returns to the set point. The deadband can be adjusted by the user through the 

user interface. According to the manufacturer, the default deadband is 5°C but heat pump 

operation was triggered by larger temperature drops during laboratory tests. At a set point 

of 49°C, a 10°C drop in temperature at the lower thermistor caused the heat pump to turn, 

while at the 57°C set point, a 15°C drop was required for the heat pump to turn on. Even 

though a relatively large draw is required to trigger the heat pump, the heating capacity of 

the heat pump is large enough to recover in a short amount of time. 

• A timer period must be defined by the user for the heat pump to turn on at all in 

this mode. However, if one or more timer period is set, the heat pump operates 

well across the operating range. 

• If the air temperature is cold enough to cause icing on the coils while the heat 

pump is operating, a defrost cycle will occur. The fan is turned off for about two 

minutes while the compressor remains on and the refrigerant reversing valve 

reheats the evaporator coil (now the condenser) to melt the ice. This process is 

repeated as needed until set point has been reached or until icing is no longer a 

problem. 

2. Auto Mode: As with Econ Mode, the heat pump turns on once the lower thermistor 

measures a temperature drop from set point of 10-15°C. A larger draw will trigger the 

electric resistance heater. The resistance heater will turn on when the upper thermistor 

measures a drop in temperature of 10-20°C and will turn off when the upper thermistor is 

at set point. The resistance heater will operate at the same time as the heat pump. If the 

resistance heater is called for, the heat pump will remain on and will continue to heat the 

tank until the lower thermistor reaches set point. Unlike Econ Mode, a timer period does 

not need to be set for the heat pump to operate. 

3. Heater Mode: This mode will only use the resistance heating element to heat the tank. 

Similar to the control of the resistance element in Auto Mode, the resistance heater will 

turn on when a drop in temperature of 10-20°C degrees is detected by the upper 

thermistor. The electric element is located in the upper third of the tank so in Heater 

Mode, only the top third of the tank will be heated. This mode will not satisfy hot water 

demands for most users and is the least efficient of all modes. Heater Mode is only

intended to be used when the heat pump is not functional. 

DOE Standard Tests

The results of the DOE standard tests show good agreement with the manufacturer’s values for 

both the first hour rating and the efficiency factor. The 1-hour DOE test was repeated with three 

different set points, 52°C, 54°C, and 57°C, to see how set point would affect the first hour rating 

results. The maximum set point is 57°C so the 52°C set point was tested for a third data point, 

even though it does not meet DOE test requirements. The two highest set points produced 

identical first hour ratings that are about 10 gallons less than the unofficial rating provided by the 

manufacturer. However, the first hour rating for the 52°C set point was only a gallon less than 

their rating. The 24-hour test was only run at a set point of 57°C and the result from that test 

agreed with published values fairly well. 
47

During the standby portion of the 24-hour test, neither the heat pump nor electric elements were 

used to maintain tank temperature. 

Table 7. Summary of manufacturer ratings and DOE Standard test estimates for Unit E.

Operating 

Mode

Set 

point 

(°C)

Mfr. First 

Hour Rating 

(liters)*

NREL - First 

Hour Rating 

(liters)

Mfr. Energy 

Factor 

Rating*

NREL -

Energy 

Factor

UA 

(kJ/hr-

°C)

Recovery 

Efficiency

Auto 57 284 246 ± 3 2.4 2.25 ± 0.02 6.2 ± 0.2 2.64 ± 0.06

Auto 54 284 245 ± 4

Auto 52 284 280 ± 5

* Unofficial Ratings

Draw Profile Tests

Draw Profile 1 was tested in two parts. During the morning segment of Draw Profile 1, four 

showers are simulated over the course of a little more than an hour. Unit E unit was able to 

maintain an outlet temperature of 49°C during the first three showers, but dropped below 40.5°C 

during the fourth shower. By the end of the fourth shower, the outlet temperature had dropped to 

about 38°C. The heat pump turned on during the second draw and remained on for the rest of the 

morning segment until set point was reached. The electric resistance heating element turned on 

during the third shower when average tank temperature started to drop. The evening draws were

completed with no significant dip in outlet temperature. The heat pump turned on about halfway 

through the evening portion and remained on until the set point was achieved after the profile 

finished. The upper resistance element also turned on for a brief time following the next to last 

draw.

Figure 33. Draw Profile 1 for Unit E.
48

Draw Profile 2 did not provide any additional insight about the performance of the previous 

HPWHs tested so it was omitted from Unit E’s test schedule.

COPhp Tests

The results of the COPhp tests are plotted below in Figure 34. As expected, COPhp increases as 

the inlet air temperatures increases and the COPhp decreases as the water temperature in the tank 

increases. These tests were run in Econ Mode to ensure heat pump-only operation. At the lowest 

air temperatures, the defrost cycle was needed to prevent icing on the coils. During the defrost 

cycle, the fan is turned off for two minutes while the compressor remained on. The periods of 

defrost can be seen in dips in the lowest wet bulb temperature COPhp curves.

