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

Field Monitoring Protocol: 

Heat Pump Water Heaters

B. Sparn, L. Earle, D. Christensen, 

J. Maguire, and E. Wilson

National Renewable Energy Laboratory

C.E. Hancock

Mountain Energy Partnership

Technical Report

NREL/TP-5500-57698

February 2013
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

15013 Denver West Parkway

Golden, Colorado 80401

303-275-3000 • www.nrel.gov

Contract No. DE-AC36-08GO28308

Technical Report

NREL/TP-5500-57698

February 2013

Field Monitoring Protocol: 

Heat Pump Water Heaters 

B. Sparn, L. Earle, D. Christensen, 

J. Maguire, and E. Wilson

National Renewable Energy Laboratory

C.E. Hancock

Mountain Energy Partnership

Prepared under Task No.: BE120401
NOTICE

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iii

Acknowledgments

The authors wish to acknowledge everyone who contributed to this report. A number of people 

from the National Renewable Energy Laboratory have contributed invaluable insights about heat 

pump water heaters, field tests in general, and using the test results in computer models 

developed for heat pump water heaters. A huge thank you to Kevin Donovan for allowing us to 

monitor his water heater and fill his garage with sensors. As always, we are grateful to Ed 

Hancock and Greg Barker for their help setting up the field test and data logging equipment. 
iv

List of Acronyms

Btu British thermal unit

COP Coefficient of performance

CT Current transformer

dB Decibel

DHW Domestic hot water

EE Electrical energy

EES Engineering Equation Solver 

GE General Electric

HPWH Heat pump water heater 

HVAC Heating, ventilation, and air conditioning 

NEEA Northwest Energy Efficiency Alliance 

NREL National Renewable Energy Laboratory 

PV Photovoltaic 

RH Relative humidity

SHR Sensible heat ratio

T&RH Temperature and relative humidity

TC Thermocouple 

UA Heat loss coefficient

 
v 

Table of Contents

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

List of Tables .............................................................................................................................................. vi

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

Monitoring a Heat Pump Water Heater ..................................................................................................... 2

Potential Research Questions: ................................................................................................................ 3

Home Characteristics and Site Information ............................................................................................. 5

Basic Field Test Information:................................................................................................................. 5

Detailed Site Information: ...................................................................................................................... 5

Data Analysis and Calculations................................................................................................................. 7

Monitoring Package.................................................................................................................................. 13

Sensor Installation Notes...................................................................................................................... 17

Short-Term Testing .............................................................................................................................. 21

Long-term Monitoring.......................................................................................................................... 21

References................................................................................................................................................. 22

Appendix A–Field Test for Protocol Validation...................................................................................... 23

Objective .............................................................................................................................................. 23

Description of Field Test...................................................................................................................... 23

Installing Monitoring Equipment ......................................................................................................... 24

Install flow meter........................................................................................................................... 24

Surface-Mounted Thermocouples.................................................................................................. 25

Temperature and Relative Humidity Sensors ................................................................................ 26

Electrical Energy Measurement..................................................................................................... 27

Sound Test ..................................................................................................................................... 29

Sample Calculations............................................................................................................................. 30

Uncertainty Calculations................................................................................................................ 34
vi 

List of Figures

Figure 1. Pipe insulation covers the surface-mounted thermocouples on the inlet and outlet water 

pipes. ................................................................................................................................................... 18

Figure 2. Surface-mounted thermocouple secured while the epoxy cures........................................ 19

Figure 3. The insulation is replaced and the thermocouple wire is directed through hole in 

insulation............................................................................................................................................. 19

Figure 4. The cover is replaced, with the lower screw not fully tightened to give thermocouple wire 

clearance. ............................................................................................................................................ 19

Figure 5. The Rheem HPWH with heater covers shown. ...................................................................... 19

Figure 6. The top heater access point with the cover removed. The fiberglass insulation can be 

removed............................................................................................................................................... 19

Figure 7. After the insulation is removed, the tank wall is accessible for installing a 

thermocouple...................................................................................................................................... 19

Figure 8. The A.O. Smith Voltex HPWH with the upper heater cover shown...................................... 20

Figure 9. The styrofoam insulation that covers the upper heater access point................................. 20

Figure 10. After the insulation is removed, the tank wall is accessible for installing a 

thermocouple...................................................................................................................................... 20

Figure 11. The AirGenerate HPWH with the heater covers shown. ..................................................... 20

Figure 12. Both heater covers removed. The upper heater is the main heater and the lower heater 

is the back up (not connected).......................................................................................................... 20

Figure 13. GE GeoSpring HPWH installed in garage closet. ................................................................ 23

Figure 14. Turn off the breaker for the HPWH........................................................................................ 24

Figure 15. Lock-out the electric panel and verify that the circuit is de-energized............................. 24

Figure 16. Connecting hose to drain in preparation for installing the flow meter. ............................ 25

Figure 17. Final plumbing configuration with the flow meter and an extra length of flexible pipe 

installed. .............................................................................................................................................. 25

Figure 18. Surface-mounted thermocouples installed on inlet and outlet water pipes..................... 25

Figure 19. Surface-mounted thermocouple at the upper tank surface. .............................................. 25

Figure 20. Surface-mounted thermocouple at the lower tank surface. ............................................... 25

Figure 21. Pipe insulation covering the surface-mounted thermocouples on the inlet and outlet 

water pipes.......................................................................................................................................... 26

Figure 22. Insulation covering the tank where one surface-mounted thermocouple was installed.26

Figure 23. T&RH sensor is mounted on the front of the HPWH to measure inlet air conditions. .... 27

Figure 24. Current transformers and voltage taps for measuring the EE consumption of the 

HPWH................................................................................................................................................... 28

Figure 25. The wires from the CTs and the voltage tap are connected to the WattNode watt-hour 

transducer. .......................................................................................................................................... 28

Figure 26. Monitoring equipment is installed and connected to the data logger. The cell modem 

and antenna are also installed. ......................................................................................................... 29

Figure 27. Performing sound test for HPWH.......................................................................................... 30

Figure 28. Daily COP vs. daily hot water draw volume. ........................................................................ 32

Figure 29. Hot water draws and electrical energy use over two days in October 2012..................... 33

Figure 30. Uncertainty propagation analysis using EES software. ..................................................... 35

Figure 31. Uncertainty for measured values. ......................................................................................... 35

Figure 32. Results from the uncertainty propagation analysis............................................................ 36

List of Tables

Table 1. Examples of Typical Monitoring Equipment and Accessories.............................................. 15

Table 2. Example of Qdraws analysis for COP calculation...................................................................... 31

Table 3. Daily COP compared to the daily hot water draw volume...................................................... 32
1 

Introduction

An integrated heat pump water heater (HPWH) uses an air source heat pump as the main heat 

source and electric resistance elements as backup heaters. HPWH technology was first 

introduced in the 1950s and, unlike some earlier versions, integrated HPWHs combine the heat 

pump, water tank, and backup resistance elements in a single package. HPWHs are expected to 

have high system coefficients of performance (COPs), the ratio of useful energy transferred to 

the electrical energy (EE) consumed, relative to traditional electric water heaters that have a COP 

< 1. HPWHs have rated COP values > 2. 

HPWHs are a good fit for water heater retrofits in existing homes, because they are about the 

same size as traditional electric water heaters, have the same plumbing and electrical 

connections, yet have the potential to save at least 50% of water heating energy relative to other 

electric water heaters (Sparn, Hudon, & Christensen, 2011). In homes and regions where natural 

gas is not available, this is the first technology with a typical water heater form factor that can 

provide an energy-efficient alternative to electric resistance water heaters. Solar thermal water 

heaters can also be an excellent alternative, but they are usually more expensive and require 

more infrastructure (Hudon, Merrigan, Burch, & Maguire, 2012). 

Like other heat pumps used for space conditioning, those used in HPWHs absorb energy from 

the surrounding air. This heat is transferred to the water in the HPWH tank. The heat pump thus 

expels cooler and drier air into its surroundings. More than other types of water heaters, HPWHs 

are strongly influenced by—and have an effect on—their surroundings. Thus, the climate and 

installation location in the home can strongly impact HPWH performance. These effects are 

complex and different for virtually every house and climate region, so field tests will be used to 

improve modeling tools that are currently under development for HPWHs. 

HPWHs have been tested in controlled laboratory environments, and their operation is well 

understood technically (Sparn, Hudon, & Christensen, 2011). However, several unique factors 

affect their performance, so monitoring installed performance of HPWHs in varied real-world 

settings will produce valuable data. 

