NAT L INST. OF STAND & TECH R.l.C

411105 SaTRbO

iMtiblK 83-2723

Performance of Add-On Type

Heat Pump Water Heaters Using

Two Different Test Methods

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Building Equipment Division

Washington, DC 20234

March 1983

Issued June 1983

Prepared for L U.S. Department of Energy

100 Washington, DC 20585

U56

03-2723

1983

(12

NBSIR 83-2723

1TATIGNAX. r.'.'HTA'Q

:<T ,-T •-•/•jiDe UB3AS*

JUN 2 7 1983

OcA 0-CO' 0\CC

.

Q- C-, VOO

3

' 9

^^

PERFORMANCE OF ADD-ON TYPE

HEAT PUMP WATER HEATERS USING

TWO DIFFERENT TEST METHODS

James E. Harris

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Building Equipment Division

Washington, DC 20234

March 1 983

Issued June 1 983

Prepared for U.S. Department of Energy

Washington, DC 20585

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

ABSTRACT

Two different makes of add-on (without tank) heat pump water heaters (HPWH)

were tested. Each of the HPWH's was subjected to a series of recovery tests

similar to the Department of Energy (DOE) recovery test for conventional

electric water heaters. The results of the tests (recovery efficiency,

standby loss, input power, storage tank capacity and energy used) were used

to compute an energy factor which could be used to calculate the estimated

annual operating cost of such HPWH’s. The energy factor was also determined

by a series of simulated use tests consisting of four equal draws totaling

64.3 gallons of hot water per day. The average energy factor derived from

the recovery tes ts was about 13 percent higher than that derived from the

simulated use tests. Based upon the results of this limited test program

it was recommended that a simulated use test be used to determine the energy

factor for HPWH's without tank.

appliances; energy; heat pump water heaters; test procedures;

testing; water heaters

Key words

:
LIST OF TABLES

Page

Table 1 . Test results of two add-on type heat pump water heaters

tested using two different test methods

9 -

Table 2. Detailed test results A-A

Table 3. Units conversion table: Sl/inch-pound/SI A-5

LIST OF FIGURES

Figure 1 . Test setup for testing add-on type heat pump water

heaters

2

Figure 2. Cool down test to determine standby loss 8

Figure 3. Energy factor as a function of tank standby loss 11

Figure A. Recovery efficiency as a function of tank water

temperature

12

Figure 5. Energy factor as a function of recovery efficiency 1A

Figure 6. Recovery efficiency as a function of starting mean tank

water temperature (for a recovery test)

A-3

IV
NOMENCLATURE

Cc

Cus

Cwh

Cy

Er

Erd

EF

EFsu

k

P

R

S

ATI

AT2

AT3

AT4

AT5

AT6

Ti

Daily hot water energy consumption (Btu/day).

Average hourly hot water storage loss (Btu/hour).

Daily water heating energy consumption (Btu/day).

Average daily energy consumption, derived from cold

tank recovery test (kilowatt hours/day).

Recovery efficiency (dimensionless).

Recovery efficiency during the differential temperature rise

from thermostat cut-in to thermostat cut-out (dimensionless).

Energy factor, the ratio of daily hot water energy

consumption to the daily energy consumption with a particular water heater (dimensionless).

Energy factor, derived from a 24 hour test with a hot tank

and simulated daily use of 64.3 gallons of hot water

(dimensionless)

.

Specific heat of water (8.25 Btu/ (gallon, °F))

Power used by heat pump water heater (kilowatts)

Recovery rate (gallons/hour)

Standby loss, expressed as a decimal loss per hour (1/hour)

Difference between the initial mean tank temperature and the

final mean tank temperature during a recovery test, (Tf, -Ti),(°F).

Difference between the average value of the mean tank

temperature and the average value of the ambient air

temperature during the standby loss test (°F).

Difference between the initial and final mean tank temperature

from the start to the end of a standby loss test, (Tf - T^), (°F).

The nominal difference between the initial mean tank temperature

and the final mean tank temperature (90°F).

The difference between the nominal mean tank temperature and

the nominal mean ambient air temperature (60°F)

.

Difference between the mean high and mean low tank water

temperature with thermostat differential;

(

T

max, cut out

Tmin, cut in^ ,(°F)

Mean tank temperature, initial, at the start of a test (°F).

v
Tf

7

Mean tank temperature, final, at the end of a test (°F).

Tr Duration of the recovery test (hours).

Ts Duration of the standby loss test (hours).

U The nominal daily hot water usage (64.3 gallons).

V Tank capacity (gallons).

Z Total electrical energy to the heat pump during a particular

test (kilowatt hours).

vi
TABLE OF CONTENTS

ABSTRACT

LIST OF TABLES

LIST OF FIGURES

NOMENCLATURE: DEFINITIONS OF TERMS USED IN EQUATIONS

1. INTRODUCTION

2. TEST PROCEDURE

3. RESULTS

3.1 COLD TANK RECOVERY TEST

3.2 SIMULATED USE TESTS

3.3 DISCUSSION OF TEST RESULTS

3.4 THE VARIATION OF ENERGY FACTOR WITH CHANGES IN STANDBY LOSS

3.5 AN ANALYSIS TO DETERMINE AN ALTERNATIVE STARTING MEAN TANK

TEMPERATURE

3.6 THE VARIATION OF ENERGY FACTOR WITH CHANGES IN RECOVERY

EFFICIENCY

4. CONCLUSIONS

5 . RECOMMENDATIONS

5.1 STORAGE TANK STANDBY LOSS AND TANK WATER STARTING

TEMPERATURE

5.2 DETERMINATION OF ENERGY FACTOR, EFsu

5.3 DETERMINATION OF RECOVERY RATE

6. REFERENCES

APPENDIX A

APPENDIX B

Page

iii

iv

iv

vi

1

1

o

9

10

10

10

13

13

15

15

16

16

17

A-l

B-l

vii

1 . INTRODUCTION

Currently there are no established Department of Energy (DOE) test procedures

for heat pump water heaters (HPWH) , There are test procedures proposed by

Gas Appliance Manufacturers Association (GAMA) [l]* and American Society of

Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) that use a recovery test from a 55°F** starting water temperature and a standby loss

test similar to the published DOE test procedure for conventional electric

water heaters [2] to calculate an energy factor. The GAMA and ASHRAE test

procedures are proposed and are not presently approved.

