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Field Testing of Pre-Production Prototype 

Residential Heat Pump Water Heaters 

Introduction 

The purpose of this Technology Installation Review is to provide an overview of the field testing of 18 

pre-production prototype units of a “drop-in” type residential heat pump water heater installed in 

a wide variety of host home situations across the United States for more than one year. It provides 

descriptions of the construction, installation, control, instrumentation, and data acquisition 

methodologies employed with the units, as well as interpretations of the measured results. 

In the United States, about half of all residential water heaters use resistance elements to heat the 

water inside a storage tank. Even the most efficient of these conventional units provides slightly less 

than one unit of useful energy (delivered to heat water) for each unit of electrical energy consumed. 

The best current conventional electric resistance unit has an energy factor of about 0.95 as determined according to the DOE Simulated Energy Use Test Procedure [Federal Register (1998)]. In 

contrast, by using electricity to “pump heat” from the surrounding air, a heat pump water heater 

can provide much more than one unit of useful energy for each unit of electrical energy consumed. 

With all losses included, the prototype heat pump water heater employed in this field test had an 

energy factor of about 2.4, according to the DOE test procedure. 

Of course, the particular combination of hot water usage (amount and pattern), ambient air temperature, ambient air relative humidity, supply water temperature, and thermostat setting employed in the 

energy factor tests would not generally be the same as those encountered in any real home situation. 

Therefore, an important goal of these field tests, conducted by the Oak Ridge National Laboratory 

[Murphy and Tomlinson (2002)] under the sponsorship of the U.S. Department of Energy, was to 

measure the performance of the prototype heat pump water heaters operating in a wide range of 

real-world situations and to develop meaningful comparisons with the performance of conventional 

electric resistance storage water heaters operating in the same situations. 

Technology Description 

The heat pump water heater unit employed in the field tests described here was a pre-production 

prototype of a drop-in replacement for conventional residential electric water heaters (Fig. 1). The 

drop-in heat pump water heater has a footprint (22.25 in. diameter) common for conventional 

50-gal water heaters and has the same requirements for power wiring (240-V, single-phase, 30-A 

circuit with ground) and for water connections (cold inlet from source to dip tube and hot outlet 

from tank to house). The temperature/pressure relief valve, the high-temperature cutout switch, and 

the upper and lower resistive elements are the same as in conventional electric water heaters. 

Internet: www.eere.energy.gov/femp/ 

No portion of this publication may be altered in any form without 

prior written consent from the U.S. Department of Energy, Energy 

Efficiency and Renewable Energy, and the authoring national laboratory. 

U.S. Department of Energy 

Energy Efficiency 

and Renewable Energy 
Technology Focus 

The main visual difference from a 

conventional unit is the addition of a 

small vapor compression heat pump 

system enclosed in a shroud on top of 

the tank. It increases the unit height 

to 60 in. and the unit weight to 180 

lb. The heat pump uses a finned-tube 

refrigerant evaporator to remove heat 

from the ambient air induced by two 

fans through a rear grille with filter. It 

consists of four primary components— 

an evaporator, a compressor, a condenser, 

and an expansion valve—combined 

with connecting tubing, a refrigerant 

working fluid, and various controls. 

In a heat pump cycle, heat is absorbed 

from the ambient air (in the evaporator, usually aided by fans) into a cold, 

low-pressure refrigerant liquid, thereby 

producing cool, low-pressure refrigerant 

vapor. The refrigerant vapor flows next 

to the compressor, where it is pressurized (with an associated temperature 

increase). From the compressor, the 

(now hot, high-pressure) vapor flows to 

the condenser, where it rejects heat to 

the water in the storage tank, thereby 

producing (warm, high-pressure) refrigerant liquid. The refrigerant liquid 

then flows to the expansion valve, which 

restricts the flow so as to produce a cold, 

low-pressure refrigerant liquid/vapor 

mixture at the exit of the valve. This 

mixture next flows to the evaporator 

to complete the cycle. 

Heat is rejected from a refrigerant 

condenser comprising a copper-tubing 

coil wrapped securely around the outside 

of the bottom third of the water tank. 

The heat is transmitted through a highly 

conductive heat transfer mastic to the 

tank wall and thereby to the water. 

Polyurethane foam insulation was blown 

into the space between the tank wall 

and the unit’s metal jacket to hold the 

condenser in place and to minimize heat 

Figure 1. Sketch of heat pump water heater field test unit. 

loss from the tank. The total refrigerant 

charge of the unit used in the tests was 

16 ounces of R-134a. Currently a 

drop-in heat pump water heater similar 

to the type tested is manufactured by 

ECR International. 