The scope of this work is to provide a standard field monitoring protocol for evaluating the 

installed performance of HPWHs in residential buildings. The report is organized to be 

consistent with the chronology of field test planning and execution. Research questions are 

identified first, followed by a discussion of analysis methods. The details of measuring the 

required information are then laid out. A field validation of the protocol at a house near the 

National Renewable Energy Laboratory (NREL) campus is included in the appendix. 
2 

Monitoring a Heat Pump Water Heater 

HPWHs have enormous potential, but their performance is significantly affected by the installed 

conditions. Several factors affect their performance and energy consumption, beyond the amount 

of hot water used by the household. 

• The installed location will influence the temperature and relative humidity (T&RH) of the 

ambient air the HPWH uses to heat the water. If the HPWH is installed in an 

unconditioned space that is too cold in the winter, the HPWH may not be able to use its 

compressor and so may use the backup electric resistance heaters to heat the water. If the 

installed location is inside conditioned space, the heat pump will always have warm air 

around it, so it is expected to increase the space heating load in the house during the 

winter and reduce the air conditioning load in the summer. 

• The volume of hot water consumed, as well as the exact draw pattern, will also impact 

HPWH performance. If several large draws occur in close succession, the heat pump may 

not be able to keep up with demand and the resistance heaters will take over. However, 

the same volume of requested hot water, spaced out more evenly over a day, could be 

satisfied with the heat pump alone. 

• Depending on where the unit is installed and how often the filter is cleaned, the 

cleanliness of the filter can affect overall performance. A very dirty filter can obstruct 

airflow and degrade performance. 

• The tank volume will affect how often the resistance heaters are needed. A larger tank 

will be able to satisfy large hot water draws, and could avoid the use of the electric 

resistance heaters even in periods of high demand. 

• Different manufacturers use different control logic that also affects when the heat pump 

is used and when the resistance heaters turn on. 

Because so many variables affect HPWH performance, monitoring the performance of HPWHs 

installed in homes around the country is essential to understanding where they are best suited. 

Laboratory tests have guided installations, but there is still much to be learned.

The high level of interaction between a home’s heating and cooling systems and the HPWH 

means that other systems will need to be monitored to fully understand its impact. The exact set 

of data required will vary for each house and the goals of that project. Data on basic house 

characteristics will also need to be collected to enable simulation of the HPWH for that house. 

By collecting data on the whole house and using a model to recreate the conditions in the house, 

the long-term effects of an HPWH can be fully quantified. 

Before any field test is conducted, the project goals need to be defined. A monitoring plan will 

be devised based on the research questions and the specifics of a particular site. Researchers 

should collect information about the home and heating, ventilation, and air conditioning (HVAC) 

equipment when they arrive at the field test site. Then the monitoring equipment and data logger 

must be installed. Before leaving the field test site, the researchers should ensure that all sensors 

were installed correctly and are functional. The research plan may prescribe some short-term 

tests that should be performed after all the monitoring equipment has been verified. Long-term 

monitoring will collect data on HPWH performance over one or more seasons, depending on the 
3 

research plan. Especially in places with a heating and a cooling season, the long-term monitoring 

should be planned to measure the HPWH performance in both. After the scheduled monitoring 

duration, the monitoring equipment will be removed from the field test site. 

Potential Research Questions 

1. What is the HPWH performance in the installed location—system COP as a function of 

ambient air T&RH, daily COP as a function of daily hot water use, and real-world 

variations in use patterns? 

2. How does the daily average system COP vary over the course of the year? How does the 

annual system COP compare to rated efficiency? How do operational efficiency and 

energy consumption compare to simulation estimates based on conditions around the 

water heater and the hot water demands of the household? Rated efficiency and 

simulation performance estimates are used when choosing to install a HPWH, so it will 

be useful to see how well these performance metrics compare to actual installed 

performance. 

3. Is the HPWH keeping up with hot water demand? Monitor outlet temperature to observe 

whether and how often it drops below 110°F. This minimum temperature threshold is 

taken from the Operating Conditions specified in the House Simulation Protocols 

(Hendron & Engebrecht, 2010). It may not be possible to determine the volume of hot 

water needs that are not met, but monitoring when and how often the hot water 

temperature drops below an acceptable level may indicate how well the HPWH is 

satisfying the home’s domestic hot water (DHW) needs.

4. Do occupants change the operating mode or temperature set point to ensure they have 

enough hot water? Does this affect the actual energy savings relative to predicted 

savings? 

5. Depending on the time of year and climate, an additional source of cold, dry air may be 

beneficial, because it could offset some of the cooling and dehumidifying loads. 

However, the HPWH will produce that same cold air in the winter. What is the overall 

impact of the HPWH on the other space conditioning equipment in the house? 

6. An HPWH produces perpetual cooling, so installing or replacing a water heater with an 

HPWH in an unconditioned or semiconditioned area of the house, such as a basement or 

a garage, may be a good option. The ambient conditions in an unconditioned space will 

determine an HPWH’s annual performance, but in new homes and retrofits, the 

temperature range in that unconditioned space is probably not well known. Are the 

monitored conditions in the unconditioned space favorable for the HPWH over a year? 

Can the heat pump operate during the entire year or are the electric resistance elements 

needed for long periods of time? 

7. For installations in conditioned spaces, is noise a concern for homeowners? An HPWH 

sounds like an air conditioner and, depending on the location, the noise could be more or 

less amplified. 

8. If the air inlet and outlet (or just the outlet) of the HPWH are ducted, the research 

questions will be more extensive. Where are the ducts pulling or depositing air? Are 

additional fans and dampers required for the duct system? How are they controlled? What 
4 

is the energy penalty or benefit? What is the first cost penalty? What energy gains are 

measured for the HPWH with the duct system, and for the HVAC system? 

As with all field tests, the research questions dictate what must be measured. The measurements

needed to answer these broad questions will be described in the following sections. The 

particular suite of sensors and controls used in any field test is tailored to each specific test. 
5 

Home Characteristics and Site Information

An HPWH field test could occur in virtually any area of the country, in a new home or a retrofit 

situation. As with any field test, some basic information about the home should be collected in 

the early planning stages to help frame the research questions and fine tune the monitoring plan. 

Also, the homeowner should be consulted at this point to clarify expectations and ensure that the 

long-term monitoring plan will not be an inconvenience. The following basic field test

information should be collected to inform the monitoring plan. 

Basic Field Test Information

• Geographical location

• Home heating and cooling type

• Original type (for retrofits) and location (conditioned/unconditioned) of water heater

• Homeowner approval for monitoring for at least one year, if long-term monitoring is 

needed for the research plan. 

• Homeowner approval for release of historical utility bills (for retrofits)

• Reasonable expectation that an HPWH is an appropriate choice of water heating 

technology based on the climate, installed location, and family size. This assessment 

should be based on simulation results, if possible. 

Once the researchers visit the field test site to begin installing the monitoring equipment, 

additional information about the home and HPWH installation is needed for modeling and to 

determine expected performance. The following detailed information should be gathered while 

researchers are on site, as needed. Many of these questions will only be relevant for field tests 

with related research goals. In retrofit cases, the expectation is that the HPWH has already been 

installed before the field test begins. In new homes, the researchers may have the opportunity to 

install monitoring equipment before construction is finished. In that case, the researchers may 

have to follow up with the homeowners after they have moved in to complete the more detailed 

survey. 

Detailed Site Information

• Equipment specifications. Make, model, and size of HPWH.

• Building use. Number of occupants, typical schedule.

• Installation date and cost. Include any known installation issues.

• Installation site for HPWH. Size and location of room, location within room.

o For retrofit projects, is the HPWH installed in same location as the previous water 

heater?

o Is the room part of conditioned space, unconditioned space, or semiconditioned 

space?

o If in a smaller room, is the door properly louvered, per manufacturer instructions?
6 

o Where is the condensate drained to? Does it require a pump? 

• Does the installation comply with manufacturer recommendations? Room size and 

clearance around the unit are important for HPWH performance. 

o Consult the Building America Measure Guideline for HPWHs for other 

installation considerations (Shapiro, Puttagunta, & Owens, 2012).

o Should the manufacturer incorporate any additional installation recommendations 

into the user manual?

• Maintenance requirements, according to the installer and/or manufacturer. Most of this 

should come from the HPWH user manual.

o Inspect the condition of the air filter and clean, if needed, prior to monitoring. 