An additional cold air test was partially completed at 4.5°C dry bulb and 3°C wet bulb. Since this 

test article claimed better cold weather performance than all the others, this additional test point 

was added. By the time the tank temperature reached 27°C it became impossible to maintain the 

air conditions due to laboratory limitations. The test did confirm that this unit can operate below 

7°C, the lower limit for ambient temperature for all of the other units, even though repeated 

defrost cycles were required.

Figure 34. COPhp traces for Unit E. 

The COPhp test run at the highest dry bulb temperature was repeated with a lower relative 

humidity to determine the impact of humidity on performance. Figure 35 shows this comparison 

for the inlet dry bulb condition at 35°C with the relative humidity at 40% and 19%. The 

corresponding wet bulb temperatures for these conditions are 24°C and 17°C, respectively. There 

was a significant performance difference between the dry and humid air conditions, especially at 
49

the lower tank temperatures. The tank recovery time is shown in Figure 36. This shows that the 

higher humidity case resulted in an increase in recovery rate from 10.8°C/hr to 11.8°C/hr. 

Figure 35. Effect of Humidity on COPhp for Unit E Figure 36. Effect of Humidity on Recovery 

Time for Unit E

 

Reduced Airflow Tests

The results of the reduced airflow tests are shown in Figure 37. Since the four previous units 

tested showed little to no impact when one-third of the inlet area was blocked, that test was 

omitted for Unit E. The two-thirds airflow blockage test resulted in no noticeable performance 

degradation. 

Figure 37. COPhp Traces Unit E with Reduced Airflow
50

System Drawbacks

Below is a list of system drawbacks that could be improved upon to enhance the capabilities of 

Unit E. It should be reiterated that the unit tested was a preproduction model and some 

functionality is expected to be modified in the full production version.

1. User interface is difficult to use. Changing operating modes is straightforward since 

each mode has its own button, but any other action, including simple tasks like changing 

the set point temperature, are impossible without the manual. 

2. Internal condensing coils do not heat the bottom of the tank. The bottom of the 

condensing coils sits about 8” above the bottom or the tank, meaning that about 8 gallons 

of water at the bottom of the tank remain cold even when the heat pump is on. This 

effectively reduces the tank capacity from 63 gallons (as measured) down to 55 gallons. 

3. Backup resistance element is too high in the tank. The backup electric resistance 

element sits about a third of the tank height from the top of the tank. When in Heater 

Mode, the resistance heater can only heat the top third of the tank. Heater mode is not 

intended to be used unless the heat pump malfunctions, but if that was to happen, this 

water heater may not be able to provide sufficient hot water. 

4. Timer feature will likely cause more harm than good. Up to three time periods can be 

set for heat pump operation when in Econ mode. If the heat pump is called for at a time 

outside of these ranges, it will not turn on. This feature is designed to allow homeowners 

to control when cold air is generated by their water heater. However, there is not a default 

timer period so if the user tries to use Econ mode without setting a time period, nothing 

will happen. 

5. Extra resistance heating element is unnecessary. Unit E comes with a second, back-up 

resistance heating element that is only used if the main resistance heater fails. In fact, the 

second resistance heater has a separate power supply that must be wired separately for 

use. This seems like an unnecessary feature that just increases its cost. Also, the back-up 

resistance element sits lower in the tank than the main resistance element. If it was 

eliminated, the main resistance element could sit lower, allowing it to heat a larger 

portion of the tank. 

6. Air filter is inaccessible. The air filter is located under the heat pump casing and cannot 

be accessed without fully disassembling the case and the outlet ducting (if in use). Users 

will likely never clean the air filter unless it is easily accessible. 
51 

Data Comparison and Analysis 

Operating Mode Control Comparison 

The following table summarizes the control logic for each operating mode for each test article. 

More detailed information about each unit’s control logic can be found in the specific water heater 

sections but the tabular format allows for quick comparison between different units. 

Table 8. Comparison of operating modes and control logic for all five units. 

Unit A Unit B Unit C Unit D Unit E 

Most efficient Mode: Efficiency eHeat EnergySaver On Econ 

Heat pump? Yes Yes Yes Yes Yes 

Control logic to turn 

on heat pump: 

ΔTtank,mfr = 

-5°C1

ΔTupper = 

-0.5°C Tlower = 22°C ΔTupper = -2°C 

ΔTlower = 

-10°C 

Resistance elements? No No Yes Yes No 

Control logic to turn 

on heating elements: - - Tlower = 22°C2

ΔTupper = 

-10°C3 - 

Hybrid Mode: Hybrid Hybrid Normal N/A Auto 

Heat pump? Yes Yes Yes - Yes 

Control logic to turn 

on heat pump: 

ΔTtank,mfr

= -5°C 

ΔTupper

= -0.5°C 

Tlower

= 22°C - ΔTlower = -10°C 

Resistance elements? Yes Yes Yes - Yes 

Control logic to turn 

on heating elements: 