In this study two different add-on HPWH's (without tank) were tested using

the proposed ASHRAE/ GAMA type of recovery test . The objective was to

determine their operating characteristics based upon these test procedures.

It was also an objective of this study to determine how the energy factors***

so determined would compare with energy factors determined by a simulated

use test. The simulated use test consisted of four equal draws totaling

64.3 gallons**** of hot water per day. These latter tests were performed in

order to simulate the HPWH installed in a home.

The test data from these different test procedures were collected and analyzed

to determine the differences in daily water heater energy consumption as computed

with the cold tank recovery test with that measured with a simulated day's HPWH

use.

2. TEST PROCEDURE

2 . 1 TEST SETUP AND LABORATORY CONDITIONS

The test setup is shown in figure 1. For all tests, the supply water temperature was conditioned to 55°F as prescribed in the test procedures proposed by

GAMA and ASHRAE. All tests were conducted in a laboratory 11.6 ft wide, 18.0

ft long, and 9.9 ft high with a resulting volume of 1879 cubic feet. There was a doorway 4.0 ft wide and 7.2 ft high with a resulting area of 28.8 square

feet. This doorway remained open for all tests. The ambient temperature ranged

from 70°F to 64.4°F, with 66°F being typical.

The test procedure using the recovery test was to start with a tank of 55°F

water and heat it up until 135°F or the maximum thermostat cutout temperature

was attained. The test procedure followed paragraphs 2.2 through 2.9 in that

order. The test procedures, test conditions and instrumentation requirements

were as specified in reference [2] . The standby loss test was conducted as specified by reference {2] and verified by the cool-down technique as shown in

figure 2.

* Note: Numbers in brackets refer to references at the end of this report.

** Note: Inch-pound units are used in this report to be consistent with DOE

and other published test procedures for other water heater types. A

conversion table is included in the Appendix (Table 3). *** Energy factor is the ratio of the daily hot water energy consumption to the

average daily energy consumption of a particular water heater and it differs

from the recovery efficiency because of the inclusion of the effects of the

system standby losses.

The value of 64.3 gallons per day is currently specified in the DOE test procedure for water heaters as representative of the typical daily household hot

water usage.

1
HOUR

METER

HEAT PUMP WATER HEATER HOT WATER DRAW SCHEDULE TEST SETUP

FIGURE 1.

2
All equations are defined in paragraph 2.2 to 2.12 and all symbols used are defined on pages v and vi.

2.2 INPUT POWER DETERMINATION

The input power, P, was determined using equation (1) where the total energy

was measured for the heat up of a tank of cold (55 °F) water to the maximum

temperature (thermostat cut-out temperature). The duration of the test was

measured in hours. The average power, in kilowatts, is the energy used, in

kilowatt hours, divided by the time in hours.

input power determination, expressed in kilowatts and defined as:

where: Z = total electrical energy to the heat pump during test

(kilowatt hours)

Tr = duration of the recovery test (hours).

2.3 RECOVERY EFFICIENCY DETERMINATION

( 1 )

The recovery efficiency, Er, was determined using equation (2) where the

resultant was the average recovery efficiency during the test which was

run in conjunction with the input power determination test in paragraph 2.2.

It was necessary to measure the mean tank temperature at the commencement and

termination of this test in order to determine the mean water temperature rise,

ATI.

Recovery efficiency, dimensionless and defined as:

(k) (V) (ATI)

Er = (Z) (3412 Btu/kWh) ' j

where: k = specific heat of water (8.25 Btu/ (gallon - °F))

V = tank capacity (gallons)

ATI = difference between the initial mean tank temperature and the

final mean tank temperature during recovery test,

(T

f - T.), (°F)

Z = as defined in 2.2

2.3.1 RECOVERY EFFICIENCY DURING THE DIFFERENTIAL TEMPERATURE RISE

The recovery efficiency during the differential temperature (the temperature

difference from the thermostat cut-in to cut-out) rise, Erd, was determined

using equation (3). The value for Erd will differ from that for Er since

recovery efficiency as a function of tank water temperature is not linear. The

value for Erd is needed in equation (4) in order to compute the standby energy

loss of the water tank and HPWH system. To determine the value for Erd, one must

know the difference in mean tank water temperature at the thermostat cut-in from

the thermostat cut-out and the energy consumed by the HPWH to effect this water

temperature rise.

3
Recovery efficiency during the differential temperature rise,

dimensionless and defined as:

Erd =

where:

_ (k) (V) (AT6)

(Z) (3412 Btu/kWh)

( 3 )

2.4

AT6 = difference between the mean high and mean low tank

water temperature with thermostat differential,

|

T

max, cut out _Tmin, cut in^ > (°F), k, V and Z are defined above

STANDBY LOSS DETERMINATION

The standby loss, S, was determined by test using the test procedure described

in reference [2] and equation (4). The second term in the numerator accounts for

the energy related to any difference between the mean tank temperature at the start

of a test and that at the end of a test. The standby loss is expressed as a decimal loss per hour and is subsequently used in equation (6) to determine the

average hourly hot water storage energy consumption.

Standby loss is the ratio of the amount of energy lost per hour

to the available energy above the datum of the average ambient air

temperature and defined as:

S =

(Z) (3412 Btu/kWh) (k) ^). (AT 3)

(k) (V) (AT2) (Ts)

(4)

2.5

where: AT3 = difference between the initial and final mean tank

temperature from the start to the end of a standby loss

test, (Tf - T± ), (°F)

AT2 = difference between the average value of the mean tank

temperature and the average value of the ambient air

temperature during the standby loss test, (°F)

Z, k and V are defined above

DAILY WATER HEATING ENERGY CONSUMPTION

The daily water heating energy consumption, Cwh, was determined by using equation

(5) which requires the recovery efficiency, Er, as determined in section 2.3. The

daily water heating energy consumption, Cwh, is then used in equation (7) to

determine the average daily water heater energy consumption.