• HPWH operating mode and set point. In a retrofit case, what was the set point of the 

old water heater, if known?

o Has the occupant had to adjust the set point or operating mode of the HPWH to 

improve comfort?

• Is inlet and/or outlet air ducted? If so, where to? Are there controls that can be used to 

change where ducting goes in summer and winter?

• Determine the installer’s experience and certifications.

• Detailed information about the home’s heating and cooling equipment (make, 

model, age) and ducts (leakage, insulation level, location). This information will be 

needed if assessing the impact of the HPWH on the space conditioning energy 

consumption. 

• System controls for space conditioning: thermostat set points, deadband, zoning, and 

ventilation control settings (any that are applicable)

• Home construction details, including blower door results if applicable, for later use 

when modeling the home to predict energy use of the HPWH and space conditioning 

equipment.

• Complete list of other retrofits occurring with HPWH installation and during the 

monitoring period. 
7 

Data Analysis and Calculations

Once the research questions for a particular project have been identified, the data analysis plan 

should be reviewed to ensure that all the quantities needed for analysis will be measured. This 

also may help researchers avoid installing unnecessary sensors. The following calculations are 

commonly used when evaluating HPWHs, but other calculations may be needed depending on 

the research objectives of the field test. 

System COP should be calculated each day for all field tests involving HPWH monitoring. All 

other calculations shown below are optional and may or may not be relevant to the research 

questions for a particular field test. Uncertainty analysis should be performed for all calculations, 

especially when the results will be presented in public reports. The appendix shows a few 

examples of uncertainty analysis. 

System Coefficient of Performance

Daily system COP is a measure of the useful energy transferred to the water divided by the EE

input to the system over the course of a day (ASHRAE, 2005). 

COP = Qthermal

Winput

= Qdraws

Winput

= ρwater ∙ Vdraws ∙ Cp,water ∙ ∆Tdraws

Winput ∙ 3600 s

hr

(1) 

where

ρwater is the density of water, 998 kg/m3. 

Vdraws is the volume of the water drawn (m3

). 

Cp,water is the specific heat of water, 4.182 kJ/kg-C. 

∆Tdraws=Tout-Tin is the difference between the outlet and inlet water temperature (°C).

Winput is the EE used by the water heater during the day (kWh)

1

. 

1 To avoid the situation where the heat pump may still be running after draws on the previous day, it may be useful 

to define 4:00 a.m. as the start and end time for each day.

The heat removed during draws should be calculated at short intervals, such as every minute, to 

ensure that any changes in ∆Tdraws over the course of the draw are taken into account. The total 

thermal energy removed from the tank in a day should be compared to the total EE needed 

during that day. That ratio will be the system COP for that day and a daily average should be 

calculated for the monitoring period. 

Heat Loss Coefficient

The heat loss coefficient (UA) of the HPWH can be measured in a short-term test or during the 

long-term monitoring phase when there are long periods of time without hot water draws. Even if 

there are no hot water draws for a day or more, the heat pump will need to come on periodically 

to compensate for standby heat loss. The rate at which heat is lost through the tank walls is 

described by the UA of the water heater. The higher the UA value, the faster heat is lost and the 

more energy is required to keep the tank hot. 
8 

It is important that the tank be heated with the heat pump prior to the standby period evaluated 

for the tank UA measurement. When the tank is heated with the heat pump, it is expected to be 

more uniformly heated than if the resistance elements are used, which will affect UA. 

Whether the UA is measured during a short-term test or during the long-term monitoring period, 

the tank needs to be conditioned so that the entire tank is at the same temperature. During a 

short-term test, this can be induced by drawing enough water to force the heat pump to come on. 

Once the heating cycle is finished, the tank’s temperature will be nearly uniform. As the tank 

cools, laboratory measurements indicate that the average tank temperature is well approximated 

by the average of temperatures in its upper and lower thirds. This is where the surface-mounted 

thermocouples (TCs) will be installed when there are upper and lower resistance heaters. If the 

HPWH under test does not have two resistance heaters, it may not be possible to measure 

average tank temperature and by extension, UA. The geometry for five of the integrated HPWHs 

currently on the market is discussed below. 

Once the tank is conditioned and the heat pump turns off, the standby period will begin. Ideally, 

this test should be done when the ambient temperature is relatively steady, such as at night. The 

standby period is defined as the amount of time between the end of one heating cycle and the 

beginning of the next. All measurements and calculations should begin about 10 min after the 

heat pump turns off, because the condenser coil will be hotter than the water in the tank for 

several minutes after the heat pump stops running. The following equation will be used to 

calculate UA after the standby period is finished (DOE, 1998). 

UA = Vst ∙ ρwater ∙ Cp,water ∙ �T�

i − T�f�

τstdby ∙ (T�

tank − T�amb) (2) 

where

Vst is the volume of the storage tank (m3

) 1, 

T�

i is the average tank temperature, (Ttank,upper+Ttank,lower)/2, at the beginning at the end of 

the standby period (°C) 2,

T�

f is the average tank temperature, (Ttank,upper+Ttank,lower)/2, at the end of the standby 

period (°C), 

τstdby is the total standby time (h),

T�

tank is the average tank temperature during the entire standby period (°C), 

T�

ambis the average ambient temperature during the entire standby period (°C). 

1 The actual tank capacity is usually lower than the rated capacity. If the manufacturer does not prove the actual tank 

volume, approximate the tank volume as 10% less than the rated value. For example, the actual volume of a 50-gal 

tank is usually closer to 45 gal. 

2 Right after the heat pump turns off, the surface of the tank will be hotter than the water, because the condenser 

coils are still hot. The average tank temperature at the beginning of the standby period should be calculated a few 

minutes after the heat pump turns off to avoid an artificially high temperature reading.
9 

Cooling Capacity

A heat pump removes heat from the air with a vapor compression cycle similar to an air 

conditioner. The total cooling capacity can be calculated to quantify the cooling effect of the 

HPWH on its surroundings. This calculation requires the airflow of the heat pump to be 

measured using a duct blaster. The cooling effect can be separated into sensible and latent 

cooling effect, and the ratio of sensible cooling capacity, qs, to the total cooling capacity, qt, is 

called the sensible heat ratio (SHR) (ASHRAE, 2005). 

qt = ρair ∙ V̇ ∙ (hinlet − houtlet) (3) 

qs = ρair ∙ V̇ ∙ Cp,air ∙ (Tin − Tout) (4) 

qt = qs + ql (5) 

SHR = qs/qt (6) 

where 

qt is the total cooling capacity of the heat pump (W), 

qs is the sensible cooling capacity of the heat pump (W),

ql is the latent cooling capacity of the heat pump (W), 

SHR is the sensible heat ratio for the heat pump (unitless),

ρair is the density of air at the average air temperature, expression given in Equation (7), 

V̇ is the volumetric flow rate of the air through the heat pump (m3

/s), 

hinlet and houtlet are the enthalpies of the air entering and leaving the heat pump, 

respectively, (J/kg). Air enthalpy can be calculated based on air temperature and 

humidity, expression given in Equation (8). 

Cp,air is the specific heat of air at the average air temperature, expression given in 

Equation (9), and 

Tin and Tout are the temperatures of the air at the inlet and outlet of the heat pump, 

respectively (°C).

Air Density (ASHRAE, 2005)

ρair = Pamb

Rda ∙ �273.15 + Tavg� ∙ (1 + 1.6078 ∙ ω) (7) 

where

ρair has units of kg/m3, 

Pamb is the ambient pressure (Pa),

Rda is the universal gas constant for dry air, 287.055 J/kg-C, 

Tavg is the average air temperature (°C), Tavg = (Tin+Tout)/2, and 

ω is the humidity ratio of the air, expression given in Equation (10). 
10 

Air Enthalpy (ASHRAE, 2005)

h = Cp,air ∙ T + ω ∙ (2501 + 1.86 ∙ T) (8)

where 

T is the temperature of the air (°C), and 

ω is the humidity ratio of the air (kg/kg).

Specific Heat of Air (ASHRAE, 2005)

Cp,air = 1005.625 + 0.023881 ∙ Tavg + 3.79x10−4 ∙ Tavg

2 − 8.3x10−8 ∙ Tavg3

1000

(9)

where

Cp,air has units of kJ/kg-C, and 

Tavg is the average air temperature (°C), Tavg = (Tin+Tout)/2. 

Humidity Ratio of Air (ASHRAE, 2005)

ω = 0.62198

RH ∙ Pws

Pamb − RH ∙ Pws

(10) 

 

where

ω has units of kg/kg, 

RH is the relative humidity of the air (unitless fraction),

Pws is the saturation vapor pressure of the air (Pa), expression given in 

Equation (11), and 

Pamb is the ambient pressure (Pa).