ΔTtank,mfr

= -10°C5

ΔTupper

= -10°C4 Tlower = 22°C - ΔTupper = -10°C 

Additional Mode: N/A High 

Demand N/A N/A N/A 

Heat pump? - Yes - - - 

Control logic to turn 

on heat pump: - ΔTupper= -0.5°C - - - 

Resistance elements? - Yes - - - 

Control logic to turn 

on heating elements: - ΔTupper= -3°C - - - 

Resistance-only 

Mode: 

Electric 

Only 

Electric 

Only Electric Only N/A Heater 

Heat pump? No No No - No 

Control logic to turn 

on heat pump: - - - - - 

Resistance elements? Yes Yes Yes - Yes 

Control logic to turn 

on heating elements: 

ΔTtank,mfr

= -2°C 

ΔTupper = 

-0.5°C Tlower = 20°C - ΔTupper = -10°C 

Table 8 Notes: 

1 Unit A uses following equation for control: 

⁄૝ ൯࢘ࢋ࢝࢕࢒ࢀ ൅ ࢘ࢋ࢖࢖࢛ࢀ כ ૜൫ ൌ ࢘ࢌ࢓,࢑࢔ࢇ࢚ࢀ

2 The upper heating element will turn on concurrently with heat pump in EnergySaver mode and Normal 

mode if tank set point is ≥54°C. 
52

3 The heating element and heat pump can run at the same time. Once called for, the heating element will 

remain on until Tupper reaches the heating element set point, which is set separately from the heat pump set 

point.

4 If heating elements are called for in Hybrid or High Demand mode, the heat pump will not turn on again. 

The heating elements will be used exclusively to finish the heating cycle.

5 Upper element and heat pump cannot operate simultaneously. When the controller calls for the upper 

element, the heat pump turns off until Tupper reaches set point. Then the heat pump will turn back on to 

finish heating the tank. 

DOE Test Results Comparison

The results from the two DOE tests, the 24-hour simulated use test and the first hour rating test, 

provide consumers with a measure of efficiency and maximum hot water delivery. A few other 

properties that are calculated through the Energy Factor calculation include the recovery 

efficiency (how efficiently heat from the heat source is transferred to the water) and the standby 

heat loss coefficient (the rate of heat transfer from the water tank per degree of temperature 

difference between the ambient air and water temperature). These properties can provide a more 

complete picture of the water heaters. Recovery efficiency, ηr, will be greater than 1 as long as the 

heat pump is used since it is a measure of how much heat is generated by the electrical energy 

input. A large value for recovery efficiency indicates an efficient heat pump. Low values for 

recovery efficiency could identify an inefficient heat pump or excessive use of the back-up 

resistance elements, since all these tests were performed in hybrid mode. The heat loss coefficient, 

UA, indicates how much heat is lost through the walls of the water tank and so a large value for 

UA would imply a poorly insulated tank. The combined effect of these properties is captured in 

the energy factor (EF) rating. Figure 38 shows the results for EF, ηr, and UA for each water heater 

as calculated from the 24-hour tests performed at NREL. 

Figure 38. DOE 24-hour test are compared for all HPWHs.
53

Results from three calculations are shown in Figure 38. Recovery efficiency is similar to Energy 

Factor, but since the Energy Factor rating takes standby heat losses and recovery efficiency into 

account, recovery efficiency is larger than energy factor. The smaller the standby losses for a 

HPWH, the closer recovery efficiency and energy factor are to one another. Unit C has the 

highest standby losses and lowest recovery efficiency of all the units tested, so it also has the 

lowest value for EF. Additional tank insulation may help the overall EF rating, but improving the 

efficiency of the heating system would have a greater impact. It should be noted that Unit C used 

its back-up elements during the 24-hour test, so it is possible that a change in control scheme 

could dramatically improve the EF rating. Unit D, on the other hand, has the highest recovery 

efficiency but also has a high standby heat loss coefficient. It still has the highest EF rating, but 

extra insulation would help reduce standby losses. The remaining three units, Units A, B and E, 

have very similar recovery efficiencies and standby heat loss coefficients, leading to EF values 

within 5% of each other. 

The results of the first hour rating test complement the results from the 24-hour test by providing a 

measure of how much hot water is available when starting with a fully heated tank of water. This 

depends heavily on the tank volume, which varies between 50 gallons and 80 gallons. For all units 

except Unit B, the first hour rating was measured for a range of set point temperatures. (Unit B 

was tested first, before it was apparent that data from multiple set points would be valuable.) The 

results from those tests are shown in Figure 39.

Figure 39. Comparison of First Hour Rating Estimates for all HPWHs.

The motivation for measuring the FHR for multiple set points came from the first round of testing 

when Units B and C were tested together. The FHR for Unit B was reasonably close to the Mfr. Rating Mfr. Rating Mfr. Rating Mfr. Rating Mfr. Rating60°C 60°C 60°C 57°C 57°C 57°C 57°C 54°C 54°C 54°C 54°C 52°C 52°C
54

manufacturer’s rating (within 10%) but for Unit C, the 58°C set point produced a first hour rating 

40% lower than the rated value. The manufacturer’s explanation for this discrepancy was that our 

set point choice was inconsistent with what was used in the official rating test. The DOE tests 

were repeated for Unit C during the third round of testing, when Unit E was being tested and a 

range of set points were used in the first hour rating test. At the recommended set point of 54°C, 

the FHR for Unit C did increase but the result is still 20% lower than the rated value. 