Daily water heating energy consumption, expressed in Btu per day and

defined as: (k)(U)(AT4)

Cwh = Er ( 5 )

Where: AT4 = the nominal difference between the initial mean tank

temperature and the final mean tank temperature, (90°F;

fixed by reference [ 2 ] )

U = the nominal daily hot water usage, (64.3 gallons)

k = is defined in 2.3

4
2.6 AVERAGE HOURLY HOT WATER STORAGE ENERGY CONSUMPTION CALCULATION

The average hourly hot water storage energy consumption, Cus, was calculated

using equation (6) with the standby loss, S, determined from the preceding

section 2.4. The value calculated is subsequently used in equation (7) to

calculate the average daily water heater energy consumption.

Average hourly hot water storage energy consumption, expressed in Btu

per hour and defined as:

Cus

(S)(k) (V) (AT5)

Erd

( 6 )

where: AT5 = the difference between the nominal mean tank temperature

and the nominal mean ambient air temperature (60 °F; fixed

by reference [2])

Z, k and V are defined above

2. 7 AVERAGE DAILY ENERGY CONSUMPTION

The average daily energy consumption Cy was calculated from equation (7).

The values of Cwh (from section 2.5), Cus (from section 2.7) and P (from

section 2.2) are used to make the necessary calculations. Equation (7) is

the same as the equation in section 4.5.4 of reference 2 except that the

terms for heat trap credits (Jh and Jc) are missing since there were no heat traps used in these tests.

Average daily energy consumption (without heat traps), expressed

in kilowatt hours per day and defined as:

CT =( 1

|

Cwh + (Cus)/ 24 hrs - Cwh \

13412 Btu/kWh ]

' day (P) (3412 Btu/kWh)

(7)

where: Cwh is defined in 2.5

Cus is defined in 2.6

P is defined in 2.2

2. 8 DAILY HOT WATER ENERGY CONSUMPTION CALCULATION

The daily hot water energy consumption, Cc, was calculated from equation (9)

using the values prescribed in reference [2].

Daily hot water energy consumption, expressed in Btu per day and

defined as:

Cc = (k) (U) (AT4) (8)

where: AT4 and U are defined in 2.5

k is defined in 2.3

5
2.9 ENERGY FACTOR CALCULATION

The energy factor, EF, was calculated from equation ( 9 ) using the values of

Cc (from section 2.8) and Cy (from section 2.8).

Energy factor derived from cold tank recovery test, dimensionless and

defined as:

EF =

Cc

(Cy ) (3412 -Btu/kWh)

( 9 )

where: Cc and Cy are defined above

2.10 RECOVERY RATE DETERMINATION

The recovery rate, R, in gallons per hour was determined by equation (10)

.

The average power, P, (from section 2.2) and Erd (from section 2.3.1 are used to calculate the recovery rate in gallons per hour. This is a performance measure of a water heater's ability to supply hot water. It is not

used in the energy determination of a HPWH.

Recovery rate, expressed in gallons per hour and defined as:

(P)(Erd) (3412 Btu/kWh) (1Q)

R =

(k) (AT4)

where: P is defined in 2.2

Erd is defined in 2.3.1

k is defined in 2.3

2.11 COLD TANK START-UP RECOVERY TEST

The procedures from sections 2.2 and 2.9 were used to determine the energy

factor of a HPWH based upon a cold tank start-up (recovery test). Since the

recovery efficiency of a HPWH varies as a function of the water temperature

and since a water heater is normally full (or nearly full) of hot water, any

test based upon a cold tank start-up may not give the same results as an actual installed water heater. An alternative test method was devised to

simulate installed conditions and this method is described in section 2.12.

2.12 ENERGY FACTOR DERIVED FROM A SIMULATED USE TEST

This test is a 24-hour test, starting with a full tank of hot water. The mean

tank temperature is determined at the start of the test in addition to the time

of the commencement of the test. Water is drawn out in a schedule pattern of

equal draws to total 64.3 gallons per day. The draw part of the test can be made

during the normal work day so that during the off-hours time of the test the HPWH

is in a standby mode. Various draw schedule tests were run with 2, 3, 4 and more

draws since by previous tests, ref 1 5 J , it was determined that there were no significant differences between such draw schedule test results and variable draws

as may be actually encountered in "typical use patterns." The draw schedule used

for most of the tests consisted of four equal draws, totaling 64.3 gallons of hot

water per day. The time for the draws and subsequent recovery was about six hours

6
These simulated usage tests are all of 24 hours duration. At the end of 24

hours, the mean temperature must be determined and the time of the test

termination recorded.

The energy factor based upon simulated use, EFsu, is calculated using equation

(11) where Cc is determined in section 2.10 and Erd is determined from section

2 . 3 .

Energy factor, derived from a 24-hour test with a hot tank and simulated

day's use of 64.3 gallons of hot water, dimensionless and defined as:

EFsu = Cc

(Z) (3412 Btu/kWh)

" (AT3)

where: Cc is defined in 2.8

Z is defined in 2.2

k and V are defined in 2.3

AT3 is defined in 2.4

Erd is defined in 2.3.1

(ID

7
MERN

TRNK

NRTER

TEMP.

Standby Loss By Cool-down of Tank:

Given: To=Temp @ Time = 0 Hours To=62.07 °C

T4=Temp @ Time = 4 Hours T4=59.98 °C

Ta=Temp # Ambient (Avg.) Ta=20. 74 °C

STANDBY LOSS BY COOL-DOWN OF TANK

FIGURE 2.

8

MEAN

TANK

WATER

TEMPERATURE,
3. RESULTS

3.1 COLD TANK RECOVERY TEST

The results of cold tank recovery tests are shown in the upper portion of

Table 1. The program to make the computations and some sample computations are included in Appendix B.

