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NBS BUILDING SCIENCE SERIES 93

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

Dynamic Performance of a

Residential Air-to-Air

Heat Pump
The Building Science Series

The Building Science Series disseminates technical information developed at the National Bureau of Standards on

building materials, components, systems, and whole structures. The Series presents research results, test methods, and performance criteria related to the structural and environmental functions and the durability and safety characteristics of building elements and systems. These publications, similar in style and content to the NHS Building Materials and Structures Reports (1938-59),

are directed toward the manufacturing, design, construction, and research segments of the building industry, standards organizations, and officials responsible for building codes. The material for this Series originates principally in the Center for Building Technology of the NBS Institute

for Applied Technology. The publications are divided into three general groups: Building Systems and Processes;

Health, Safety and Comfort; and Structures and Materials. For further information regarding these publications

please contact the Program Planning and Liaison Unit, Center for Building Technology, Institute for Applied

Technology, National Bureau of Standards, Washington, D.C. 20234.

J

[See mailing list announcement on last page]
National U;---

Dynamic Performance of a

Residential Air-to-Air Heat Pump

George E. Kelly and John Bean

Center for Building Technology

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued March 1977
Library of Congress Cataloging in Publication Data

Kelly, George E., 1944- Dynamic performance of a residential air-to-air heat pump.

(National Bureau of Standards Building Science Series; 93) Supt. of Docs, no.: €13.29/2:93

1. Heat pumps —Testing. I. Bean, John, joint

author. II. Title. III. Series: United States. National Bureau of

Standards. Building Science Series; 93. TA435.U58 no. 93 [TH7638]

690'.021s [621.4'025] 76-50050.

National Bureau of Standards Building Science Series 93

Nat. Bur. Stand. (U.S.), BIdg.Sei.Ser. 93,18 pages (Mar. 1977) CODEN: BSSNBV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON; 1977

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price 45 cents Stock No. 003-003-01691-8 There is a minimum charge of $1.00 for each mail order
Contents

Ha«r

1. Introduction 1

2. Experimental Equipment 2

3. Experimental Procedure 3

4. Experimental Results 6

5. Comparison With Present Present day Fossil-Fuel Heating Equipment 10

6. Discussion 11

7. References , 12

iii

Dynamic Performance of a Residential air-to-air Heat Pump"

George E. Kelly

Mechanical Systems Section

and

John Bean

Thermal Engineering Section

Center for Building Technology

National Bureau of Standards

Washington, D.C. 20234

Information is presented on the dynamic performance of a 5-ton air-to-air heat pump, which was

installed in a residence in the Washington. D.C. area. The effect of part-load operation on the heat pump's COOLING and HEATING coefficients of performance (COP) was determined. When the pump

operated in the heating mode at outdoor temperatures below 40 °F (4.4 °C). a considerable discrepancy was found to exist between the measured performance and the performance data supplied by the manufacturers. This discrepancy is apparently due to the adverse effects of frost buildup and

defrosting of the outdoor coil. The seasonal performance factor (SPF) of the heat pump was estimated and then traced back to the power plant to obtain an "EFFECTIVE SPF" which is then compared with

the performance which might be expected from fossil-fuel heating equipment.

Key words: Cooling and heating coefficients of performance; effective heating COP: heat pumps: heat pumps and energy conservation; part-load performance; seasonal performance factor.

1. Introduction

With approximately 11 percent of the total energy

consumed in the United States [1]' going into the space heating of residences, there is an urgent need

for information on the energy effectiveness of the

various systems used for residential heating. These

systems include the traditional fossil-fuel heating

equipment, such as gas-or-oil fired furnaces and

boilers, electrical resistance heating systems, such

as electric furnaces and baseboard units, and air-toair heat pumps. Unfortunately, at the present time

there exists very little published data on the partload and seasonal performance of either the fossil- fuel or air-to-air heat pump systems.

In an attempt to obtain quantitative information on the dynamic performance of air-to-air heat

pumps, a 5-ton heat pump was installed in a 20- year-old house (known as the Bowman house) in the Washington, D.C. area. This heat pump experiment was part of a larger program to experimentally measure the energy savings which could be

achieved through retrofitting. The approach used

consists of measuring the heating and cooling

requirement of an old home and then making

various energy saving modifications [2]. By measuring the energy requirements of the house after each

*Tliis research was sponsored by the Federal Energ) AdministratiiMi.

' Figures in brackets indicate tiie hleralure relerences at the end ol this paper.

modification, an estimate of the energy savings

resulting from each change could be made. The

house studied was a single-story frame ranch-type

residence which was built in the early 1950 s and

had a floor area of approximately 2500 sq ft (232.25

m2).

The Bowman house was originally heated by an

oil-fired, warm-air furnace and this furnace was

used in determining the pre-retrofit heating requirement of the house during the 1973—74 winter period.