All units tested after Units B and C were tested over a range of set points to see if there was any 

effect on the first hour rating. Results for Unit A were higher than the manufacturer’s rating with 

little difference across set points. Similarly, the set point had little impact on the FHR for the Unit 

D. Units C and E are both restricted to a maximum set point of 58°C (135°F) so the lowest set 

point tested was 52°C even though it is outside of the acceptable range for the first hour rating 

test. Interestingly, the lowest set point for Unit E was closest to the unofficial manufacturer’s 

rating and Unit C’s lowest set point temperature decreased even below the original test value at 

58°C set point temperature. 

COPhp Test Results Comparison

The COPhp tests were performed to determine the performance of the HPWH operating with the 

heat pump alone. This allows the units to be compared against each other as they operate under 

various average tank temperatures. Figure 40 shows a comparison of the COPhp curves for the five 

integrated HPWHs for the inlet wet-bulb air temperature of 14°C (67°F). 

Figure 40. COPhp Comparison for Five HPWHs at 14°C Inlet Wet Bulb Air Temperature

This comparison shows that the COPhp curves for Units B and D are very similar, both in shape 

and magnitude. Unit A is also similar to Units B and D in the lower range of average tank 

temperatures, but the performance drops off more significantly at the higher tank temperatures. 

Unit E performed very well at lower average tank temperatures, but the slope of the curve is
55

greater, resulting in lower performance than the other units at the higher average tank 

temperatures. It should be noted that the shapes of the curves appear to depend on the type of 

refrigerant used. Units A, B and D use R134-a and their COPhp curves have a linear characteristic 

shape. Units C and E use R410-a and their COPhp curves decay exponentially. Another 

explanation for the similar shapes of Unit C and E is that the heat transfer from the refrigerant and 

the water is more direct, whereas the other three units have to transfer heat through the wall of the 

copper tubing and the wall of the tank before getting to the water. It should also be noted that a 

typical range of average tank temperatures for a water heater in a home is between 40°C and 

50°C. Over this range, Units B and D have the best performance in terms of energy efficiency. 

Another key performance consideration is the heating capacity of the heat pump. Figure 41 shows 

a comparison of the heating capacity for the five integrated HPWHs for the inlet wet-bulb air 

temperature of 14°C.

Figure 41. Heating Capacity Comparison for Five HPWHs at 14°C Inlet Wet Bulb Air Temperature

This comparison shows that the Units B and D have the lowest heating capacity, so although they 

are very efficient at the higher average tank temperatures, they will take the longer to reheat than 

the other units. The heating capacity curve for Unit C, much like the COPhp curve, is irregular 

until the average tank temperature reaches about 25°C because the heat pump could not sustain 

operation due to icing at the lowest tank temperatures. 

In the end, a homeowner should weigh energy efficiency with thermal performance. For homes

with low hot water demand, the high efficiency, low heating capacity units should be able to 

satisfy demand and provide highly efficient water heating. For high use situations, heating 

capacity and tank volume may become more important factors.
56

Draw Profile System COPsys Comparison

COPsys values were calculated for the morning and evening portions of Draw Profile 1 and for 

Draw Profile 2. The purpose of these calculations is to determine realistic performance numbers 

for the units tested. 

As mentioned previously, the morning portion of Draw Profile 1 was designed to represent a 

high demand situation. All test articles, except for Unit A, resorted to electric resistance

supplemental heaters. The evening portion of Draw Profile 1 was designed to represent the 

evening hot water use for a typical household. These draws are spaced further apart than the 

morning draws and therefore represent a medium demand situation. For the smaller tank units, 

Units B, C and E, the electric resistance elements were needed to supplement the heating 

process. 

Draw Profile 2 represented a low demand situation spanning across six hours. None of the 

HPWHs used back-up electric resistance heat to supplement the reheat cycle. Since COPsys can 

be estimated using the COPhp curves for Draw Profile 2, it was decided not to run this profile for 

Unit E.

Table 9: Draw Profile System COPsys Values

HPWH Mfr. COPsys

(DP1 – Morning)

COPsys

(DP1 – Evening) COPsys (DP2)

Unit A 3.55 ± 0.08 3.42 ± 0.08 2.62 ± 0.06

Unit B 1.21 ± 0.03 1.81 ± 0.05 3.46 ± 0.06

Unit C 1.44 ± 0.04 2.61 ± 0.06 2.53 ± 0.05

Unit D 3.85 ± 0.06 5.37 ± 0.05 3.09 ± 0.06

Unit E 1.68 ± 0.05 2.77 ± 0.07 N/A

The results from the Draw Profile COPsys tests are shown in Table 9. It should be noted that these 

tests include the standby losses incurred by the tanks during the test. The morning results show 

that the units with the smallest tanks, Units B and C, had the lowest COPsys values. These units 

had a difficult time keeping up with demand and had to rely extensively on the back-up electric 

resistance heating elements. Unit D had the highest COPsys for the morning draw profile. This 

can be attributed to both its large storage tank and small heating capacity of its heat pump. The 

other two units performed as expected based on the sizes of their storage tanks.