Control System 

Each field unit employed a microprocessor-based control system. The 

microprocessor received and processed 

input from seven indicators (four 

thermistors, two switches, and 

one voltage divider) as enumerated in 

Table 1. All thermistors were sampled 

32 times during each software cycle and 

averaged to minimize electrical noise 

distortions of temperature indications. 

Based on the processed values of the 

7 inputs and on the values of the 

16 adjustable control parameters 

stored in the electrically erasable 

programmable read-only memory 

(EEPROM), the control logic of the 

software determined which, if any, 

of five devices should be activated by 

means of associated relays. The controlled 

Table 1. Inputs to the heat pump water heater control board 

Input Indicator 

Thermostat setting Voltage divider 

Lower tank temperature Thermistor 

Upper tank temperature Thermistor 

Evaporator temperature Thermistor 

Compressor discharge 

temperature 

Thermistor 

Mode selection Toggle switch 

Condensate pan level (overflow) Float switch 

. 

2 –– FEDERAL ENERGY MANAGEMENT PROGRAM 
Technology Focus 

Table 2. Summary of unit installation situations 

Unit State City Residents Water 

source 

Unit location Previous 

heater 

1 Alabama Douglas 2A County U. basement Hybrid 

3 Florida Melbourne 2A, 1C City Garage None 

4 Alabama Verbena 2A, 5C County C. laundry 80 gal 

5 Florida Milton 2A, 2C City Garage 50 gal 

6 Tennessee Knoxville 2A, 2C City Garage 80 gal 

7 Connecticut Cromwell 2A City S. basement Hybrid 

8 Washington Seattle 2A, 2C City U. basement 52 gal 

9 Connecticut East Hampton 2A, 1C Well U. workshop 50 gal 

10 Tennessee Lenoir City none City Closet None 

11 Florida Pensacola 2A, 3C City Garage 50 gal 

12 Oregon Hillsboro 2A, 2C Well U. basement 80 gal 

13 Georgia Danielsville 2A, 2C Well S. utility 50 gal 

14 North Carolina Wake Forest 2A, 1C City U. workshop 50 gal 

15 Oregon Portland 2A Well Garage 50 gal 

16 Georgia Gainesville 2A, 3C City U. utility 50 gal 

18 Georgia Conyers 2A, 2C City U. basement 50 gal 

19 Texas Smithville 2A County C. laundry 50 gal 

20 South Dakota Madison 2A, 2C City C. basement 50 gal 

Fan operation was limited to periods 

when the compressor was active. If at 

any time after compressor startup the 

evaporator temperature fell below the 

fan 1 upper temperature limit minus 

the fan hysteresis value, fan 1 was activated until the evaporator temperature 

equaled or exceeded the fan 1 upper 

temperature limit. If, at any time after 

compressor startup, (with fan 1 already 

on) the evaporator temperature fell 

(additionally) below the fan 2 upper 

temperature limit minus the fan 

hysteresis value, fan 2 was activated 

until the evaporator temperature 

equaled or exceeded the fan 2 upper 

temperature limit. Thus compressor 

operation was accompanied, at various 

times, by no fan operation, fan 1 

operation only, or fan 1 and fan 2 

simultaneous operation. 

If heat was called for in the lower section 

C = conditioned, U = unconditioned, S = semi-conditioned 

devices were the three heating devices 

(compressor, upper resistive element, 

and lower resistive element) and the two 

auxiliary devices (fan 1 and fan 2). To 

indicate which heating device was active, 

three colored light-emitting diodes were 

installed in a vertical column within 

the shroud so as to be visible through 

the evaporator air exit grille. When 

the upper resistive element was active, 

the top (green) diode was lit from the 

control board. The middle (yellow) 

diode indicated compressor activity and 

the bottom (red) diode indicated lower 

resistive element activity. 

Control Logic 

The control system logic incorporated 

hysteresis to avoid excessive equipment 

cycling and permitted only one heating 

device to operate at any one time. To 

minimize hot water runouts, the dominant heating device was set up to be 

the upper resistive element. Therefore, 

whenever the upper tank temperature 

fell below the thermostat setting minus 

the upper tank temperature hysteresis 

value, the upper resistive element was 

activated until the upper tank temperature exceeded the thermostat setting. 