The energy factor, EF, computed from equation (10) for unit A is 1.719, while

the value for unit B is 1.703. The mean of these two values is 1.711. This

test method is different from the normal usage pattern of water heaters in

actual use and since there is no draw of hot water, it may result in values

that are not representative of actual service conditions.

Table 1

Summary of test results

Cold Tank Recovery Tests

UNIT EF P (KW) Cwh (Btu) ATI (°F) Cy (kWh) Ta (°F)

Standard

EF Deviation

"A”* 2.210 1.296 21,602 85.6 8.159 66.9 1.719 0.0100

2.179 1.225 21,920 80.7 8.218 67.0 1.703 0.0255

MEAN 1.711

SIMULATED USE TESTS, TWENTY-FOUR HOURS WITH DRAWS TOTALING 64.3 GALLONS

UNIT Z (kWh) Tf (°F) Ti (°F) Cy (kWh) Ta (°F) EFsu Standard

Deviation

"A"it 11.256 132.964 136.477 10.557 69.3 1.421 0.0860

"tf'itlt 8,483 132.188 134.276 8.668 69.6 1.614 0.1441

MEAN 1.518

* These data are the mean of 6 tests, see table 2 for detailed data for each test.

** These data are the mean of 9 tests, see table 2 for detailed data for each test.

it These data are the mean of 3 tests, see table 2 for detailed data for each test.

itlt These data are the mean of 6 tests, see table 2 for detailed data for each test.

3.2 SIMULATED USE TESTS

The results of simulated use tests are shown in the lower portion of table 1.

The energy factor, EFSUj derived from simulated day’s use of 64.3 gallons of

hot water by a draw test and standby portion to simulate the HPWH installed

in a home for unit A was 1.421 while the value for unit B was 1.614. The mean

of these two values is 1.518.

9
3.3 DISCUSSION OF TEST RESULTS

These two units, A and B, showed very similar test results when the cold

water recovery test was used as the basis for energy factor determination.

When these two units were tested with a simulated use test method, the

energy factor of unit A was 88.7 percent of the value of unit B.

The mean value of the energy factors of units A and B is 1.711 when a cold

tank recovery test is used; the mean value of the energy factors of these same units is 1.518 when the simulated use test method is used. The former

energy factor (1.711) is 112.7 percent of the latter result (1.518).

It is difficult to accurately simulate actual home use conditions in the

laboratory; however the simulated use test is more nearly

representative of actual conditions than the cold tank recovery test. Earlier

testing (ref. 5 ) has shown that the variations in draw schedules do not have

a significant effect upon the energy factor when the same daily gallons of hot

water are used with an integral type HPWH. It is suspected that this is a general phenomenon with water heaters since reference [6] also found only small

variations in the energy factor with various draw schedules with a solar water

heater

.

3.4 THE VARIATION OF ENERGY FACTOR WITH CHANGES IN STANDBY LOSS

Figure 3 shows that the energy factor (EF) derived from a cold tank recovery

test is significantly affected by the standby loss(es) of the system. This curve is developed by holding the values of Er, Cwh, P, k, U, AT4, and V constant

and allowing S to vary from 0.0 to 0.05 per hour. This results in the variation

of energy factor from 2.196 to 1.054 as shown. Equations (5), (6), (7), and (10) are used to develop this curve. The proposed GAMA test procedure, ref [1],

specified that S = 0.7 percent per hour. Figure 3 shows this point relative to

S from 0 to 5 percent loss per hour. A consequence is that with an identical

unit under test, the energy factors would vary from 1.98 to 1.05 when the standby loss varied from 0.5 percent per hour to 5.0 percent per hour.

The specified value of S = 0.7 percent per hour is probably lower than that of

water heater tanks that might be used with these add-on type HPWH's. By comparison,

a "high efficiency" electric water heater with foam insulation has a loss of about

1.0 percent per hour; the tank used for these tests had a standby loss of 1.3

percent per hour.

3.5 ANALYSIS TO DETERMINE AN ALTERNATIVE STARTING MEAN TANK TEMPERATURE

An analysis to compensate for the differences in test results by using a different starting water temperature for the recovery test method is included

in Appendix A. This analysis showed that if the mean energy factor determined

by a 24-hour simulated use test is used to compute the average daily water

heater energy consumption (Cy) then this value can be used to compute the daily

water heating energy consumption (Cwh), using an assumed standby loss of 1.3

percent per hour and an assumed power of 1.21 kilowatts. This is used to

10
ENERGY

FRCTOR,

STANDBY LOSS, S

ENERGY FACTOR AS A FUNCTION OF STANDBY LOSS

(WITH POWER ASSUMED 0 1.21 kW; Cus=25 ,096(S) ; Cwh=31 ,739)

FIGURE 3,

11
(COEFFICIENT

OF

PERFORMANCE,

COP)

(FOR A HPWH, WITHOUT TANK)

RECOVERY EFFICIENCY AS A FUNCTION OF MEAN TANK WATER TEMPERATURE

FIGURE 4.

12
compute the average hourly hot water storage energy consumption (Cus) . When

Cus is known, then the value of the recovery efficiency (Er) can be calculated.

After Er is determined, then the equivalent mean tank starting temperature can

be calculated when the characteristics of the HPWH system are known by an empirical determination. In the example in Appendix A, it was determined that

the starting tank water temperature should be 114. 3°F to give the same energy

factor using a recovery test vs. the simulated use test. A recovery test was run with a 114°F starting tank water temperature and the resultant energy factor was 1.510 which compares well with mean energy factor of 1.518 obtained by the simulated

use tests.

3.6 THE VARIATION OF ENERGY FACTOR WITH CHANGES IN RECOVERY EFFICIENCY

Figure 5 shows that the energy factor (EF) derived from a cold tank recovery

test is significantly affected by the recovery efficiency (Er) . This curve

is developed by holding the values of Cwh, P, k, S, U, AT4 and V constant and

allowing the values of Er to vary from 1 to 3. Figures 4 and 6 show the variation

of recovery efficiency (Er) with variable mean tank water temperature.