For the evening portion of Draw Profile 1, all the HPWHs except Unit A performed better than 

they did in the high demand situation of the morning draws. Unit B has the lowest COPsys due to 

its control logic, which did not switch back to the heat pump once the electric resistance 

elements are enabled, due to the combined size of the draws. Units C and E had modest increases

in COPsys during the evening portion of Draw Profile 1 because the electric resistance heating 

elements were used less than for the morning draws. Unit A experienced a slight decrease in

COPsys. It is expected that these numbers are similar because Unit A did not need its electric 

resistance heat for either of these profiles. It is slightly greater for the case of the morning draws 

because average tank temperatures dropped lower than in the case of the evening draws because 
57

of the high density of draws, resulting in a higher COPsys. Unit D had the greatest COPsys during 

the evening segment because it did not turn on its heating element and its heating capacity is low. 

The COPsys values for Draw Profile 2 are similar to the COPsys values that can be obtained from 

the COPhp curves that define the heat pump performance. There is also little difference in 

performance across the four units that performed Draw Profile 2. 

Recovery Rate Comparison

One concern about heat pump water heater technology is the slow rate of recovery to set point that 

could result in insufficient hot water to meet demand. As shown in Table 10, the recovery rate for 

a standard electric resistance water heater with 4.5kW of heating capacity is 22.3°C/ hr. This 

means that a standard 50 gallon water heater could recover from a series of large draws within an 

hour. 

Table 10: Recovery Rate Comparison vs. Electric Resistance Water Heater

HPWH Recovery Rate 

(°C/hr)

Percent 

Reduction vs. 

Electric (%)

Electric Resistance* 22.3 -

Unit A 8.5 62

Unit B 6.3 72

Unit C 13.2 41

Unit D 5.4 76

Unit E 9.6 57 *Note: From Energy Plus Simulation using 50 Gallon Electric Resistance Water

 Heater with a heating capacity of 4.5kW.

When comparing recovery rate between the standard electric water heater and the HPWHs, the 

difference is significant. Table 10 shows the recovery rates for the heat pump water heaters when 

tested at 14°C inlet air wet-bulb temperature. The data used to calculate these values was taken 

from the Operating Mode Tests when water was drawn until the heat pump alone turned on and 

then the tank was allowed to recover. The data from these tests was used in place of the COPhp

tests since the HPWH were allowed to operate as they would normally, where some controls were 

overridden during the COPhp tests. The intent was to capture heat pump-only operation, unless 

that operation rarely occurs, as with Unit C. The results show that recovery rates for HPWH vary 

between 5.4°C and 13.2°C, which correspond to a reduction in recovery rate relative to an electric 

resistance water heater of between 41% and 76%. 

These reductions are significant, but are not expected to be detrimental to the technology if the 

storage tank is sized appropriately for the household. Households with large demands should use a 

HPWH with a large storage tank so that hot water is less likely to run out. In addition to the tank 

size, control logic is important, as was mentioned in the section describing the Operating Mode 

Tests for each unit. As new manufacturers enter the market and as newer versions of the models 

tested become available, it will be difficult to know how the control logic is programmed before 

purchasing a HPWH. However, as future generations of HPWHs make their way to the market, 

the control logic will inevitably improve and cater to a range of hot water demands.
58 

Cooling Capacity and SHR Comparison 

The operation of the heat pump removes heat from the air through a vapor compression cycle that 

is similar to a conventional air conditioner. Therefore, operating the heat pump will have a certain 

cooling effect on its surroundings. This total cooling effect can be separated into a sensible and 

latent cooling capacity for each unit, as well as a sensible heating ratio, which is defined as 

follows: 

ܵܪܴ ൌ ݍ௦ ݍ௧ ൗ

where qs is the sensible cooling capacity and qt is the total cooling capacity. Table 11 shows the 

sensible and total cooling capacities for each of the HPWHs tested and the corresponding SHR at 

an inlet air wet bulb temperature of 14°C (57°F) and an average storage tank temperature of 

48.9°C (120°F). Note the cooling capacity numbers and SHR will change with wet bulb 

temperature and tank temperature. 

 Table 11: Cooling Capacity and SHR Comparison 

HPWH Mfr. Sensible Cooling 

Capacity (W)

Total Cooling 

Capacity (W) SHR

Unit A 1680 1704 0.986 

Unit B 672 768 0.875 

Unit C 905 1350 0.670 

Unit D 1015 1027 0.988 

Unit E 1048 1051 0.997 

Biquadratic Coefficients for HPWH Modeling 

The performance mapping data acquired during testing can be used to develop and validate 

simulation models that represent HPWH technology. One way to represent the data for COPhp and 

compressor heating capacity is by using biquadratic curve fits. For heat pump water heaters, these 

curves are defined using the following equation: 