If the upper tank temperature criterion 

was satisfied, lower tank heat was called 

for whenever the lower tank temperature fell below the thermostat setting 

minus the upper/lower tank temperature differential minus the lower tank 

hysteresis value. If lower tank heat was 

called for, the preferred device to 

activate was the compressor. 

of the tank, but the compressor could 

not operate, the lower resistive element 

was activated. Once activated, the lower 

resistance element continued operation 

until: 

1.	 the condition(s) preventing 

compressor operation were 

eliminated (causing compressor 

activation); (see discussion in Data 

Analysis section for description of 

some such conditions); 

2. 	 the lower tank temperature 

exceeded the thermostat setting 

minus the upper/lower tank 

temperature differential; or 

3. 	 the upper tank temperature fell 

below the thermostat setting minus 

the upper tank temperature 

hysteresis value (causing upper 

resistive element activation). 

FEDERAL ENERGY MANAGEMENT PROGRAM –– 3 
Technology Focus 

Field Demonstration 

Test Site Characteristics 

The cooperation of ten electric utilities 

was crucial in identifying potential 

host residences for the field test units, 

coordinating their installation, arranging 

for dedicated data acquisition telephone 

lines, and assisting with troubleshooting 

exercises. The testing benefited greatly 

from the wide range of situations made 

accessible by this cooperation and 

the willingness of the homeowners 

to participate. 

As indicated in Table 2, 18 municipalities 

in 10 states, ranging from Connecticut 

to Washington to Texas to Florida, 

were represented among the test sites. 

Seventeen of the houses had two adults, 

and the number of children in the 

households ranged from none to five. 

The remaining house actually had no 

residents but served as a Habitat for 

Humanity construction office. In this 

case, scheduled hot water draws were 

accomplished by a solenoid valve 

activated by a timer. Four houses 

were served by well water and the 

remainder by central (city or county) 

water systems. 

Six units were located in basements, of 

which one was conditioned (with direct 

access to the heating/ventilating/airconditioning systems), four were 

unconditioned, and one was semiconditioned. Five units were installed in 

(unconditioned) garages. Two units were 

sited in (conditioned) laundry rooms 

and two more in (unconditioned) 

workshops. An additional two units 

were installed in utility rooms 

(one unconditioned and one semiconditioned). The unit in the Habitat 

house was installed in a small closet 

with two exterior walls, one interior 

wall, and one closed interior door. 

Two of the homes had no previous 

occupants and therefore no previous 

water heater. Ten residences had 

previously had conventional 50-gal 

resistance electric water heaters. Four 

of the homes had previously had largercapacity conventional electric water 

heaters (three 80-gal, one 52-gal). Two 

of the residences had previously had 

add-on electric heat pump water heaters 

(from other manufacturers) in electric 

resistance hybrid arrangements of 

different capacities. 

Field test project personnel were able 

to observe installation procedures at 13 

of the sites and to verify initial proper 

performance of the field test units. The 

remaining five sites (Washington, Texas, 

South Dakota, and two in Oregon) 

were monitored during installation, 

and remote assistance was provided by 

field test project personnel as required. 

Upon installation, for safety reasons, 

the thermostat setpoint temperature 

was 120°F. However, residents were 

permitted to adjust this value as they 

saw fit throughout the test period by 

means of a knob on the front of 

each unit. 

Testing Setup 

Installation 

Because of its integral design (heat 

pump and storage tank in a single, preassembled package), installation of the 

unit was truly “drop-in.” That is, both 

the water and electrical connections 

were identical to those of a conventional 

electric resistance storage water heater. 

The only additional installation step was 

connection of a condensate drain line 

from the evaporator pan. 

Data Acquisition 

Strategy 

Sensors were installed on each heat 

pump water heater to provide input to 

its associated data logger, which would 

preprocess and temporarily store the 

data in final storage. The data logger 

was connected to a modem that was 

connected to a dedicated phone line. 

To store data, on a regularly programmed weekly schedule, the central 

personal computer (with modem) 

called each data logger (through its 

associated modem) to download the 

data accumulated since the last download. To accomplish other functions 

related to installation, troubleshooting, 

or mode switching, manual calls using 

the personal computer were initiated to 

the relevant data logger. 

Distinct strategies were employed to 

conserve data logger storage space, 

extend required download intervals, 

and minimize download time. First, the 

measurements were divided into three 

time-interval groups: “rapid” (every 2 

seconds), “moderate” (every 30 seconds), 

and “slow” (every 10 minutes) so as to 

optimize the data streams required for 

performance assessment. Second, data 

transmittal to the final storage area of 

the data logger was programmed to 

be event-triggered. That is, data were 

transmitted from input storage to final 

storage only when hot water was being 

drawn (as indicated by the flow meter) 

and/or power was being drawn (as 

indicated by the total power transducer). 

Instrumentation 

Temperature, water flow, humidity, 

power, and condensate flow measurements were implemented with sensors 

. 