Saturation Vapor Pressure (ASHRAE, 2005)

Pws = exp �

C1

TK

+ C2 + C3TK + C4TK

2 + C5TK3 + C6TK4 + C7 ln(Tk)� (11) 

where

Pws has units of Pa, and 

TK is the absolute air temperature (Tk=T+273.15) (K).

Cn coefficients given below. 

For TK ≤ 273.15K: For TK > 273.15K:

C1 = -5.6745× 103 C1 = -5.8002 × 103

C2 = 6.3925 C2 = 1.3915

C3 = -9.6778 × 10-3 C3 = -4.8640 × 10-2

C4 = 6.2216 × 10-7 C4 = 4.1765 × 10-5

C5 = 2.0748 × 10-9 C5 = -1.4452 × 10-8

C6 = -9.4840 × 10-13 C6 = 0

C7 = 4.1635 C7 = 6.5460
11 

Winput

Qair

Qloss

Qdraws

Effect of Heat Pump Water Heater on Heating and Cooling Loads 

Calculate the effect of the HPWH on the space conditioning load by looking at the energy 

balance. The net effect on the space, Qnet,space, includes the heat loss and the cooling load from 

the heat pump. The energy balance is shown below: 

Winput + Qair = Qdraws + Qloss (12) 

⇒ Qnet,space = Qloss − Qair = Winput − Qdraws (13) 

⇒ Qnet,space = Winput – ρwater ∙ Vdraws ∙ Cp,water ∙ ∆Tdraws (14) 

where

Qair is the energy from the air that is added to the tank (kJ), 

Qdraws is the hot water energy removed from the tank during draws (kJ), 

Qloss is the energy lost from the tank via convection (kJ), 

Qnet,space is the net energy added to the space (kJ), 

Winput is the EE used by the water heater to reheat the tank (kJ),

ρwater is the density of water, 998 kg/m3

, 

Vdraws is the volume of the water drawn (m3

),

Cp,water is the specific heat of water, 4.182 kJ/kg-K, and 

∆Tdraws=Tout-Tin is the difference between the outlet and inlet water temperature (°C).

 

The result of this energy balance will calculate the net effect of the HPWH on the space around 

it. If the heat pump is being used to heat the water, the heat added to space, Qnet,space, should be a 

negative number, indicating that heat is removed from the space. If only the resistance heaters 

are used, Qnet,space will be a positive number, indicating that heat is added to space by heat loss 

from the tank. As with the COP calculation, the Qdraws term is calculated on shorter time steps 

and totaled over an entire day. Winput is equal to the total EE used on the same day. 

If the heat pump is primarily used, this will create a net cooling effect on the space. If the HPWH 

is located in conditioned space, it could help offset some of the cooling load in the summer, but it 

could also add to the heating load in the winter. That impact should be compared to the heating 

or cooling energy used in the house. If those systems are being monitored, a daily comparison 

can be employed to provide a frame of reference. For instance, it would be important to know if 

the amount of heat being removed from the air by the HPWH is equivalent to 1% or 20% of the 

daily heating load. If the other HVAC equipment is not being monitored, monthly bills can be 
12 

used and compared to the month-long effect of the HPWH on the space. It is unlikely that the 

entire cooling effect would need to be made up by the heating system during the heating season, 

or that the exhaust air would provide all the building’s cooling needs during the cooling season.

Qnet,space calculates the net cooling effect on the space from the HPWH, but includes both 

sensible and latent cooling, where thermostats respond to sensible heat only. To extract the 

sensible cooling effect, multiply the SHR by Qnet,space, if SHR has been calculated. However, 

SHR is close to 1 for the HPWHs tested at NREL in 2011 (Sparn, Hudon, & Christensen, 2011). 
13 

Monitoring Package 

The following measurements should be tailored for the specific field test, based on the research 

questions for that project. All are optional, except those needed for system COP. 

The sampling rate required for the data depends on the goals of the field test. If HPWH energy 

consumption is the paramount concern, making measurements once every minute may be 

sufficient. If more detailed information about hot water use at the fixture level is desired, higher 

frequency data acquisition, on the order of 1 Hz, may be required. Increasing the rate of data 

logging increases the size of the resulting data files and may require more frequent upload 

depending on the data logger’s onboard storage capacity. Also, unnecessarily high (temporal)

resolution data will increase the time required for data processing.

It is also possible to take higher frequency data for a subset of channels for a short time. If highresolution data are needed at the fixtures only when hot water is being used, some data loggers 

can save high-resolution data only during the times when a draw is occurring. 

Electrical Energy Measurements

1. Whole-house EE (if applicable)

2. Photovoltaic (PV) EE production (if applicable; 

needed for whole-house measurement)

3. HPWH EE

4. EE for HPWH fan (optional, if HPWH is ducted) 

5. EE for HPWH circulation pump (optional, if applicable) 

6. EE for DHW recirculation pump (if applicable)

7. EE for space conditioning electric heaters (if applicable)

8. EE for air conditioning or heat pump (if applicable) 

9. EE for dehumidifier (if applicable)

10. EE for furnace fan or air handler (if applicable)

Temperature and Humidity Measurements

1. HPWH inlet air temperature and humidity

2. HPWH outlet air temperature and humidity (placed very near the air outlet) 

3. Same room air temperature and humidity (placed away from the HPWH, in a well-mixed 

location—this measurement will be especially important for any ducted or enclosed 

situations)

4. Air temperature and humidity near the thermostat (when HPWH is installed in 

conditioned space)

CAUTION! Working on a 

high-voltage system is 

extremely hazardous. Work 

inside of a circuit breaker 

panel must be performed by a 

qualified electrician.
14 

Temperature Measurements

1. Inlet water temperature (immersion or surface-mounted) 

2. Outlet water temperature (immersion or surface-mounted) 

3. Tank temperature (surface-mounted, affixed to outside of tank, under insulation; if

possible, install 2 sensors: one near the upper electric element and one near the lower 

element)

4. Surface temperature of the evaporator coils at the refrigerant outlet (optional, most 

applicable for installations that may have freezing concerns)

Flow Rate Measurements

1. Water flow rate on either the inlet or the outlet side (used to determine draw volumes) 

2. Natural gas flow rate/consumption for the gas furnace (if applicable) 

3. HPWH condensate volume or flow rate (optional) 

Other Measurements

1. Outlet airflow rate (short-term test for HPWHs with a single airflow rate)

2. Noise measurement (when HPWH is located in a place where noise would be noticed, 

most likely relevant only when installed in conditioned space) 

Weather Station 

1. Outdoor air temperature and humidity

2. Solar radiation (optional, useful only when HPWH is installed in an attic or a garage)

Table 1 lists some common sensors that could be used to take the above data points. These 

sensors have been used in past Building America research projects, but equivalent sensors with 

similar operating ranges and accuracies would also be acceptable. 
15 

Table 1. Examples of Typical Monitoring Equipment and Accessories1. 

Monitored 

Parameter

Sensor 

Description

Sensor Manufacturer

and Part Number Sensor Range Sensor Accuracy

Electrical Power Measurements

Whole House 

Electrical Power

Watt node power 

meter

Continental Control 

Systems (CCS) WNB-3Y208-P 100HZ output± 0.5%

Split core current 

transformer CCS CTS-0750-100 0–100 A ± 1.0% for > 10% of rated current

HPWH Electrical 

Power

Watt node power 

meter

CCS WNB-3Y-208-P 

100HZ output ± 0.5%

Split core current 

transformer CCS CTS-0750-30 0–30 A ± 1.0% for > 10% of rated current

Electrical Power 

for HPWH Fan

Watt node power 

meter

CCS WNB-3Y-208-P 

100HZ output ± 0.5%

Split core current 

transformer CCS CTS-0750-5 0–5 A ± 1.0% for > 10% of rated current

Electrical Power 

for Recirculation 

Pump

Watt node power 

meter

CCS WNB-3Y-208-P 

100HZ output ± 0.5%

Split core current 

transformer CCS CTS-0750-5 0–5 A ± 1.0% for > 10% of rated current

Power for 

Electric Heaters 

Watt node power 

meter

CCS WNB-3Y-208-P 

100HZ output ± 0.5%

Split core current 

transformer CCS CTS-0750-30 0–30 A2 ± 1.0% for > 10% of rated current

Electrical Power 

for Air 

Conditioning or 

Heat Pump

Watt node power 

meter

CCS WNB-3Y-208-P 

100HZ output ± 0.5%

Split core current 

transformer

CCS CTS-0750-30, CCS 

CTS-0750-15

0–30 A 

(comp)2

0–15 A (AH)