The furnace was removed in the spring of 1974 and

a heat pump installed in its place, with little modification made to the existing duct system. The

heat pump was instrumented and used to measure

the pre-retrofit cooling requirement of the house

during a test period in the summer of '74. During the 1974—75 winter, various modifications were made to the house to reduce the energy

needed for heating and cooling purposes. First,

weather stripping and caulking were apphed around

the doors and windows, after which the heating

requirement of the house was determined. The next stage of retrofitting consisted of installing storm

windows and then remeasuring the heating requirement of the house. The third and last stage of the

post-retrofit heating test consisted of adding additional insulation to the 4 inches already existing in the attic, blowing insulation into the walls, and then

measuring the new heating requirement ot the

house. The post-retrofit cooling requirement of the house was measured during the summer of 1975.

1
The data presented in this paper pertain to measurements of the heat pump's performance

made during the pre-retrofit cooling test, the first stage of the post-retrofit heating test (after caulking

and weather stripping) and after the second stage of

the post-retrofit heating test (alter the installation of storm windows). The performance of the air-to-air heat pump after the placement of additional insulation in the attic and the blowing of insulation into the walls will not be discussed, since this modification resulted in a greatly oversized heat pump with

different performance characteristics. The pre-retrofit cooling test lasted for a period of four days,

and 373 hours of heating data were obtained for the combined first and second stages of the post-retrofit

heating test.

2. Experimental Equipment

The 5-ton heat pump installed in the Bowman

house was a commercially available, split system

employing thermostatic expansion valves. It was

sized to meet the calculated pre-retrofit cooling

recjuirements of the hoiTse. which was determined

using ASHRAE procedures [3]. Figure 1 is a schematic of the indoor section, which contained the

indoor ct)il, indoor blower, supplemental resistance

heaters, and the expansion valve employed when the

heat pump was used to cool the house. The outdoor

section, shown in figure 2, contained the outdoor

coil, outdoor fan, compressor, accumulator, swit- chover valve, and the expansion valve used during

the heating process. The heating duct system is shown schematically

in figure 3. Half the house was over a basement

and the other half over a crawl space, and there were separate supply and return ducts tor each half

of the house. The return air was ted into a return

air plenum and then into the indoor air handler,

where it was either heated or cooled. Upon leaving

the indoor air handler, the air entered the two

supply ducts, where air-flow monitoring devices were used to measure the mass flow rate of air delivered to each half of the house. Each of these

air-flow monitoring devices was purchased commercially and consisted of honeycomb straighteners and

a rake of Pitot tubes located at centers of equal

area. By measuring the difference between the

average total and the average static pressure in each supply duct and knowing the barometric

pressure and the temperature of air in each duct,

the mass flow rate of air delivered to each half of

the house could be calculated. Two 36-junction

copper-constantan tliermopiles, which were located

as shown in figure 3, were used to measure the

temperature rise or drop in the air delivered to each

supply duct as it passed through the air handler.

The output of each thermopile was fed into an

electronic integrator, which allowed for calculation

of the average temperature difference between the return plenum and each supply duct over a period

of time. The main supply ducts, as well as the

branches leading to the individual room registers.

INDOOR COIL

SUPPLEMENTAL

RESISTANCE HEATERS

ogo

o°o

o

o

COOLING m « HEATING

CHECK VALVF

FlGLRE 1. Schematic of heat pump's indoor unit.

FiGL RE 2. Schematic of heat pump's outdoor unit.

CRAWL SPACE

BASEMENT INDOOR

AIR HANDLER

AIR FLOW

MONITORING DEVCE

RETURN AIR PLENUM

O - 36 JUNCTION THERMOPILE

X . 36 JUNCTION THERMOPILE

H IGl'RE 3. Schematic of duct system in the Bowman house.

2
were insulated in order to cut down the heat losses from these ducts. During the summer cooling test, the condensate

produced by the indoor coil was collected in a large

container which was located on top of a mechanical

scale. By weighing the condensate hourly the latent cooling done by the heat pump was determined.

The relative humidity sensor in the return air plenum was used to compare the measured coohng

performance of the heat pump with that predicted

in the manufacturer's specifications.

Watt-hour meters were used to measure the

energy input to the compressor, outdoor fan and the indoor blower and, during the winter test, to the

supplemental resistance heaters. Although the watt- hour meters used during the cooling test had to be

read manually, the ones employed on the heating

test were equipped with an internal set of contacts. A pulse counter-printer was used to count the

number of contact closures. A signal from an

external source would cause the pulse counterprinter to print out and then reset to zero. Strip-chart recorders and copper-constantan ther- mocouples were used to continuously record the air temperature in the return air plenum and in both

supply ducts. These recordings allowed for constant monitoring of the heat pump and provided information on the length of ON-time and OFF-time. In

addition, the return-air temperature measurement

was necessary in order to accurately compare the measured heating and coohng performance of the heat pump with the manufacturer's performance

data. The outdoor temperature and relative humidity were also recorded hourly. The copper-constantan

thermocouple and relative humidity (RH) sensors, used to measure the outdoor temperature and the humidity were housed in a small outdoor instrument

shelter.