4 –– FEDERAL ENERGY MANAGEMENT PROGRAM 
Technology Focus 

on each heat pump water heater 

unit. The data logger, power supply, 

modem, power transducers, and pressure 

transducer were housed in a covered 

instrument box attached by aluminum 

brackets to the side of each unit. 

Either an “open” (resistance mode) or a 

“closed” (heat pump mode) signal was 

sent from the control input/output 

channel on the logger to the mode 

switch input terminals of the heat pump 

water heater control board to set the 

desired operational mode. Successful 

implementation of this control strategy 

enabled both local and remote switching 

between operational modes. Remote 

mode switching was accomplished 

during phone connection to the data 

logger by manually changing the value 

of the associated flag in the custom data 

logger program. 

Laboratory Checkout 

Because the field test units were preproduction prototypes, and because 

some operational difficulties had been 

encountered previously with an earlier 

shipment of 10 units employed in durability tests [Baxter and Linkous (2002)], 

intensive checkout exercises were undertaken in the laboratory (Fig. 2) before 

shipment to the field test homes. Two 

tandem computer systems were used to 

monitor the performance of each unit 

during checkout, one for the installed 

instrumentation package (with communication through the data logger) 

and one for the heat pump water heater 

control system (with communication 

through the on-board microprocessor 

port). When necessary, changes to the 

EEPROM settings and control program 

were implemented through this 

second link. 

With the monitoring systems in place, 

an extensive series of water heat-up and 

draw sequences was undertaken to 

exercise the various sensors and control 

systems. Anomalies were detected, 

causes identified, and solutions implemented until proper operation was 

verified. As was the case for the 10 

durability tests, the most common 

problems were associated with 

unreliable splices in thermistor leads. 

Other detected anomalies were related 

to control board defects such as shorted 

mode switches, loose wiring, a failed 

light-emitting-diode connector, and a 

broken voltage regulator. In addition, 

two operational problems were found 

in the control program software. When 

all appropriate corrective measures had 

been completed, a final set of tests was 

conducted to verify correct remote communication and control operation using 

the central computer/modem/telephone 

line/modem/data logger system. 

Data Processing 

From the weekly downloads of data 

from the final storage of each logger, 

various algorithms were used to prepare 

weekly summaries. Specific calculated 

results from “rapid” data included hot 

water consumption, associated delivered 

heat value, source (inlet) water temperature, and maximum supply (outlet) 

water temperature. From “moderate” 

data, electricity consumption, 

compressor run time, upper resistive 

element run time, lower resistive 

element run time, beginning average 

tank temperature, and ending average 

tank temperature were calculated. These 

last two values were used to adjust the 

weekly hot water consumption values 

to account for additional heat stored 

in or removed from the tank. The effective weekly coefficient of performance 

(COP) was calculated (in nondimensional units) by dividing the net heat 

delivered to the hot water system by the 

electrical energy consumed. 

Calculated values from “slow” data 

included ambient temperature range 

and run-time average, thermostat 

setting range, minimum evaporator 

temperature, and maximum compressor 

discharge temperature. 

Data Analysis and Interpretation 

Average weekly performance data taken 

from the 18 field test units during 

operation in the heat pump mode are 

presented in Table 3. The variations in 

volume of hot water consumed, useful 

energy delivered in that water, electrical energy consumption, and associated 

heating COPs for the systems illustrate 

the wide range of operating situations 

encountered at the field test sites. 