± 1.0% for > 10% 

of rated current

Electrical Power 

for Dehumidifier

Watt node power 

meter

CCS WNB-3Y-208-P 

100HZ output ± 0.5%

Split core current 

transformer CCS CTS-0750-15 0–15 A2 ± 1.0% for > 10% of rated current

Electrical Power 

for Dehumidifier

Watt node power 

meter

CCS WNB-3Y-208-P 

100HZ output ± 0.5%

Split core current 

transformer CCS CTS-0750-15 0–15 A2 ± 1.0% for > 10% of rated current

 

1 Pieces of measurement equipment that have been commonly used in Building America tests are given here as 

examples only. We do not recommend or endorse any manufacturer or product. 2 Current range for electric heaters, air conditioner or heat pump and air handler, and dehumidifier is approximate 

and should be reconsidered for the specific field test installation, as the size and capacity for this equipment can 

vary.
16 

Temperature and Humidity Measurements

HPWH Inlet Air 

Temperature 

and RH

Temperature and 

humidity sensor Vaisala HMP60 –40°–60°C0%–100% RH

± 0.6 °C,

± 3% RH

HPWH Outlet Air 

Temperature 

and RH

Temperature and 

humidity sensor Vaisala HMP60 –40°–60°C0%–100% RH

± 0.6 °C,

± 3% RH

Room Air 

Temperature 

and RH

Temperature and 

humidity sensor Vaisala HMP60 –40°–60°C0%–100% RH

±0.6 °C,

± 3% RH

Air Temperature 

and RH Near 

Thermostat

Temperature and 

humidity sensor Vaisala HMP60 –40°–60°C0%–100% RH

± 0.6 °C,

± 3% RH

Temperature Measurements

Inlet Water 

Temperature 

Immersion TC

or surfacemounted TC

Omega TMQSS-125U-6 

or CO1-T –270°–400°C Greater of ± 0.5°C or 0.4%

Outlet Water 

Temperature 

Immersion TC

or surfacemounted TC

Omega TMQSS-125U-6 

or CO1-T –270°–400°C Greater of ± 0.5°C or 0.4%

Tank 

Temperature 

T type surfacemounted TC Omega CO1-T –270°–400°C Greater of 

± 0.5°C or 0.4%

Thermally 

conductive epoxy Omega OB-101-1/2 N/A N/A

Evaporator Coil 

Surface 

Temperature

T type surfacemounted TC Omega CO1-T –270°–400°C Greater of 

± 0.5°C or 0.4%

Flow Rate Measurements

Water Flow 

Rate (Inlet or 

Outlet)

Turbine Flow Meter Omega FTB-4605 0.15–13.0

gpm

± 1.5% of reading

Natural Gas 

Flow Rate for 

Gas Furnace

Diaphragm Gas 

Flow Meter

Elster American Meter 

AC-250 0–250 SCFH ± 1 CF/pulse

Condensate 

Flow Rate Tipping Bucket Campbell Scientific 

TB4-25 1–500 mm/h ± 2%

Other

Outlet Airflow 

Rate Duct Blaster The Energy 

Conservatory 10–1500 cfm ± 3%

Noise 

Measurement Sound Level Meter Extech 407736 35–130 dB ± 1.5 dB
17 

Weather Station

Outdoor Air 

Temperature 

and RH

Temperature and 

humidity sensor Vaisala HMP60 -40-60°C0-100% RH

± 0.6°C,

± 3% RH

Radiation Shield Campbell Scientific 

41303-5A N/A N/A

Solar 

Radiation Pyranometer Campbell Scientific 

CS300-L50 0-2000 W/m2 ± 5% FS

Sensor Installation Notes

Water Flow Meter Installation

A few considerations should be taken into account when deciding which water line to install the 

flow meter on. To ensure that the flow meter is subject to laminar flow, it should be installed in a 

location that has 10 pipe diameters of straight pipe upstream of the flow meter and 5 pipe 

diameters of straight pipe downstream. For example, if the flow meter is being installed in 1-in. 

diameter pipe, there should be 10 in. of straight pipe before the flow meter and 5 in. of straight 

pipe after. This helps to ensure consistent readings, but is not always achievable. 

Also, installing the flow meter on the outlet pipe is preferred because the inlet pipe can be 

subject to thermal expansion flow discrepancies where the hot water in the tank expands and 

pushes water out the inlet. However, some water heaters are installed with heat traps, which 

eliminate this issue. When flow meters are installed on the outlet pipe, the plastic components 

found in turbine flow meters can experience fatigue after being immersed in hot water. 

Measuring Inlet and Outlet Water Temperatures: Immersed or Surface-Mounted 

Thermocouples? 

In cases where the HPWH was installed before the monitoring equipment, it may be preferable to 

use a surface mounted TC for the inlet and outlet water temperature measurements to keep the 

pipes intact. Past laboratory tests have shown that surface-mounted TCs measure temperatures to

within 2% of temperatures measured by immersed TCs. One of the pipes (inlet or outlet water 

line) will need to have an in-line flow meter installed, so an immersion TC can be installed at the 

same time, if desired. However, these TCs will be used to calculate a change in temperature, so 

the type of TC used on the inlet and outlet should match. 

Insulating Around Thermocouples

A surface-mounted TC on the inlet and outlet water lines needs to have insulation wrapped 

around the pipe and the TC, as shown in Figure 1 with surface-mounted TCs. 
18 

Figure 1. Pipe insulation covers the surface-mounted TCs 

on the inlet and outlet water pipes

The surface-mounted TCs installed to measure tank temperature also need to be insulated. In 

most cases, the TC will be installed under existing insulation, which can be replaced. Once 

power to the unit has been turned off, remove the covers for the electric resistance heaters 

and the piece of insulation underneath (specific notes for the HPWHs tested at NREL in 2011 

are given below). Affix the TC to the tank surface with thermally conductive epoxy. Wait 

until the epoxy is dry before replacing the insulation and the cover. 

Installing Tank Thermocouples for the Different HPWH Brands

In 2011, NREL tested the first five integrated HPWHs on the U.S. market (Sparn, Hudon, & 

Christensen, 2011). There have been one or two new entrants to the market, but the water heaters 

listed below make up most of the options available. If a field test involves an HPWH that is not 

discussed here, the researchers will need to investigate how many electric elements there are and 

make suitable adjustments to the tank temperature measurement method if needed. 

• General Electric (GE) GeoSpring HPWH (first generation): Two electric resistance 

elements provide access to the tank wall at two places. Removing the heater covers 

requires a security or tamperproof Torx bit. The Styrofoam insulation should be put back 

after the TCs are installed, as shown in Figure 2 through Figure 4. 

• Rheem Hybrid HPWH: Two electric resistance elements provide access to the tank wall 

at two places. The heater covers can be removed with a standard Phillips head 

screwdriver. The fiberglass insulation should be returned to the heater access port after 

the TCs are installed. 

NREL Image Gallery 23088Credit: Bethany Sparn
19 

Figure 2. Surface-mounted 

TC is secured while the 

epoxy cures.

Figure 3. The insulation is 

replaced and the TC wire is 

directed through a hole in the 

insulation. 

Figure 4. The cover is 

replaced, with the lower 

screw not fully tightened 

to give TC wire 

clearance.

Figure 5. The 

Rheem HPWH 

with heater covers 

shown.

Figure 6. The top heater 

access point with the cover 

removed. The fiberglass 

insulation can be removed.

Figure 7. After the insulation 

is removed, the tank wall is 

accessible for installing a 

TC. 

• A.O. Smith Voltex HPWH: Two electric resistance elements provide access to the 

tank wall at two places. The heater covers can be removed with a standard Phillips 

head screwdriver. The Styrofoam insulation should be put back after the TCs are 

installed (see Figure 8 through Figure 10). 

NREL Image Gallery23089, 23090, and 23091Credit: Bethany Sparn

NREL Image Gallery 23092, 23093, and 23094Credit: Bethany Sparn
20 

Figure 8. The A.O. 

Smith Voltex HPWH 

with the upper heater 

cover shown.

Figure 9. The Styrofoam 

insulation that covers the 

upper heater access point.

Figure 10. After the 

insulation is removed, the 

tank wall is accessible for 

installing a TC. 