13
2

L.1

Ld

CZ

o

o

cr

L.

>-

(J)

O'

u

u . 5

WHEN THE FOLLOWING VALUES ARE

ASSUMED AND ARE HELD CONSTANT:

P=1 .21 kW

S-1. 3%/Hour

V=50.7 Gallons

CPI 1 i i i i i 0 .5 1 1.5 2 2.5 3

RECOVERY EFFICIENCY, Er

ENERGY FACTOR AS A FUNCTION OF RECOVERY EFFICIENCY

FIGURE 5.

14
4. CONCLUSIONS

It is concluded that the recovery test from a cold tank start-up results

in a higher energy factor value than from a simulated use test with a standby time and draws of hot water totaling 64.3 gallons per day.

When an analysis was made to find starting mean tank water temperature

to compensate for this difference and use a recovery test to determine

the energy factor, it was found that the starting temperature should be

about 114°F. This higher starting mean tank temperature was used for a recovery test and the resultant energy factor compared well with the

simulated use test results.

There is a problem using the current DoE water heater test procedure to

test a HPWH in that it gives a higher energy factor than that of a simulated use test procedure. This is because the performance of a HPWH

is affected by the mean tank water temperature (see figure 4).

The specification of a hot water storage tank’s standby loss has a significant effect on the resulting energy factor. Since there is a wide range

of standby losses of water heater storage tanks, this matter requires

more investigation to see if these effects can be reduced.

There is a need to have a uniform test method that is agreed to by manufacturers, associations and consumers that will result in accurate information on the cost of operation of HPWH’s. There are many variables that

affect the overall system efficiency (energy factor). Many of the variables are a compromise so that the operating 'costs of heating water for an "average household" is also a compromise. Consumers need information that

is current, creditable and reasonable to enable them to make water heater

purchasing decisions that are based upon tests that are representative of

actual installed operating conditions.

5 . RECOMMENDATIONS

5.1 STORAGE TANK STANDBY LOSS AND TANK WATER STARTING TEMPERATURE

It has been determined that it is possible to use a recovery test with a starting tank water temperature different from 55° to yield an equivalent

energy factor to that of a draw test of 64.3 gallons with a 55°F supply

water temperature. Tests in this report indicate that the required intermediate temperature should be about 114. °F; however, since these tests

were rather limited, more information is needed to determine:

1. For HPWH's in general, what is the appropriate starting water

temperature for use in a recovery test in order to yield the same energy factor as that obtained in a 24-hour simulated use

test with a 64.3 gallon draw schedule and a supply water temperature

of 55°F.

15
2. Since it is probable that most add-on type HPWH's will be used

with conventional electric water heater storage tanks and because

the standby loss of the tank is an important variable in the

energy factor determination, any specified value of standby loss

must be carefully determined. Reference [7] states "A value of

2%/h for cooldown loss is typical of EWH [Electric Water Heaters]

with fiberglass insulation." (page 18, paragraph 4.5). Tests at

NBS indicate that high efficiency foam type of EWHs have standby

losses of 1% per hour. Until such time as a more accurate determination of average standby loss is made it is recommended that

the value of 1.5% per hour should be used.

5.2 DETERMINATION OF ENERGY FACTOR, EFSu

It is recommended that the test to determine the energy factor (EFSu)

should be by a simulated use test of 24 hours duration with four equal

draws (totaling 64.3 gallons of hot water per day) and that such a determination be made using equation (11) of this report. It should be noted that

normally these four draws can be made within a period of six hours or less,

which means that this recommended test can be run during normal working

hours. It is also recommended that the first draw be started after the

thermostat cut-out; that the second draw be commenced after the thermostat

cut-out after the first draw recovery is complete; that the third draw

be started after the thermostat cut-out after the second draw recovery is

completed; that the fourth and final draw be started after the thermostat

cut-out after the third draw recovery is completed and that the final draw

be trimmed so that the total of all draws is 64.3 gallons. The mean tank

temperature at the beginning and end of the 24-hour test should be determined and the energy (in kWh ) be measured during this test. The

supply water should be at 55°F.

5.3 DETERMINATION OF RECOVERY RATE

It is recommended that the recovery rate (R) in gallons per hour be determined

by use of equ (8) except that the value of Erd should be used in place of

Er so that:

=

(P) (Erd) (3412 Btu/kWh)

(8.25 Btu/gal °F) (90°F)

or simplified:

R = (4.595) (P) (Erd) (gal/hour)

16
6. REFERENCES

[1] GAMA Proposed Draft Efficiency Test Procedure for Heat Pump Water

Heaters (dated 9/81) $ 1901 N. Ft. Myer Drive, Arlington, VA 22209

[2] Federal Register , Vol. 42, No. 192, October 4, 1977, pages 54100-54119,

as amended by Federal Register , Vol 43, No. 203, October 19, 1978,

pages 48986 - 48987 and Federal Register , Vol. 44, No. 175, September 7,

1979, pages 52632 - 52640.

[3] Wan, Andrew C. , "Energy Test Method Development for Electric Heat

Pump Water Heaters," NBSIR 79-1951, National Bureau of Standards,

Washington, D.C., January 1980.

[4] Palla, Robert L. Jr. , "Evaluation of Energy-Conserving Modifications

for Water Heaters," NBSIR 79-1783, National Bureau of Standards,

Washington, D.C., July 1979.

[5] Harris, James E. , Letter Report for Department of Energy, "Energy Use

of a Heat Pump Water Heater by Various Draw Schedules," National Bureau

of Standards, Washington, D.C., October 1982.

[6] Fisher, Ronald A., Jr. and Fanney, A. Hunter, "Thermal Performance

Comparison for a Solar Hot Water System Subjected to Various Hot Water

Load Profiles," to be published in the ASHRAE Journal.