Major Causes of Reduced Performance 

A key factor that contributed to reduced 

COP performance in some units was 

a relatively large amount of resistive 

element (upper and/or lower) operating 

time. The primary reason for increased 

upper element operation was increased 

concentration (amount and time) of 

household hot water draws, particularly evident in units 8, 4, and 6. A less 

common cause was extended periods 

between draws. In this situation, 

average tank temperature slowly 

declined because of standby losses, 

and, over time, tank thermal stratification essentially disappeared as a result 

of thermal diffusion. As a result, the 

upper resistive element was activated 

by the falling upper tank temperature 

before the compressor or lower element 

FEDERAL ENERGY MANAGEMENT PROGRAM –– 5 
Technology Focus 

Table 3. Summary of average weekly heat pump water heater field test performance 

Unit Gal kBtu kWh COP % savings 

13 (Danielsville, GA) 448 241 28.9 2.44 60 

12 (Hillsboro, OR) 374 218 29.2 2.19 60 

18 (Conyers, GA) 306 125 16.8 2.18 61 

07 (Cromwell, CT) 270 135 18.4 2.15 58 

20 (Madison, SD) 309 180 24.7 2.14 59 

03 (Melbourne, FL) 499 197 27.1 2.13 59 

11 (Pensacola, FL) 612 281 39.4 2.09 57 

09 (East Hampton, CT) 511 310 44.3 2.05 56 

16 (Gainesville, GA) 411 207 29.7 2.04 58 

14 (Wake Forest, NC) 458 229 33.1 2.03 58 

19 (Smithville, TX) 373 138 20.7 1.95 57 

05 (Milton, FL) 451 219 33.8 1.90 53 

01 (Douglas, AL) 444 228 35.6 1.88 52 

04 (Verbena, AL) 794 388 60.8 1.87 50 

06 (Knoxville, TN) 652 345 55.0 1.84 53 

10 (Lenoir City, TN) 522 226 40.9 1.62 44 

08 (Seattle, WA) 805 423 80.8 1.53 40 

15 (Portland, OR) 95 56 16.4 1.00 45 

Simple averages 463 230 35.3 1.95 54 

days. In Fig. 3, the dramatic variation 

of hourly hot water consumption for 

Saturday of that week is demonstrated. 

This pattern of variation is followed 

closely by the hourly electricity 

consumption given in Fig. 4. Data from 

the same unit operating in heat pump 

mode two weeks later show a similarly 

uneven pattern of hot water usage in 

Fig. 5. However, the corresponding 

electricity consumption data presented 

in Fig. 6 show a decidedly more uniform demand pattern, with consistent, 

low power draws (from the compressor 

and fans). Table 4 presents the measured 

performance during one week of 

operation in each mode for this unit 

and the calculated electricity savings 

(about 61% in this case) for the heat 

pump mode relative to the resistance 

mode. 

A limited amount of resistance mode 

data was acquired from 16 of the field 

test units. As expected, most of the 

was activated by the drop in lower tank 

temperature. This condition was most 

frequently encountered in unit 15. 

Also for this unit, extremely low heat 

demand magnified the detrimental 

effects of given standby losses. 

Increased lower resistive element operating time was caused by conditions that 

kept the control system from allowing 

compressor operation. In the case of 

unit 10, the substantial cooling of air in 

the small closet (especially during cold 

weather) caused the evaporator temperature to reach its lower limit, shutting 

down the compressor and activating the 

lower resistive element. In the case of 

unit 8, concentrated hot water draws 

combined with low inlet water temperature and low ambient temperature 

(during cold weather periods) prevented 

the compressor discharge temperature 

from reaching the required value in 

the allowed time, shutting down the 

compressor and activating the lower 

element. Less frequent causes of lower 

element activation were excessive 

compressor discharge temperatures 

(especially in high ambient and high 

thermostat setpoint temperature situations) and on/off compressor timer 

violations. 

Mode Comparisons 

Figure 2 illustrates daily hot water 

consumption for one week of unit 9 

operation in the resistance mode. It is 

clear from these data that the heaviest 

usage occurred during the two weekend 

heating by far in this conventional 

mode was accomplished by the lower 

resistive element. Of course, the size of 

the deviation in COP from unity in this 

case was an indication of the relative size 

of standby losses characteristic of each 

location, as mentioned earlier. 

The average weekly data for heat pump 

and resistance modes are plotted as 

electrical energy consumption versus 

delivered heat energy in Fig. 7. For 

any given amount of delivered heat 

energy, the energy savings that could be 

achieved by the use of heat pump mode 

compared with resistance (conventional) 

mode can be directly estimated as the 

vertical distance between data for the 

two modes. Based on this data, unitby-unit estimates of percentages of 

electrical energy savings were calculated. 

The results, presented in Table 3, 

ranged from 40 to 61%. 

. 

6 –– FEDERAL ENERGY MANAGEMENT PROGRAM 
Technology Focus 

Gallons

Figure 2. Sample daily hot water consumption for Unit 9 in resistance mode. Gallons 

“double-humped” weekday profile given 

in Fig. 8. The same methods were used 

to generate the analogous profile for the 

same six units operating in heat pump 

mode. This profile, also presented in 

Fig. 8, had a generally smaller amplitude 

than the corresponding resistance mode 

profile, reflecting the higher COPs 

associated with the heat pump mode. 

Winter data from the same units and 

modes were analyzed in the same 

manner to provide the results shown in 

Fig. 9. In this case, the heat pump mode 

profile had a much smaller amplitude 

than the resistance mode profile over 

virtually the entire average weekday. 