• AirGenerate AirTap Integrated HPWH: The AirGenerate HPWH has two electric 

resistance heaters, but only the top one is wired for regular use. The second backup 

heater below the main electric heater can be wired for use in the event that the heat 

pump and main electric element fail. The covers can be removed with a standard 

Phillips head screwdriver. The upper heater access port is encased in plastic and does 

not provide access to the tank (see Figure 11). The lower heater’s access port has a 

thin layer of insulation covering the tank, but that can be pulled back to expose the 

tank wall (see Figure 12). With only a single access point located below the midpoint 

of the tank, measuring average tank temperature will not be possible with this 

particular HPWH without modifying the tank. 

Figure 11. The 

AirGenerate HPWH 

with the heater 

covers shown.

Figure 12. Both heater covers 

removed. The upper heater is the 

main heater and the lower heater 

is the backup (not connected).

NREL Image Gallery 23095, 23096, and 23097Credit: Bethany Sparn

NREL Image Gallery 23098 and 23099Credit: Bethany Sparn
21 

• Stiebel Eltron Accelera 300 HPWH: The Stiebel Eltron HPWH has a single electric 

resistance element that points down into the tank from the top. There are no points to 

access the tank wall, so measuring average tank temperature will not be possible with this 

particular HPWH.

Short-Term Testing

1. Sound level measurement. One common consumer concern with HPWHs is noise. After 

the HPWH is fully installed and operating, take sound measurements at several locations 

at a 1-m radius from the HPWH and average the readings. The sound measurement 

should be taken at a height of 1.5 m (head height) off the ground. See the Northwest 

Energy Efficiency Alliance (NEEA) Northern Climate Specification for more details

(NEEA, 2011). Repeat measurement 3 times while heat pump is running. To measure the 

sound generated by the HPWH alone, ensure that all nearby appliances are off and no one 

enters the space while testing is in progress. A background sound reading should also be 

taken while the HPWH (and everything else) is off for comparison. 

2. Airflow rate measurement. If cooling capacity is desired, the airflow rate must be 

measured. For most HPWHs, the airflow rate is constant during operation. Other than the 

first-generation GE HPWH, all others operate a single flow rate. Use the duct blaster to 

measure the outlet flow rate. Repeat the test 3 times during heat pump operation to verify 

constant flow rate. 

Because duct blasters are intended for use with flat, rectangular ducts, it may be 

necessary to construct a plenum between the HPWH and the duct blaster. See the MiniSplit Heat Pump Field Monitoring Protocol for detailed instructions (Christensen, Fang, 

Tomerlin, & Winkler, 2011). 

Long-Term Monitoring

1. EE and hot water use should be monitored over heating and cooling seasons. 

2. Daily system COP should be calculated as a function of inlet wet bulb temperature. 

Correlation to daily hot water volume should also be noted, as should the relationship to 

the mains water temperature. The factor that has the largest impact on system COP will 

depend on the specific situation, so all should be considered during the analysis. 

3. Cooling capacity can be measured if airflow rate has been characterized. 

4. Calculate the heat removed from the surrounding space by looking at the difference 

between EE added to the water heater and useful energy transferred to the water, as 

shown in Equation 14. The heat removed from the surrounding space will not be 

equivalent to the impact on the heating and cooling systems, but it will give the 

maximum possible impact on peak heating and peak cooling days. Depending on climate 

and season, differentiating between sensible and latent heat removed from the space may 

be relevant. 
22 

References

Christensen, D., Fang, X., Tomerlin, J., & Winkler, J. (2011). Field Monitoring Protocol: MiniSplit Heat Pumps. TP-5500-49881. Golden, CO: National Renewable Energy Laboratory. 

Hendron, R., & Engebrecht, C. (2010). Building America House Simulation Protocols. 

NREL/TP-550-49426. Golden, CO: National Renewable Energy Laboratory. 

Hudon, K., Merrigan, T., Burch, J., & Maguire, J. (2012). Low-Cost Solar Water Heating 

Research and Development Roadmap. TP-5500-54793. Golden, CO: National Renewable 

Energy Laboratory. 

NEEA. (2011). A Specification for Residential Heat Pump Water Heaters Installed in Northern 

Climates: Version 4. Portland, OR: Northwest Energy Efficiency Alliance. 

Shapiro, C., Puttagunta, S., & Owens, D. (2012). Measure Guideline: Heat Pump Water Heaters 

in New and Existing Homes. Golden, CO: National Renewable Energy Laboratory. 

Sparn, B., Hudon, K., & Christensen, D. (2011). Laboratory Performance Evaluation of 

Residential Integrated Heat Pump Water Heaters. TP-5500-52635. Golden, CO: National 

Renewable Energy Laboratory.

Taylor, J. (1997). An Introduction to Error Analysis. Sausalito, CA: University Science Books.
23 

Appendix: Field Test for Protocol Validation

Objective

The protocol described in this report was used for a validation field test at a local home to ensure 

that the methods are reliable and the documentation is thorough. The main objective of this field 

validation was to verify that monitoring recommendations, which were based on NREL’s past 

experiences with testing HPWHs in a laboratory setting, were feasible for a field installation.

Some of the recommended calculations and uncertainty analysis were also performed to provide 

researchers with examples based on actual field test data.

Description of Field Test 

The field test was conducted at a townhouse in Arvada, Colorado, beginning in October 2012. A 

first-generation, 50-gal GE GeoSpring HPWH was installed in the house one year before the 

field test took place. The water heater is located in a closet inside the home’s two-car garage (see 

Figure 13). The water heater shares the closet with the furnace and air handler. 

Figure 13. GE GeoSpring HPWH installed in garage closet

The HPWH is not in the home’s conditioned space, so there was no need to monitor the energy 

consumption of the furnace or air conditioning to determine the impact on the other space 

conditioning equipment. Because the water heater is installed in the garage in a cold climate, the 

homeowner closes the closet doors (which do not have louvers) in the winter so that the air 

drawn by the heat pump is warm from the furnace. This configuration (when the closet doors are 

closed) does not meet the manufacturer’s recommendations for adequate airflow.

The homeowner keeps the HPWH in hybrid mode, which means the heat pump is the primary 

heat source, unless it cannot keep up with demand or if the ambient temperature drops below NREL Image Gallery 23100 Credit: Ed Hancock
24 

45°F or rises above 110°F. The tank set point was 120°F, but the homeowner indicated that he 

may increase the set point to 125°F or 130°F during the winter months to help compensate for 

colder mains temperatures.

Installing Monitoring Equipment

All the sensors used in this field test match the recommendations listed in Table 1, though not all 

the sensors listed there were needed. 

At the start of the field test, the electricity for the HPWH was turned off (see Figure 14). The 

breaker for the HPWH was turned off, the breaker was locked out, and a volt meter was used to 

verify that the circuit was de-energized (see Figure 15). Once the electrical hazard was removed, 

monitoring equipment could be installed. 

Figure 14. Turn off the breaker for 

the HPWH.

Figure 15. Lock out the 

electricity panel and verify 

that the circuit is deenergized

Alternatively, unplug the HPWH if it is installed using cord-and-plug instead of being hardwired. 

Lock out the plug while the HPWH is being worked on. 

Install Flow Meter 

Installing the in-line flow meter requires one of the water lines to be disconnected. In many cases 

where the water lines are soldered together, this step will require a plumber. In this particular 

field test, there were threaded connections between the inlet and outlet water lines and the 

home’s plumbing. If this is the case for your test site, it may be helpful to have a variety of

fittings and pipe lengths readily available to avoid multiple trips to the hardware store. The inlet 

pipe had a valve to turn off the water, so the flow meter was installed on the inlet side. 

To reduce the amount of water spilled, some water was drained from the tank after the inlet valve 

was closed. A hose was attached to the tank drain and a fixture inside the house was opened and 

a few gallons of water were drained from the tank (see Figure 16). Then the flexible hose on the 

water inlet was disconnected from the tank and the flow meter was installed. An additional NREL Image Gallery 23101 and 23102 Credit: Lieko Earle 
25 

section of flexible hose was needed after adding the length of the flow meter to the inlet pipe (see 

Figure 17). 

Figure 16. Connecting the 

hose to the drain in 

preparation for installing the 

flow meter

Figure 17. Final plumbing 

configuration with the flow meter 

and an extra length of flexible 

pipe installed

Surface-Mounted Thermocouples

Four surface-mounted TCs were installed for this field test on the inlet water pipe (Figure 18), 

the outlet water pipe, the upper tank surface (Figure 19), and the lower tank surface (Figure 20). 