[7] Wan, C. Andrew, Palla, Robert L. , Jr., and Harris, James E. , "Development

of an Energy Test Method for a Dedicated Water-Heating Heat Pump,"

NBSIR 81-2372, National Bureau of Standards, Washington, D.C.

,

November 1981.

17

APPENDIX A

This is an analysis to determine the starting mean tank water temperature

that would result in the same energy factor by use of the cold tank re- covery test as would be obtained by the 24-hour simulated test (with

standby time and a draw schedule with a 55°F supply water temperature to

simulate usage.

From the simulated use test, the mean value of EFsu was found to be 1.518.

Rearrange equation (10) and solve for Cy:

Cy

(EFsu) (3412 Btu/kWh)

and substituting from equation 9 for Cc and inserting known values:

Cv = 8.25 Btu/gal °F)(64.3 gal/day) (90°F)

y (1.518) (3412 Btu/kWh)

Cy = 9.218 kWh/day.

Use eq. (6) and assume: standby loss in 1.3 percent per hour, temperature

difference (AT5) is 60°F, the tank capacity (V) is 50 gallons, and the

recovery efficiency during the differential temperature rise (Erd) is 1.57,

to determine the value of Cus:

„ (S) (k) (V) (AT5) T _ , J , Cus = —- - - v. —. Insert known and assumed values:

Erd

_ (0.013/hour) (8.24 Btu/gal °F) (50 gal) (60°F) LUS

1.57

(Note: The product of the terms of the numerator of this

equation represents the tank cooldown loss and must be

divided by Erd: 1.57 to represent the electrical energy

equivalent needed to replace this thermal loss).

Cus = 204.9 Btu/hour

Insert this value and the assumed power of 1.21 kilowatts into equation (7)

above:

A-l
Note: This is per 4.5.4 of ref [2] and Jh and Jc are zero since no

heat traps were used.

Collect terms and simplify to:

Cy " (3412 Btu/kWh) (Cvh + A91B "

Substitute computed value (9.218 kWh/day) for Cy and, collect terms and

solve for Cvh:

(9.218 kWh/day) (3412 Btu/kWh) - 4918 Btu = Cwh - 0.04964 Cwh

0.9504 Cwh = 26,534 Btu/day

Cwh = Btu/day

0.9504

Cwh = 27,920 Btu/day

Now use this value with equation (5) and rearrange to solve for Er:

r

, (k)(U)(AT4) Oh —

Er = ^

and substitute known or computed values:

_ (8.25 Btu/gal “FH64.3 sal/day (90°F) Er

27,920 Btu/day

Er = 1.710

Now use the characteristics of the HPWH tested and shown by the solid line

in figure 6. This HPWH has been determined by test to exhibit the characteristics to enable a graphical solution for a starting tank water temperature

of

:

T = 114. 3°F

This starting water temperature would apply only to the particular HPWH

tested under the specified conditions. It would not necessarily apply to

all HPWH's. Each HPWH type would require the duplicate testing of a recovery

test and a draw schedule test as cited in this analysis to ensure the suitability of a different starting water temperature to simulate a daily usage

of 64.3 gallons of hot water heated from a 55°F supply. However, after this

draw test was done, all future testing of identical models (e.g. for quality

control test) could be done using a recovery test without any draw and use

the specific starting water temperature found for that model.
AVERAGE

Er

FROM

START

TO

END

OFA

RECOVERY

TEST

STARTING MEAN TANK WATER TEMPERATURE

(FOR A HPWH , WITHOUT TANK)

RECOVERY EFFICIENCY AS A FUNCTION OF STARTING MEAN TANK WATER TEMPERATURE

(FOR A RECOVERY TEST)

FIGURE 6,

A3
TABLE 2

Detailed Test Results

COLD TANK RECOVERY TESTS

i Unit Z (kWh) Tr (hrs) Tf ( °F) Ti(°F) 1 Er Cvh (Btu) P(kv) ATI (°F) Cy(kWh/day) EF

A 4.734 3. 688 140.27 54.00 2.234 21,370 1.284 i 86.27 8.092 1.729

A 4. 731 3.673 140.63 55.00 2.219 21,518 1.288 85.63 8. 133 1.720

A 4. 703 3.553 140.09 55.94 2.193 21,766 1. 331 84.15 8.216 1. 703

A ! 4.779 3. 696 140. 31 54.19 2.209 21,614 1.293 86.12 8.161 1. 715

A 4. 789 3. 720 140.49 55.00 2.189 21,810 1.287 85.49 8.212 1. 704

A 4. 752 3.669 140.68 54.73 2.217 21,532 1.295 85.95 8.140 1. 719

B 4.771 3.660 134.02 52.74 2.089 22,858 1.304 81.28 8.503 1.646

B 4.542 3.713 135.99 55.34 2.177 21,933 1.223 80.65 8.221 1.702

B 4.475 3. 653 136.25 56. 72 2.179 21,914 1.225 79.53 8.216 1.703

B 4.581 3.800 135.86 55.55 2.149 22,215 1.206 80.31 8.289 1. 688

B 4.585 3. 758 135.47 51.80 2.237 21,342 1.220 83.67 8.060 1.736

B 4.420 3. 667 134.66 55.04 2.208 21,620 1.205 79.62 8.129 1.721

B 4.420 3.619 135. 71 56.03 2.210 21,604 1.221 79.68 8.131 1.721

B 4.550 3. 767 135.83 54. 71 2.182 21,883 1.210 81.12 8.202 1.706

B 4.545 3. 751 135.92 55.13 2.179 21,910

i 1.212 80.79 8.210 1.704

SIMULATED USE TESTS,

TWENTY-FOUR HOURS WITH DRAWS TOTALING 64.3 GALLONS

Unit Z (kWh) Tf (°F) Ti(°F) EFsu

A 9.024 139.68 140.68 1.535

A 9.168 131.85 139.68 1.415

A 11.106 131.75 130.49 1.273

B 7.851 127.40 135.09 1.635

B 8.811 134.87 135.95 1.570

B 10.166 134.99 129.98 1.442

B 7.623 133.82 135.92 1.790

B 7.699 131.93 133.82 1.777

B 10.267 137.24 127.40 1.494

A-4
Table 3 Units Conversion Table: SI/INCH-POUND/SI

To Convert Multiply By To Obtain

gpm 0.06309 L/s

(Gal Ions /minute) (Liters/second)