The striking exception was during the 

very early morning hours, when, in heat 

pump mode, the compressor and fans 

continued to run to complete recharging 

of the hot water in the tank. As can be 

seen from a comparison of Figs. 8 and 

9, the winter data profile for each mode 

had the same general shape, but exhibited 

a larger amplitude, when compared 

with the summer case. This trend was 

expected as the result of greater demand 

for delivered hot water heat and possibly 

reduced efficiency during cold weather. 

Several primary factors can contribute 

to greater demand in the winter period. 

Lower inlet water temperature (except, 

possibly, where the source is a deep well) 

means that more heat is required to raise 

the water temperature the thermostat 

setpoint temperature. With lower inlet 

water temperature, cold water/hot water 

mixing applications (for showers, etc.) 

will require a greater proportion of hot 

Figure 3. Sample hourly hot water consumption for first Saturday; Unit 9; resistance mode. 

Electrical Demand Comparisons 6-week period in the resistance mode to 

produce day-by-day profiles [Tomlinson 

Summer electrical demand data were and Murphy (2004)]. The relatively 

broken down into 15-min intervals and consistent weekday patterns were then 

aggregated for six units operating over a averaged to produce the characteristic 

water and/or a higher thermostat setting 

to deliver water at a given temperature. 

If hot water pipes are exposed to lower 

ambient temperatures, more heat is lost 

to the surroundings as the hot water 

travels through them, also requiring 

greater hot water flow (or longer pre-run 

FEDERAL ENERGY MANAGEMENT PROGRAM –– 7 
Technology Focus kWh

Figure 4. Sample hourly electrical energy consumption for pattern in previous figure. Gallons 

Figure 5. Sample hourly hot water consumption for second Saturday; Unit 9; 

heat pump mode. 

Seasonal Effects of Ambient 

Temperature and Supply Water 

Temperature 

Even with seasonal variations in demand, 

seasonal variations in COP were 

expected to be smaller in field situations 

where both the ambient temperature 

and the supply water temperature were 

generally constant throughout the year. 

Unit 13 was installed in a conditioned 

utility room (with relatively small 

fluctuations in ambient temperature) 

and was supplied by well water (characterized by relatively small fluctuations 

in supply water temperature). Weekly 

values of minimum evaporator temperature, average ambient air temperature, 

and normalized (to average) COP 

acquired over 75 weeks are shown in 

Fig. 10. Periods with incomplete or 

atypical data (e.g., vacations) were not 

included. 

The data for the ambient air temperature 

measured near the heat pump water 

heater vary between 64 and 72°F during 

this period. The measured evaporator 

temperature data generally follow the 

same pattern, varying between 44 and 

54°F during the same period. Of course, 

the evaporator temperature is displaced 

considerably (approximately 19°F) 

below the ambient air temperature to 

accommodate the required heat transfer 

from the ambient air to the evaporator. 

The corresponding normalized COP 

data in the same figure show relatively 

intervals to bring delivery temperature tion also increase. Occupants may also 

up to the desired level) and/or a higher take longer showers and baths to warm 

thermostat setting to deliver water at up from exposure to cooler surroundings. 

the desired temperature. If the ambient 

temperature decreases and/or the thermostat setpoint temperature increases, 

standby losses associated with conducsmall fluctuations (from 9% below to 

10% above the average) during the 

same period. 

Unit 14 was installed in an unconditioned garage and was supplied by a city 

water system. This situation subjected 

the heat pump water heater to substantial 

seasonal fluctuations in both the ambient temperature and the supply water 

. 

8 –– FEDERAL ENERGY MANAGEMENT PROGRAM 
– –

Technology Focus 

temperature. The corresponding weekly 

averages acquired over a period of 104 

weeks are shown in Fig. 11. In contrast 

to Fig. 10, significant seasonal variations 

are evident. The weekly average ambient air temperatures measured near the 

heat pump water heater range from 37 

to 82°F during this period. Minimum 

evaporator temperature data follow a 

similar pattern, varying between 27 

and 58°F during the same period. The 

normalized COPs also show relatively 

large fluctuations (from 38% below to 

28% above the average). Each of the 

three parameters is in phase (that is, 

low COP coincides with low ambient 

temperatures and low evaporator 

temperatures), and they show cyclic 

variations closely correlated with the 

change in seasons and in outdoor 

average air temperatures. The average 

COP for this unit was lower than that 

for unit 13 for several reasons: 

1. the overall average ambient 

temperature and minimum 

evaporator temperature for unit 14 

were lower (see discussion below); 

2. the water use pattern for the unit 14 

kWh 

Figure 6. Sample hourly electrical energy consumption for pattern in previous figure. 