The thermally conductive epoxy takes some time to cure, so whenever possible, the TCs should 

be put in place and taped so the installer does not have to hold it in place while the epoxy cures. 

This was done on the inlet and outlet water pipes, but was not possible for the TCs used to 

measure tank temperature because the TC had to be recessed into a small hole and there was not 

a suitable surface to tape it to. 

Figure 18. Surface-mounted TCs

installed on the inlet and outlet water 

pipes 

Figure 19. Surfacemounted TC at the 

upper tank surface

Figure 20. Surfacemounted TC at the lower 

tank surfaceNREL Image Gallery 23103 and 23104

Credit: Ed Hancock, Bethany Sparn

NREL Image Gallery23105, 23106, and 23089Credit: Bethany Sparn, Ed Hancock
26 

Insulation should be installed around any surface-mounted TC to ensure that the TC is measuring 

the water temperature, with minimal influence from the room air temperature. The surfacemounted TCs that were installed on the inlet and outlet water pipes were insulated with pipe 

insulation (shown in Figure 21) and the insulation for the electric element access ports were

replaced after the TCs were glued to the tank (Figure 22).

Figure 21. Pipe insulation 

covering the surface-mounted 

TCs on the inlet and outlet water 

pipes 

Figure 22. Insulation covering 

the tank where one surfacemounted TC was installed

Temperature and Relative Humidity Sensors

Three T&RH sensors were installed to monitor air conditions during this field test. Performance 

of the heat pump is largely driven by the wet bulb temperature of the air when the coils are wet, a 

quantity that depends on the air T&RH. The air entering the heat pump, the air leaving the heat 

pump, and the air inside the garage were monitored using T&RH sensors. In this particular 

location, the homeowner kept the closet doors closed during the colder periods of the year, so the 

garage temperature and the inlet air temperature were often significantly different. 

For the T&RH sensors measuring inlet and outlet air conditions, a wire mount was taped to the 

HPWH body and the sensor was taped to the wire mount, as shown in Figure 23. This brand of 

HPWH pulls air in through the side grilles and from under the annulus. The furnace was located 

just to the right of the HPWH, as shown in Figure 26, so the T&RH sensor was mounted at the 

front intake area to measure the average inlet air temperature. The side of the wire mount that is 

taped to heat pump casing is bent into a wide U shape to provide a stable base for the tape. The 

wire used was 14-gauge galvanized steel wire and the tape used was a 10-mil polyvinyl chloride 

pipe wrap tape, but a number of other tape and wire types would also work. A similar procedure 

was used for the T&RH sensor for the outlet air attached to the back of the heat pump casing. NREL Image Gallery 23088 and 23090 Credit: Bethany Sparn
27 

Figure 23. T&RH sensor is mounted on the front of the HPWH to measure inlet air conditions.

Electrical Energy Measurement

Most field tests will include EE measurements for the whole house and the HPWH, in addition to 

any other EE measurements that are dictated by the research questions. The HPWH was installed 

outside the conditioned space, so it was not necessary to measure the EE consumption of any 

other space conditioning equipment. Also, a PV system was installed during the retrofit project 

that included the HPWH installation, and monitoring equipment was installed to measure the EE

generated by the PV system and the EE consumed by the house. Because those measurements 

were already being taken and the data were available, there was no need to duplicate them. So it 

was only necessary to install monitoring equipment for the EE consumption of the HPWH. 

A disconnect switch for the HPWH was in the same closet as the HPWH, providing a convenient 

location to install current transformers (CTs). The WattNode Watt-hour transducer was mounted 

in a separate box above the disconnect switch. CT wires and voltage sensing wires were run from 

the disconnect box and the WattNode box through a short length of conduit. 

NREL Image Gallery 23107Credit: Bethany Sparn
28 

Figure 24. CTs and voltage taps 

for measuring the EE

consumption of the HPWH

Figure 25. The wires from the CTs and the 

voltage tap are connected to the 

WattNode Watt-hour transducer. 

Once all the monitoring equipment was installed, all the sensor wires were terminated at a 

Campbell data logger, which was enclosed in box along with its power supply. Water was turned 

back on first; a fixture inside the house was also turned on to get the air out of the supply line. 

Anywhere the plumbing was altered was monitored closely for leaks until the researchers could 

be sure that all the connections were sound. The first installation of the flow meter was not 

watertight and so was reconfigured. Once the integrity of the water lines has been verified and all 

the covers exposing electrical connections have been replaced (the WattNode box, the HPWH 

disconnect switch, and the covers for the two electrical resistance heaters in the tank), the 

breaker to the HPWH can be turned back on. The HPWH will likely turn back on immediately. 

A cellular modem was connected to the Campbell Scientific data logger to provide remote access 

to the data. The antenna was mounted near the WattNode box, because that elevated location 

provided the best signal. The remote connection was tested to ensure that the signal was strong 

and data were updated as expected. The values being reported by all the recently installed 

sensors should be verified as well. It should be simple to determine if the T&RH readings are 

reasonable. If the power consumption of the water heater is not known, check the user manual 

before the field test to determine a reasonable range. In general, the heat pump will use 700–

1000 W and the electric resistance elements will increase the power consumption to 2000–4500 

W. 

NREL Image Gallery 23108 and 23109Credit: Lieko Earle
29 

Figure 26. Monitoring equipment is installed and connected to the data logger. 

The cell modem and antenna are also installed.

Sound Test

After all the monitoring equipment was installed, a sound test was performed (see Figure 27). A 

baseline sound test was performed while the HPWH was still disconnected from power. Based 

on the NEEA test specification, sound readings were taken at two places on a radius 1 m from 

the water heater. Sound was measured at only two points, because the area around the water 

heater was mostly taken up by the surrounding walls and furnace. The garage door was also 

closed and all HVAC equipment was off during this baseline measurement to ensure the lowest 

ambient noise. Then the water heater was turned back on. Because we drew some hot water and 

then had the water heater turned off for several hours, the heat pump turned on immediately to 

reheat the tank. The fan came on first for a few minutes, followed by the compressor with the 

fan. Once both the compressor and fan were on, the second set of sound measurements were 

taken at the same two points. 

The average baseline or ambient noise was measured to be 33 dB, which is measured for 

comparison purposes. The sound measured when the heat pump was on was 58.5 dB. The 

Cell modem 

antenna

Cell modem

Data logger 

box

Signal wires

NREL Image Gallery 23110Credit: Bethany Sparn
30 

manufacturer claims that the average operating sound level should be 58 dB, so our measurement 

is well-aligned with expectations. 

Figure 27. Performing a sound test for the HPWH

Sample Calculations

A couple weeks after the monitoring equipment was installed, data were analyzed to ensure the 

readings were in line with expectations. A few of the common calculations are shown here for 

examples. 

Daily System COP

The daily system COP is calculated for each day, with the day defined to begin and end at 4:00 

a.m., which should ensure that all heating to compensate from the previous day’s draws have 

been completed. A single day’s calculation is shown below, specifically to show how the thermal 

energy should be calculated. 

COP = 

ρwater ∙ Vdraws ∙ Cp,water ∙ ∆Tdraws

Winput

Constants: 

ρwater = 998 kg/m3

. 

Cp,water = 4.182 kJ/kg-C. 

NREL Image Gallery 23111Credit: Lieko Earle
31 

To capture the difference in temperature between the inlet and outlet water lines, it is necessary 

to calculate Qdraws (the numerator of the COP equation) at small intervals. In this case, Qdraws was 

calculated every minute, as shown in Table 2. A small chunk of a day of data is shown to 

illustrate how this is done. 

Table 2. Example of Qdraws Analysis for COP Calculation

Data Calculations

Timestamp Tinlet (°C) Toutlet(°C) Vdraws

(m3) ΔTdraws (°C) Qdraws= ρwaterCp,waterVdrawsΔTdraws (kJ)

9:18 20.3 45.7 0.00 25.4 0.0

9:19 20.3 45.5 0.60 × 103 25.2 63.2

9:20 19.6 47.1 4.60 × 103 27.5 527.6

9:21 19.4 47.1 4.50 × 103 27.7 521.0

9:22 19.8 47.1 4.40 × 103 27.2 500.1

9:23 19.5 47.0 4.40 × 103 27.6 506.1

9:24 19.4 47.0 4.40 × 103 27.6 506.7

9:25 19.8 47.0 4.40 × 103 27.2 499.5

9:26 19.9 47.0 4.40 × 103 27.0 496.2

9:27 19.9 46.9 4.40 × 103 27.0 495.6

9:28 19.8 46.8 4.35 × 103 27.0 489.8

9:29 19.8 46.7 4.40 × 103 26.9 494.0

9:30 19.8 46.6 4.45 × 103 26.8 497.4

9:31 19.8 46.5 4.40 × 103 26.6 489.0

9:32 19.8 46.3 3.25 × 103 26.5 359.2

9:33 19.9 46.0 0.00 26.1 0.0

... ... ... ... ... ...