௕௪ܶ כ ଷܥ ൅ ௕௪ܶ כ ଶܥ ൅ ଵܥ ൌ ܾ݈݅ܽ݁ݎܸܽ ݉ܽ݊ܿ݁ݎ݋݂ݎܲ݁

ଶ ൅ ܥସ כ ܶ௪௔ ൅ ܥହ כ ܶ௪௔ଶ ൅ ܥ ଺כ ܶ௪௕ כ ܶ௪௔

where Twb is the inlet air wet bulb temperature and Twa is the average water temperature in the 

storage tank. The coefficients used to define the normalized COPhp and heating capacity curves 

for the test articles can be found in Tables 12 and 13. The COPhp and heating capacity equations 

were normalized by rated COPhp and rated heating capacity, respectively. The rated values for 

each unit can be found in Table 14. The rated conditions are defined at a Twb of 14°C (57°F) and a 

Twa of 48.9°C (120°F). 

Table 12: Coefficients for Normalized COPhp Curve 

HPWH Mfr. C1 C2 C3 C4 C5 C6

Unit A 1.229E+00 5.549E-02 1.139E-04 -1.128E-02 -3.570E-06 -7.234E-04 

Unit B 1.192E+00 4.247E-02 -3.795E-04 -1.110E-02 -9.400E-07 -2.657E-04 
59

Unit C 6.945E-02 6.601E-03 1.598E-04 8.842E-04 8.170E-06 3.255E-05

Unit D 9.814E-01 5.334E-02 -2.802E-04 -3.073E-03 -1.384E-04 -2.897E-04

Unit E 2.168E+00 8.124E-02 4.786E-04 -4.870E-02 4.284E-04 -1.499E-03

Table 13: Coefficients for Normalized Heating Capacity Curve

HPWH Mfr. C1 C2 C3 C4 C5 C6

Unit A 7.055E-01 3.945E-02 1.433E-04 2.768E-03 -1.069E-04 -2.494E-04

Unit B 5.050E-01 5.116E-02 -2.026E-04 5.444E-03 -1.154E-04 -2.472E-04

Unit C 6.879E-01 1.987E-02 7.659E-04 2.621E-03 5.323E-05 -5.210E-04

Unit D 5.101E-01 3.588E-02 5.563E-05 4.828E-03 -1.348E-04 7.738E-05

Unit E 9.285E-01 4.088E-02 2.737E-04 -3.625E-03 -6.521E-05 -2.986E-04

Table 14: Rated COPhp and Heating Capacity

HPWH Mfr. Rated COPhp

Rated Heating 

Capacity (W)

Unit A 2.43 2350

Unit B 2.76 1380

Unit C 2.42 2670

Unit D 2.77 1820

Unit E 2.02 2040

It should be noted that these curves do not include the stand-by losses associated with the storage 

tanks. The stand-by losses were removed from the COPhp calculation for the purposes of 

modeling inputs. Also note, it is important to use the storage tank UA values that were provided 

previously to include the effect of stand-by losses when running an energy simulation.

A biqradratic curve fit was chosen to represent the COPhp and heating capacity data so it could be 

easily incorporated into the energy simulation software, EnergyPlus (EnergyPlus 2010). A HPWH 

model currently exists in EnergyPlus, and has been evaluated with the data from the results for 

Unit B (Hudon 2012). The results of this evaluation show that the current model configuration in 

EnergyPlus does not accurately capture the energy used by the HPWH. Efforts are underway to 

improve this model for eventual incorporation into the whole-house energy simulation tool, BEopt 

(NREL 2011).
60

Conclusions

The laboratory results presented in this paper show that the heat pump water heater is a 

promising emerging technology with the potential to significantly reduce the energy associated 

with generating hot water relative to standard electric water heaters. We conclude from these 

results that a HPWH can be used to save energy in all U.S. climates when compared to an 

electric resistance water heater, and that this technology would provide the most benefit in 

warmer climates due to the improved performance and space cooling/dehumidification effects.

The results of the COPhp tests show that heat pump performance is reduced at lower air 

temperatures, but even at these conditions, power consumption is roughly one-third to half that 

of a conventional electric resistance water heater when using the heat pump exclusively.

The performance results for HPWHs are promising, but another important factor is how they will

be perceived by homeowners. One issue homeowners might encounter when using this 

technology is its lower rate of recovery in heat pump-only operation. This issue can be mitigated 

by choosing a tank size that is appropriate for the hot water demand of the household. It is 

suggested that existing water heaters be replaced only by HPWHs with the same or increased 

tank capacity. In addition, the various operating modes are designed to be changed based on the 

user’s current need. Homeowners should become familiar with the user manual for the unit they 

purchase so the correct operating mode is selected. 

Homeowners and installers must also be aware of the physical challenges associated with 

installing these units as well as their effect on the surrounding environment. HPWHs are larger 

than conventional water heaters and require a sizeable volume of air from which to draw heat. 

This imposes limits on the installation location and should be considered before purchasing a 

unit. If the physical requirements can be met, the surrounding environment must also be 

considered. In regions with a long heating season, it is not recommended that HPWHs be 

installed in conditioned space, particularly if space heating is provided via electric resistance.