3. Experimental Procedure

In both the coohng and heating modes of operation, the performance of the heat pump is described

by its coefficient of performance or COP. When the

heat pump is used to cool the house, the coefficient of performance, which in this case may be referred

to as the COOLING COP, is the ratio of the total heat extracted from the return air to the total energy supphed to the heat pump. When the heat

pump is used for heating, its coefficient of perfor- mance or HEATING COP is the ratio of the total heat dehvered by the heat pump system to the total energy input to the system. In this paper, the heat

pump system is defined to include the input and

output of the first step of the supplemental resist- ance heaters when this step operates during a defrost period. During periods of operation not involving defrost, the input and output to the

supplemental heaters are not included in the calculations to determine the HEATING COP. This

definition of the heat pump system was adopted

because it was felt that the heat pump should be

penalized for the energy required to temper the

supply air during the periods that heat is being

extracted from the return air in order to defrost the outside coil. During defrost, the first stage of

supplemental resistance heaters in the Bowman

house installation was capable of just canceling the coohng effect of the indoor coil, which resulted in a supply air temperature approximately equal to the return air temperature and neither heating or

cooling being done to the interior space. When the heat pump cools the interior space, the

total coohng, ^''(t), done over a time period t is given by the equation [1]

(1) where QHt) is the total sensible cooling and QHt)

is the total latent cooling done during the time

period r. The quantities Q^si^) and QHt), in addition

to depending on the time period r, depend upon

the outdoor dry-bulb temperature, the indoor wet- and dry-bulb temperatures, the rate of air flow

through the indoor unit, the performance of the

heat pump's various components, and the cooling

load of the house.

The quantity Qs(t) is determined using:

(2)

where C,/is the average specific heat of the air- water mixture leaving the indoor unit for typical

ON-cycle conditions,

i = l,2 pertains to the supply ducts passing

through the basement and crawl space,

m, is the average mass flow rate of the air- water mixture in supply duct i during ONperiods.

Ar,-(T) ^Ti{t) dt.

timi' period t

in the equation defining Ar,(T) indicates

that integration is to be carried out during

periods when the heat pump is operating.

^Tiir) is the absolute value of the change in drybulb temperature experienced by the air entering supply duct i at time r as it passes through the indoor unit.

It has been assumed that there is no cooling of the interior living space during the OFF-cycles.

since the indoor blower and heat pump ceased

operating at the same time and the location of the

indoor unit in the basement prevented any natural

convective cooling.
During each of the four days in the pre-retrofit

coohng test, the mass flow rate in each supply

duct was periodically measured while the heat

pump was on. These values were then averaged to find an average air flow rate, m, for each supply

duct which was used in eq (2) to determine the

sensible cooling provided by the heat pump. The

standard deviations of these measured mass flow

rates were approximately 2 percent and 3 percent

of the average mass flow rates for the two supply

ducts. The quantities Ar, were measured with

thermopiles and then integrated over time using

electronic integrators. During the OFF-periods, the

signals to the integrators were shorted, which

effectively set (AD, equal to zero and this was

equivalent to integrating only over the ON-periods.

The total latent cooling provided by the heat

pump over a time period r was determined using

the equation:

QiiT) - ^W{T) (hg{T) - hf(T)) (3)

where AJ^(t) is the weight of condensate collected

in this time period, and hg (T) and hf (D are the

specific enthalpies of saturated water vapor and

saturated liquid, respectively, at the average evaporator coil temperature T. The COOLING COP was calculated using:

0''{t) COOLING COP = (4)

1 (r)

where /(t) is the total measured energy input to the heat pump during the period of time, r, used

to determine Q''(t). For evaluating the COOLING

COP, data were collected hourly on Ar,, AJF and /

and these hourly values were then combined to obtain values for time periods r, which either

contained an integral number of complete ON-OFF

cycles or consisted of all the consecutive hours in a 24-hour period for which a cooling load existed.

Two cooling tests were also run with the heat

pump operating in a steady-state manner. One test was for a three-hour period and the other was for a one-hour period. In order to achieve approximately

steady indoor conditions, the doors and some of

the windows in the house were partially opened

and the heat pump allowed to operate until

conditions sta bilized. Data were then collected

hourly on AT",, AJF, and /, and the steady-state

COOLING COP was calculated for each of the

four one-hour periods.