L/s 15.85 gpm

(Liters /second) (Gallons /minute)

g 3.785 L

(Gallons) (Liters)

L 0.2642 g

(Liters) (Gallons)

in 2.54 cm

(Inches) (Centimeters)

cm 0.3937 in

(Centimeters) (Inches)

ft 0.3048 m

(Feet) (Meters)

m 3.281 ft

(Meters) (Feet)

BTU 1.055 kJ

(British Thermal Units) (kilojoules)

kJ 0.9479 BTU

(kilojoules) (British Thermal Units)

Temperature (T), Conversion Equations:

For Temperature Use:

°F to *C: l(T)°F - 32°F] (5/9) (T)°C

°C to °F: [ (T)°C] (9/5) + 32°F - (T)°F

For Temperature Differentials or Tolerances Pee:

*F to *C: (T)°F(5/9) - (T)°C

•C to °F: (T)°C(9/5) - (T)°F

A- 5

Appendix B

BASIC PROGRAM TO COMPUTE ENERGY EQUATIONS

(; denotes print command)

LIST

“10 REM THIS IS A BASIC PROGRAM TO COMPUTE ENERGY EQUATIONS

-20 REM AND TO COMPUTE ENERGY FACTOR. NEEDED INPUTS ARE:

-30 REM ENERGY (WATT-HOURS), MEAN TANK (FINAL) AND (INITIAL)

-40 REM TEMPERATURES, WITH A 50.7 GALLON STORAGE TANK WITH

-45 REM STANDBY LOSS OF 1.37. PER HOUR. ALL TEMPS., DEG. (C).

-50 5 "ENERGY USED DURING RECOVERY TEST (WATT-HOURS) ?"

-60 INPUT El

-70 ' "DURATION OF RECOVERY TEST IN HOURS? " -30 INPUT T9

-90 P=(E1/T9)/J000

-100 ? "FINAL MEAN TANK WATER TEMPERATURE, DEGREES C?"

-110 INPUT K

-120 ? "INITIAL MEAN TANK WATER TEMPERATURE, DEGREES C ?"

-130 INPUT L

-140 T 1 =K—

L

-150 E2=(220. 66)*(T1 )/El

-160 REM E2=ER=REC0VERY EFFICIENCY

-170 REM C 1 =CWH=DA I LY ENERGY TO HEAT WATER

-180 Cl=47743/E2

-190 ; "RECOVERY EFFICIENCY, ER="!E2

-200 ? "DAILY WATER HEATING ENERGY, BTU/DAY , CWH="’C1

-210 REM S=STANDBY LOSS AS A DECIMAL LOSS PER HOUR

-220 S=. 13E-1

230 REM C4=CUS=HC«URLY STANDBY LOSS, BTU/HOUR.

-240 REM C5=CUS ( 24—CWH/3412P

)

-250 C4=(S)*( 8. 25*50. 7*60)

-260 C5= ( C4 ) * ( 24—C 1 / ( 34 1 2*P )

)

-270 REM C6=CY= DAILY WATER HEATING ENERGY, KWH/DAY

-280 C6= ( Cl +C5 ) / ( 3412

)

-290 REM E4=EF=ENERGY FACTOR

-300 E4= (13. 993 ) / ( C-6 ) -310 *, "ENERGY USED IN RECOVERY TEST (WATT-HOURS) :"?E1

-320 ? "AVERAGE POWER DURING RECOVERY TEST (KW):"5P

-330 ? "DURATION OF RECOVERY TEST, HOURS : " ; T9

-340 ? "WATER TEMPERATURE RISE DURING THE RECOVERY TEST, C DEG.:"?T1

-350 ? "FINAL MEAN TANK TEMPERATURE , DEG. C, DURING RECOVERY TEST:"?K

-360 5 "INITIAL MEAN TANK TEMP., DEG.C, DURING RECOVERY TEST:" ?L

-370 ; "HOURLY STANDBY LOSS, BTU/HOUR = CUS : " ? C4

-380 ? "DAILY STANDBY LOSS, BTU/DAY : " ? C5

-390 5 "DAILY WATER HEATER ENERGY, KWH = CY : " 5 C6

400 5 "ENERGY FACTOR = EF : " ; E4

-410 END

—BASICI

I

I B-l
Appendix B

SAMPLE CALCULATIONS, UNIT "A"

RUN

-ENERGY USED DURING RECOVERY TEST (WATT-HOURS) ?

->4779

-DURATION OF RECOVERY TEST IN HOURS?

—>3. 696

-FINAL MEAN TANK WATER TEMPERATURE, DEGREES C?

—>60. 17

-INITIAL MEAN TANK WATER TEMPERATURE, DEGREES C ?

— 12 . 93

-RECOVERY EFFICIENCY, ER= 2.2089086

-DAILY WATER HEATING ENERGY, BTU/DAY , CWH= 21613.841

-ENERGY USED IN RECOVERY TEST (WATT-HOURS) : 4779

-AVERAGE POWER DURING RECOVERY TEST ( KW ) : 1.2930195

-DURATION OF RECOVERY TEST, HOURS : 3.696

-WATER TEMPERATURE RISE DURING THE RECOVERY TEST, C DEG.: 47.84

-FINAL MEAN TANK TEMPERATURE , DEG. C, DURING RECOVERY TEST: 60.

-INITIAL MEAN TANK TEMP., DEG.C, DURING RECOVERY TEST: 12.33

-HOURLY STANDBY LOSS, BTU/HOUR = CUS : 326.2545

-DAILY STANDBY LOSS, BTU/DAY : 6231.7489

-DAILY WATER HEATER ENERGY, KWH = CY : 8. 1610755

-ENERGY FACTOR = EF : 1.7146024

-BAS I Cl I —

>

RUN

-ENERGY USED DURING RECOVERY TEST (WATT-HOURS) ?