The garage could be an ideal location for this technology if freezing temperatures are not 

common. In addition, installation in an unconditioned basement can be considered as long as 

cooling the basement air is not a concern. Operating the heat pump can dehumidify the air 

around it, which could be an advantage in a basement environment. One other installation 

consideration is the presence of a drain. Operating the heat pump will result in condensation 

building up on the evaporator coils that needs to be directed to a nearby drain.

Some of the issues that were found with the HPWH technology can be improved upon as future 

generations are released. Three areas where improvements could be made include control logic, 

user interfaces, and ease of serviceably. It was found that the COPsys of the HPWHs operating in 

their Hybrid modes varied across the models tested, mostly as a result of the differences in 

control logic. For example, some control schemes appeared to sacrifice efficiency in favor of 

shorter recovery times while other strategies maintained efficiency by minimizing the use of the 

electric elements, limiting the volume of hot water available during high demand times. In 

addition to control logic, some of the units have user interfaces that are not intuitive or require 

toggling through multiple menus to make changes. This issue could be frustrating to a user, and 

result in a homeowner not changing modes at all. Also, if there is an issue with the operation of 

the heat pump, it is possible for the homeowner not to realize it because hot water will still be 
61

available as long as the back-up elements are functional. An effective means of detecting and 

informing the homeowner of faults or necessary service by an HVAC technician should be 

included on every unit.

Selecting the correct HPWH for a specific application can be challenging since advantages and 

disadvantages exist for each of the units that were tested. The results of this research are intended 

to provide guidance to homeowners and builders who are considering purchasing and installing a 

HPWH in a residential home. The following paragraph summarizes the findings of this research 

for the specific units that were tested.

In general, we found that Unit A performed well and was able to provide ample hot water 

because of its large tank volume and responsive control logic. The drawbacks of this unit include 

its high airflow rate and size, which may result in installation difficulty. Installation location 

must be considered carefully for this unit. Unit E performed well at low air and water 

temperatures and its exhaust airstream can be easily ducted. This makes it a good option for cold 

climates. The drawbacks to this unit are the non-intuitive user interface, the inaccessible air 

filter, and lack of heat pump access for servicing. Unit B also performed well, but was not as 

consistent at delivering hot water to the end user because of its smaller tank volume. For low 

demand situations or when tank size is an installation concern, Unit B would be a good option. 

Unit D had the highest Energy Factor and was the most efficient of the HPWHs tested. The 

drawbacks to this unit are that the tank is the slowest to recover, the electric resistance element 

does little to help in high-demand situations and the set point is difficult to change. In addition, it 

only has one operating mode. Unit C performed well, but only within a limited range of 

operating temperatures. Due to this limitation, this unit is expected to suffer reduced efficiency 

when used in cold or hot climates unless installed in conditioned space. Due to its small storage 

tank and water circulation pump, this unit had difficulty maintaining hot water in high demand 

situations. 
References

Ashdown, B.G. 2004. Heat Pump Water Heater Technology: Experiences of Residential 

Consumers and Utilities. Oak Ridge National Laboratory, Oak Ridge, TN.

Baxter, V.D., Tomlinson, J.J., Murphy, R.W., Ashdown, B.G., Lapsa, M.V. 2005. Residential 

Heat Pump Water Heater (HPWH) Development Status – USA. Proceedings of 8th International 

Energy Agency Heat Pump conference, Las Vegas, NV, May 16-19, 2005.

Calm, J.M. 1984. Heat Pump Water Heaters. Electric Power Research Institute (EPRI), Palo Alto, 

CA.

Consumer Energy Center. 2011. Water Heaters. Retrieved May 16, 2011, from 

http://www.consumerenergycenter.org/home/appliances/waterheaters.html

Department of Energy (DOE). 1998. 10 CFR Part 430 Energy Conservation Program for 

Consumer Products: test Procedure for Water Heaters, Federal Register Vol. 63, No. 90.

EnergyPlus. 2010. Version 6.0 Documentation. 

Hendron, B., Burch, J, and G. Barker. 2010. Tool for Generating Realistic Residential Hot Water 

Event Schedules. National Renewable Energy Lab (NREL), Golden, CO. NREL/CP-550-47685.

Hudon, K., Sparn, B., Christensen, D., and Maguire, J. 2012. Heat Pump Water Heater 

Technology Assessment Based on Laboratory Research and Energy Simulation Models. 

ASHRAE Winter Conference 2012.

Hashimoto, K. 2006. Technology and Market Development of CO2 Heat Pump Water Heaters 

(ECO CUTE) in Japan. Central Research Institute of Electric Power Industry. Nagasaka, Japan.

Levins, W.P. 1982. Estimated Seasonal Performance of a Heat Pump Water Heater Including 

Effects of Climate and In-House Location. Oak Ridge National Laboratory, Oak Ridge, TN. 

Morrison, G.L. 2003. Seasonal Performance Rating of Heat Pump Water Heaters. School of 

Mechanical and Manufacturing Engineering, University of New South Wales, Sydney, Australia.