	 	 	 	 

	 	 	 	 

Heat pump mode	 

(01/01 01/07)	 

Resistance mode	 

12/18 12/24)	 

Resistance mode 

(adjusted) 

Heat energy delivered (kBtu)	 	 

Electrical energy consumed (kWh)	 

Heating coefficient of performance	 

285	 	 

36.5	 	 

2.29	 	 

272	 	 

89.9	 	 

0.89	 	 

285 

94.2 

0.89 

	 	 	 

Savings using heat pump mode	 94.2 

−36.5 

57.7 kWh 

(61.3%)	 	 

family included more heavy hot water Table 4. One-week measured heat pump mode/resistance mode performance/savings 

use instances than the pattern for the comparison for unit 9 

other family (causing more usage of 

the upper resistive element); and 

3. the thermostat was set somewhat 

higher by the unit 14 family than 

by the unit 13 family (resulting in 

higher condenser temperature 

operation for unit 14). 

It should be noted that, for unit 14, 

the displacement of the evaporator 

temperature below the ambient air 

temperature varied from 27°F at higher 

air temperatures to 9°F at lower air 

Figure 7.

Electrical Energy Consumption (kWh)

 Electrical energy consumption 

versus delivered heat energy for 

resistance and heat 

FEDERAL ENERGY MANAGEMENT PROGRAM –– 9 
Technology Focus 

Average Electrical Demand (kW) 

average. In fact, the extremes of the 

valleys coincide with the lowest ambient 

air and evaporator temperatures seen. In 

these situations, to prevent frost buildup 

on the evaporator coil, the heat pump 

water heater controls automatically 

terminated heat pump operation 

(compressor and fans) and activated the 

lower resistance element. The resulting 

electric resistance heat operation substantially reduced the weekly average 

COP below that achievable with the 

heat pump system alone (if frost were 

not a problem). 

Summary of Results Figure 8. Combined six-unit summer weekday average electrical demand 

(15-minute intervals). 

Average Electrical Demand (kW) 

Figure 9. Combined six-unit winter weekday average electrical demand 

(15-minute intervals). 

Eighteen pre-production prototype 

drop-in residential heat pump water 

heaters were instrumented, pre-tested 

for operational problems in a laboratory 

environment, installed in a wide variety 

of host homes across the United States, 

and monitored for more than one year 

to determine performance and energy 

savings. Each 50-gal integral unit, 

incorporating a heat pump system with 

a storage tank in a single, pre-assembled 

package, served as a drop-in replacement 

for a conventional electric resistance 

storage water heater. Results of the 

testing indicated that performance was 

sensitive to hot water usage (amount and 

pattern), ambient temperature, inlet 

water temperature, and thermostat setting. 

temperatures. This reduced difference 

in heat transfer temperature reflects 

the reduced capacity of the heat pump 

system to remove heat from the air at 

low ambient air temperatures. Although 

the heat pump’s power requirement also 

drops at low ambient air temperatures 

because of reduced compressor load, 

the normalized COP data show that the 

reduction in heat removal capacity is 

greater than the power reduction. 

Another characteristic of the normalized COP data for unit 14 in Fig. 11 

is that the depth of the “valleys” at low 

ambient temperatures is greater than the 

height of the “peaks” at high ambient 

temperatures, compared with the 

Measured energy savings ranged from 

40 to 61% compared with an efficient 

conventional electric resistance storage 

water heater. Aggregate demand peaks 

were also found to decrease substantially 

compared to conventional units. 

. 

10 –– FEDERAL ENERGY MANAGEMENT PROGRAM 
Technology Focus 

Normalized coefficient of performance, COPnrm

Figure 10. Weekly average ambient temperature, minimum evaporator 

temperature, 

Normalized coefficient of performance, COPnrm 

Anticipated Commercialization 

Two firms that manufacture a large 


share of the water heaters sold in North 


America have formed teams to develop 


designs for heat pump water heaters. 


These water heaters are expected to 


appear on the market in the near future 


at a price yielding paybacks no longer 


than three years, which would make 


them very attractive for any application. 


This technology offers the promise of 


significant reductions in energy usage 


and peak demand, which should be of 


interest both to electric utilities and 


their customers. The HPWH will be 


useful in any type of housing, and 


especially if installed in large numbers, 


such as in military family housing 


or public housing, can contribute 


significantly to reducing energy costs 


and meeting federal energy goals.


For more information, contact one of 


the following researchers in the ORNL 


Buildings Technology Center: 


Van Baxter: 865-574-7639,


baxtervd@ornl.gov; or 


Richard Murphy: 


865-576-7772, 


murphyrw@ornl.gov.