->4787

-DURATION OF RECOVERY TEST IN HOURS?

—>3. 720

-FINAL MEAN TANK WATER TEMPERATURE, DEGREES C?

—>60. 27

-INITIAL MEAN TANK WATER TEMPERATURE, DEGREES C ?

—>12. 78

-RECOVERY EFFICIENCY, ER= 2.1890836

-DAILY WATER HEATING ENERGY, BTU/DAY, CWH= 21809.582

-ENERGY USED IN RECOVERY TEST (WATT-HOURS) : 4787

-AVERAGE POWER DURING RECOVERY TEST (KW): 1.286828

-DURATION OF RECOVERY TEST, HOURS : 3.72

-WATER TEMPERATURE RISE DURING THE RECOVERY TEST, C DEG.: 47.49

-FINAL MEAN TANK TEMPERATURE , DEG. C, DURING RECOVERY TEST: 60.

-INITIAL MEAN TANK TEMP., DEG.C, DURING RECOVERY TEST: 12.73

-HOURLY STANDBY LOSS, BTU/HOUR = CUS : 326.2545

-DAILY STANDBY LOSS, BTU/DAY : 6209.5135

-DAILY WATER HEATER ENERGY, KWH = CY : 8.2119273

-ENERGY FACTOR = EF : 1 . 7039849

—BASIC 1

1

B-2
Appendix B

SAMPLE CALCULATIONS, UNIT "B"

-BASICI

I

RUN

-ENERGY USED DURING RECOVERY TEST (WATT-HOURS) ?

->4553

-DURATION OF RECOVERY TEST IN HOURS?

->3.767

-FINAL MEAN TANK WATER TEMPERATURE, DEGREES C?

->57.683

-INITIAL MEAN TANK WATER TEMPERATURE, DEGREES C ?

->12.617

-RECOVERY EFFICIENCY, ER= 2.1317164

-DAILY WATER HEATING ENERGY, BTU/DAY, CWH= 21383.229

-ENERGY USED IN RECOVERY TEST (WATT-HOURS) : 4558

-AVERAGE POWER DURING RECOVERY TEST (KW): 1.2099314

-DURATION OF RECOVERY TEST, HOURS : 3.767

-WATER TEMPERATURE RISE DURING THE RECOVERY TEST, C DEG.: 45.066

-FINAL MEAN TANK TEMPERATURE , DEG. C, DURING RECOVERY TEST: 57.633

-INITIAL MEAN TANK TEMP., DEG. C, DURING RECOVERY TEST: 12.617

-HOURLY STANDBY LOSS, BTU/HOUR = CUS : 326.2545

-DAILY STANDBY LOSS, BTU/DAY : 6100.7638

-DAILY WATER HEATER ENERGY, KWH = CY : 3.2016405

-ENERGY FACTOR = EF : 1.7061221

-BASICI

I

->

RUN

-ENERGY USED DURING RECOVERY TEST (WATT-HOURS) ?

->4545

-DURATION OF RECOVERY TEST IN HOURS?

->3.751

-FINAL MEAN TANK WATER TEMPERATURE, DEGREES C?

->57.733

-INITIAL MEAN TANK WATER TEMPERATURE, DEGREES C ?

->12.850

-RECOVERY EFFICIENCY, ER= 2.1790721

-DAILY WATER HEATING ENERGY, BTU/DAY, CWH= 21909.784

-ENERGY USED IN RECOVERY TEST (WATT-HOURS) : 4545

-AVERAGE POWER DURING RECOVERY TEST (KW): 1.2116769

-DURATION OF RECOVERY TEST, HOURS : 3.751

-WATER TEMPERATURE RISE DURING THE RECOVERY TEST, C DEG.: 44.8

-FINAL MEAN TANK TEMPERATURE , DEG. C, DURING RECOVERY TEST: 5

-INITIAL MEAN TANK TEMP., DEG. C, DURING RECOVERY TEST: 12.35

-HOURLY STANDBY LOSS, BTU/HOUR = CUS : 326.2545

-DAILY STANDBY LOSS, BTU/DAY : 6101.093

-DAILY WATER HEATER ENERGY, KWH = CY : 3.2095135

-ENERGY FACTOR = EF : 1.7044349

-BAS I Cl I .—>

-

B-3

Nj

co
NSS-114A (rev. 2-ac) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

4. TITLE AND SUBTITLE

1. PUBLICATION OR

REPORT NO.

NBSIR 83-2723

2. Performing Organ. Report No. 3. Publication Date June 1983

PERFORMANCE OF ADD-ON TYPE HEAT PUMP WATER HEATERS USING TWO DIFFERENT TEST

METHODS

5. AUTHOR(S)

James E. Harris

6. PERFORMING ORGANIZATION (If joint or other than N BS, see instructions) 7. Contract/Grant No. national bureau of standards

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

United States Department of Energy

Washington, DC 20585

10. SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

Two different makes of add-on (without tank) heat pump water heaters (HPWH) were tested. Each of the HPWH's was subjected to a series of recovery tests

similar to the Department of Energy (DOE) recovery test for conventional

electric water heaters. The results of the tests (recovery efficiency,

standby loss, input power, storage tank capacity and energy used) were used

to compute an energy factor which could be used to calculate the estimated

annual operating cost of such HPWH’s. The energy factor was also determined

by a series of simulated use tests consisting of four equal draws totaling

64.3 gallons of hot water per day. The average energy factor derived from

the recovery tests was about 13 percent higher than that derived from the

simulated use tests. Based upon the results of this limited test program

it was recommended that a simulated use test be used to determine the energy

factor for HPWH's without tank.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

Appliances; energy; heat pump water heaters; test procedures; testing;

water heaters

13. AVAILABILITY

^£X] Unlimited

| | For Official Distribution. Do Not Release to NTIS

I Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

f(Xl Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

30

15. Price$8.50

USCOMM-DC 6043-P80