NREL. 2011. Building Energy Optimization (BEopt) software. Golden, Colorado. 

http://www.nrel.gov/buildings/energy_analysis.html#beopt

Taylor, J.R. 1997. An Introduction to Error Analysis. University Science Books, Sausalito, CA. 
63

Appendix A – Draw Profiles

Table 15. Draw Profile 1 details. Morning segment includes the four showers and the evening segment 

contains remaining draws.

Draw Profile 1

Start 

time

End 

Time

Flowrate 

(lit/min) Liters/draw Notes

6:10 6:19 7.6 68.1 Shower 1

6:30 6:35 7.6 37.9 Shower 2

6:50 7:00 7.6 75.7 Shower 3

7:10 7:18 7.6 60.6 Shower 4

17:03 17:04 3.8 3.79

Food Preparation

17:04 17:05 1.9 3.79

17:05 17:06 3.8 1.89

17:09 17:10 3.8 3.79

17:10 17:11 5.7 3.79

17:12 17:14 3.8 5.68

18:10 18:11 3.8 7.57

Hand Washing 18:11 18:12 1.9 3.79

18:12 18:13 5.7 1.89

18:13 18:14 3.8 5.68

18:45 18:48 7.6 3.79

18:55 18:57 7.6 22.71 Dishwashing

19:10 19:12 7.6 15.14

19:50 19:54 11.4 15.14 Bath

20:50 20:51 1.9 45.42

Face Washing 20:51 20:52 3.8 1.89

20:52 20:53 1.9 3.79

20:54 20:55 3.8 1.89

Figure 42. Graphical representation of Draw Profile 1.
64

Table 16. Draw Profile 2 details.

Draw Profile 2

Start 

Time End Time Flowrate (lit/min) Liters/draw

6:00:00 6:00:30 3.8 1.893

6:10:00 6:10:30 3.8 1.893

6:20:00 6:21:00 3.8 3.785

6:35:00 6:35:30 3.8 1.893

6:50:00 6:50:30 3.8 1.893

7:05:00 7:06:00 3.8 3.785

7:10:00 7:11:00 3.8 3.785

7:55:00 7:55:30 3.8 1.893

8:05:00 8:05:30 3.8 1.893

8:20:00 8:20:30 3.8 1.893

8:35:00 8:35:30 3.8 1.893

8:45:00 8:45:30 3.8 1.893

8:50:00 8:50:30 3.8 1.893

9:05:00 9:05:30 3.8 1.893

9:10:00 9:10:30 3.8 1.893

9:15:00 9:15:30 3.8 1.893

9:25:00 9:26:00 3.8 3.785

9:30:00 9:31:00 3.8 3.785

9:35:00 9:36:00 3.8 3.785

9:40:00 9:41:00 3.8 3.785

10:50:00 10:51:00 3.8 3.785

10:55:00 10:56:00 3.8 3.785

11:00:00 11:00:30 3.8 1.893

11:05:00 11:05:30 3.8 1.893

11:30:00 11:31:00 3.8 3.785

11:35:00 11:36:00 3.8 3.785

11:40:00 11:41:00 3.8 3.785

11:45:00 11:45:30 3.8 1.893
65

Figure 43. Graphical representation of Draw Profile 2.
Appendix B – Laboratory Schematics

Figure 44. Water-side Schematic Ice Machine

Neslab 750HX Chiller

Chromalox 

CMX-250-9

Heater

Pump

Mains Water

Water Dump

Water Dump

Glycol to Water 

Cooling HX

Ball

Valve

Solenoid

Valve

Solenoid

Valve

Supply Water Tank

HPWH 1

6 TC

HPWH 2

6 TC

On/Off

Solenoid

Valve

Turbine 

Flowmeter

Coriolis meter 

for Condensate

T

P,T

T

Coriolis meter 

for Condensate

Back 

Pressure 

Valve

TT

P

T

P

T

P

Turbine 

Flowmeter

Flow Control

Valve

T P

Turbine 

Flowmeter

Ball

Valve

Ball

Valve

Turbine 

Flowmeter

On/Off

Solenoid

Valve

Flow Control

Valve
67

Figure 45. Air-side Schematic

Outside Fan

Heaters

Evaporative pads

Heaters

18 in Ducts

Heater

HPWH1

test chamber

HPWH2

test chamber

LFE

LFE

Excess Air

Excess Air

Neslab 750 Chiller

Outside Fan Outside Fan

To Outside

To Outside

INLET 1

OUTLET 1 OUTLET 2

T

Flow Nozzles

VFD

VFD

Liquid-to-air 

cooling HX

P, T, H

P, T, H

Flow Nozzles Flow Nozzles

P, T, H
68

Appendix C – Heating Capacity Curves

Figure 46. Heating Capacity as a Function of Average Tank Temperature for Unit A

Figure 47. Heating Capacity as a Function of Average Tank Temperature for Unit B
69

Figure 48. Heating Capacity as a Function of Average Tank Temperature for Unit C

Figure 49. Heating Capacity as a Function of Average Tank Temperature for Unit D
70

Figure 50. Heating Capacity as a Function of Average Tank Temperature for Unit E