9:44 20.1 41.6 0.00 21.4 0.0

9:45 19.9 44.7 1.80 × 103 24.8 186.5

9:46 19.9 45.6 1.00 × 103 25.7 107.1

9:47 19.9 45.5 0.00 25.6 0.0

9:48 19.9 45.3 0.45 × 103 25.3 47.6

9:49 20.1 45.8 2.30 × 103 25.7 246.8

9:50 20.1 45.4 0.00 25.3 0.0

... ... ... ... ... ...

12:15 17.2 22.5 0.00 5.3 0.0

12:16 18.1 36.1 2.00 × 103 18.0 150.3

12:17 19.2 48.9 0.35 × 103 29.6 43.3

ΣQdraws = 7226.9

At every time step, the product of the volume of hot water drawn during that minute and the 

difference in temperature between the outlet and inlet water lines is multiplied by the density and 

specific heat of water to calculate the heat removed from the tank during each minute. The sum 

of Qdraws over the entire day is used as the numerator in the COP equation. Similarly, Winput is 
32 

found by taking the sum of the EE consumed by the HPWH during the day. When evaluating the 

entire day, the results for the numerator and denominator for the COP calculation are: 

Qdraws = 22823.5 kJ (the total from the full day, not just the segment shown in Table 2) 

Winput = 2.61 kWh 

⇒ COP = 

22823.5 kJ

�2.61 kWh ∙ 3600s

h �

= 2. 43

The COP on this day was 2.43, which is the highest observed thus far during the field test. 

During the first two weeks of monitoring, the average daily system COP is 1.89. 

Coefficient of Performance Versus Draw Volume

A number of factors affect system COP, including ambient air temperature and humidity, daily 

draw volume, and inlet temperature. In this case, for the short period of data analyzed, the draw 

volume stands out as a major factor. Table 3 and Figure 28 show the relationship between daily 

draw volume and daily system COP. One day had no hot water draws at all, leading to a daily 

COP of 0.

Table 3. Daily COP Compared to the 

Daily Hot Water Draw Volume

COP Draw Volume

(liters)

10/5/2012 2.431 208.4

10/6/2012 2.022 137.5

10/7/2012 1.898 155.8

10/8/2012 1.983 111.0

10/9/2012 2.025 138.4

10/10/2012 1.976 115.0

10/11/2012 1.745 72.3

10/12/2012 0.000 0.0

10/13/2012 1.390 71.7

10/14/2012 1.540 60.1 Figure 28. Daily COP versus daily 

hot water draw volume

Heat Added to Space, Qnet,space

Even though the HPWH was installed outside the conditioned space and the cooling produced by 

the heat pump does not affect the space conditioning load, the cooling effect was calculated here 

for examples. On days with the largest draw volumes, the heat removed from the space is greater 

because the heat pump has to run longer to compensate for the draws. 

Qnet,space = WInput – ρwater ∙ Vdraws ∙ Cp,water ∙ ∆Tdraws
33 

This calculation uses the same measurements that were used to calculate COP, so this example 

will use the same day of data used to calculate COP. As previously described, 

Qdraws = ρwater ∙ Vdraws ∙ Cp,water ∙ ∆Tdraws= 22823.5 kJ 

Winput = 2.61 kWh 

⇒ Qnet,space = �2.61 kWh ∙

3600s

h � − 22823.5 kJ = −13434. 3 kJ

As expected, the heat added to the space is a negative number, indicating that heat pump has 

removed the heat from the air. Even though the heat was not removed from the conditioned 

space, it is still useful to put this cooling effect into perspective. The heat removed from the 

space is roughly 13,000 Btu over the course of an entire day. For comparison, window air 

conditioners can have the capacity to remove 13,000 Btu/h. Furnaces, on the other hand, have the 

capacity to produce heat at a rate of 50,000–100,000 Btu/h. So, even though the heat pump is 

removing heat from the air to heat the water, the amount of energy needed to compensate for this 

heat loss will be only a fraction of the total heating load during the winter. 

Standby Heat Loss Coefficient

As seen in Table 3, one day during the current monitoring period had no hot water draws, but the 

heat pump came on to compensate for tank losses. This is an ideal situation for calculating the 

tank’s UA. During this day and into the next, there were three periods of time between heating 

cycles without draws, as shown in Figure 29. 

Figure 29. Hot water draws and EE use over two days in October 2012

Tank UA was calculated for each period and averaged. The UA calculation for one of these 

drawless periods is shown below. 

UA = Vst ∙ ρwater ∙ Cp,water ∙ �T�

i − T�f�

τstdby ∙ (T�

tank − T�amb)
34 

T�

i = (Ttank,upper + Ttank,lower)/2 at beginning of the standby period. 

T�

f = (Ttank,upper + Ttank,lower)/2 at end of the standby period. 

T�

tank is the average of the upper and lower tank TCs over the standby period. 

T�

tank is the average ambient temperature over the standby period. 

τstdby is the length of the standby period in hours. 

Constants: 

ρwater = 998 kg/m3 

Cp,water = 4.182 kJ/kg-C 

Vst = 45 gal = 0.1703 m3 

 

For the standby period beginning at 5:00 p.m. on October 12, 2012 and ending at 5:35 a.m. on 

October 13, 2012: 

τstdby = 12.6 h 

T�

i = 49.4°C 

T�

f = 46.3°C 

T�

tank = 47.9°C 

T�

amb = 22.8°C 

⇒ UA =

(0.1703 m3) ∙ �998 kg

m3� ∙ �4.182 kJ

kgC� ∙ (49.4°C − 46.3°C)

(12.6hr) ∙ (47.9°C − 22.8°C) = 7. 0 kJ

hrC

For this particular standby period, the tank UA was calculated to be 7.0 kJ/hrC. The average UA 

from all three standby periods analyzed was 7.0 kJ/hrC. This is consistent with the tank UA 

measured during NREL’s laboratory testing of this make and model of HPWH, which was 7.1 

kJ/hrC. 

There are a few things to note from this exercise. The tank volume, Vst, is listed as 45 gal even 

though the tank is rated as 50 gal. The actual tank volume was measured during our laboratory 

tests to be 45 gal. This is consistent with the rule of the thumb that the actual tank volume is 

likely 10% less than the rated volume. If a field test involves a new HPWH model and the flow 

meter is installed while the tank is empty, it would be good to measure the actual tank volume 

during the initial filling. 

Also, it is important to mark the beginning of the standby period for the calculation about 10 min 

after the heat pump turns off to ensure that the tank temperature measurements are not artificially 

high. 

Uncertainty Calculations

Three calculations were presented in the previous section, and uncertainty analysis for those 

calculations is shown in Figure 30 through Figure 32. Error propagation was done using a 

program called Engineering Equation Solver (EES), but could also be done manually using the 

general formula for error propagation (Taylor, 1997). 
35 

The values for specific heat and density of water are assumed to be constant in all of the 

calculations. The tank volume and the duration of the standby period used in the UA calculation 

are also assumed to be constant. The uncertainty for each of the measured values comes from 

manufacturer information about the sensor. The stated error for each sensor, shown in Table 1, is 

used as the uncertainty for that measurement. 

Figure 30. Uncertainty propagation analysis using EES software

Figure 31. Uncertainty for measured values
36 

Figure 32. Results from the uncertainty propagation analysis

The error propagation shows that the uncertainty for the calculated values ranges between 3% 

and 5% of the results. The temperature measurements account for most of the uncertainty 

associated with each calculation. Thus, all TCs and TC wires should be rated to special limits of 

error to ensure the lowest error in the temperature measurements. 

Uncertainty analysis can be used to help guide the choice of sensors used in a field test. For 

instance, if a different flow meter were being considered, a preliminary uncertainty analysis 

could be performed to see how different flow meters with different rated accuracies would affect 

the uncertainty in the COP calculation. It is always desirable to use sensors with high accuracy, 

but since higher accuracy equates to a higher cost, the uncertainty analysis can help determine if 

a higher level of accuracy has an appreciable effect on the desired calculation.