Figure 11. Weekly average ambient temperature, minimum evaporator 

temperature, and normalized COP for Unit 14. 

FEDERAL ENERGY MANAGEMENT PROGRAM –– 11 
.

Technology Focus

92

William Lintner

New Technology Demonstration

 Program Manager

Federal Energy Management 

 Program

U,S. Department of Energy

1000 Independence Ave, SW, EEWashington, DC 20585

Phone: (202) 586-3120

Fax: (202) 586-3000

william.lintner@ee.doe.gov

Steven A. Parker

Pacific Northwest National

 Laboratory

P.O. Box 999, MSIN: K5-08

Richland, WA 99352

Phone: (509) 375-6366

Fax: (509) 375-3614

steven.parker@pnl.gov

Technical Contact

Michael Kintner-Meyer

Pacific Northwest National

Laboratory

P.O. Box 999, MSIN: K5-08

Richland, WA 99352

Phone: (509) 375-4306

Fax: (509) 375-3614

Michael.Kintner Meyer@pnl.gov

Produced for the U.S. Department

of Energy, Energy Efficiency and

Renewable Energy, by the Pacific

Northwest National Laboratory

DOE/EE-0000

xxxx 2006 

General Contacts

A Strong Energy Portfolio 

for a Strong America 

Energy efficiency and clean, renewable 

energy will mean a stronger economy, 

cleaner environment, and greater 

energy independence for America. 

Working with a wide array of state, 

community, industry, and university 

partners, the U.S. Department of 

Energy’s Office of Energy Efficiency 

and Renewable Energy invests in a 

diverse portfolio of energy technologies. 

For More Information 

EERE Information Center 

1-877-EERE-INF 

1-877-337-3464 

www.eere.energy.gov/femp 

General Contacts 

Will Lintner 

Team Lead, RSTT 

Federal Energy 

Management Program 

U.S. Department of Energy 

1000 Independence Ave, SW 

EE-92 

Washington, DC 20585 

Phone: (202) 586-3120 

Fax: (202) 586-3000 

william.lintner@ee.doe.gov 

Technical Authors 

For more information, contact one 

of the following researchers in the 

ORNL Buildings Technology Center: 

Van Baxter: 865-574-7639, 

baxtervd@ornl.gov; or 

Richard Murphy: 865-576-7772, 

murphyrw@ornl.gov. 

-

DOE/EE-0317 

May 2007 

References 

Federal Register 1998. Vol. 63, No. 90/ May 11/ Rules and Regulations/ 

pp 26012 – 26015. 

Murphy, R. W., and J. J. Tomlinson. 2002. Field Tests of a “Drop-In” Residential 

Heat Pump Water Heater. ORNL/TM-2002/207. Oak Ridge National Laboratory, 

September. 

Tomlinson, J. J. and R. W. Murphy. 2004. Measured Performance and Impacts of 

“Drop-In” Residential Heat Pump Water Heaters. ASHRAE Transactions, Vol 110, 

Part 2, June. 

Log on to FEMP’s Web site for information 

about New Technology Demonstrations 

www.eere.energy.gov/femp/ 

You will find links to 

• A New Technology Demonstration Overview 

• Information on technology demonstrations 

• 	Downloadable versions of publications in 

Adobe Portable Document Format (pdf) 

• A list of new technology projects under way 

• 	Electronic access to a regular mailing list for 

new products when they become available 

• 	How Federal agencies may submit requests 

to us to assess new and emerging 

technologies 

DISCLAIMER 

This report was sponsored by the United States	 

Department of Energy, Energy Efficiency and	 

Renewable Energy, Federal Energy Management	 

Program. Neither the United States Government	 

nor any agency or contractor thereof, nor any of	 

their employees, makes any warranty, express	 

or implied, or assumes any legal liability or	 

responsibility for the accuracy, completeness,	 

or usefulness of any information, apparatus,	 

product, or process disclosed, or represents	 

that its use would not infringe privately owned	 

rights. Reference herein to any specific 	 

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by trade name, trademark, manufacturer, or 	 

otherwise does not necessarily constitute or	 

imply its endorsement, recommendation, or	 

favoring by the United States Government or	 

any agency or contractor thereof. The views 	 

and opinions of authors expressed herein do 	 

not necessaily state or reflect those of the	 

United States Government or any agency or	 

contractor thereof.									 

U.S. Department of Energy 

Energy Efficiency 

and Renewable Energy 

No portion of this publication may be altered in any form without 

prior written consent from the U.S. Department of Energy, Energy 

Efficiency and Renewable Energy, and the authoring national laboratory.