When the heat pump operated in the heating

mode, the total heat Q"{t), delivered to the interior living space during the time period r, is given by:

Q"{r) = S \ it) m,- (t) Ar, it) dt - (5a)

i=l time period t

2 - C; X ^Tiir) (5b)

i=l

^ + <Cp) I f m,it)^TAt)dt

OFF

period

in T

where Cpi(t) and m,(t) are the specific heat and mass flow rate at time t of the air-water mixture

entering supply duct i, Cp, m„ AT", (r) and AF, (t) are the same as defined for equation 2, and (C^)

is the specific heat of the air-water mixture leaving

the indoor unit for typical OFF-cycle conditions.

The average mass flow rate during ON-periods,

m,, was determined by periodical!;' checking the mass flow rate in supply duct i throughout the

heating test. The standard deviations of the meas- ured mass flow rates were approximately L6

percent and 1.1 percent of the average mass flow

rates for the two supply ducts. The quantities

Ar, (r) and AT2 (t) were determined in the same manner as for the cooling test, with the signals to the integrators being shorted during OFF-periods.

The convective heating, which corresponds to the

second term on the right-hand side of eq (5b), was

estimated by running a series of tests to measure

the change with time in rhjit) and ATi(t) after the

heat pump shut off. The mass flow rate was

measured at various cross sectional positions in each supply duct using a hot-wire anemometer,

and ATjlt) was obtained using the two 36-junction

thermopiles and strip-chart recorders. It was found

that the convective heating term could be approximated in this series of tests by an equation whose

independent variable was the length of the OFFperiod, provided that the OFF-period was less than

30 minutes. When the OFF-period exceeded 30

minutes, a constant value was used. This approximation was then used to estimate the convective

heating provided during OFF-periods throughout

the entire post-retrofit heating tests, since it was

impractical to measure this quantity for each

individual cycle. Although the use of one approximation for all OFF-periods could introduce a small

error, the effect of this error on the heating COP

was small since the convective heating term was a small percentage of the total heat term, Q" (r).

In t he post-retrofit heating test, the quantities

AFi (r) and AT2 (r). and the energy inputs to the

indoor blower (/g). the outdoor fan and the compressor (/o), and supplemental resistance heaters

(//,), were obtained for each ON-OFF cycle. This was achieved by having the pulse counter-printer,

attached to the watt-hr meters, and the two

integrators, used to integrate the signals from the

thermopiles, print out and reset to zero each time

the heat pump came on. The total heat delivered

during each ON-OFF cycle was then computed

and used to calculate the HEATING COP for each

cycle using the equation:

4
HEATING COP -

where t, is length of the ON-OFF cycle and

(6)

Ig + Iq when no defrosting takes place

during the ON-OFF cycle + ^0 + ^ Ri when a defrost process

occurs during the ON-cycle.

The quantity If^ is the energy input to the first step of supplemental resistance heaters and is equal to I^, 1^/2 or /r/S, depending on whether

one, two or three steps of resistance heaters were

operating during the defrosting process. Four steady-state tests were also performed to measure the COP of the Bowman house heat

pump when it was operating continuously in the

heating mode. Two of the tests were done when

the outdoor temperature was above 50 °F (10 °C)

and they consisted of operating the heat pump for approximately a half hour until steady-state conditions were reached and then measuring the energy

input and heat output over a half-hour test period.

The same procedure was followed in the other two

steady-state tests, except that since they were

performed at lower outdoor temperatures the heat

pump was first made to go through a defrost cycle.

This removed any frost which may have accumulated on the outdoor coil and resulted in a measured COP which represented the maximum

performance achievable by the heat pump at the given indoor and outdoor test conditions. As in the

steady-state cooling tests, the doors and windows

in the Bowman house were partially opened in order to achieve the approximately steady indoor

conditions necessary for the performance of these

four tests. During the pre-retrofit cooling test, the postretrofit heating test and the steady-state heating

and cooling tests, a considerable number of additional measurements were made. These measure- ments included hourly readings of the outdoor drybulb temperature and continuous strip-chart re- cordings of the dry-bulb temperature and relative humidity of the return air as it entered the indoor

unit. These latter measurements were used to obtain the average outdoor dry-bulb and the aver- age indoor dry-bulb and wet-bulb temperatures

existing during periods for which COOLING AND

HEATING COP's were calculated. These average

indoor and outdoor conditions were then used in conjunction with the manufacturer's performance

data to obtain the cooling and heating COP's

which the manufacturer claimed the heat pump

should achieve under these conditions, if it had

been operated in a steady-state manner. By divid- ing the measured COOLING or HEATING COP

by the manufacturer's respective steady-state cool- ing or heating COP at the same indoor and

outdoor conditions, an indication of the effect of

part-load operation on the heat pump's perfor- mance was obtained.

The air flow rates encountered in the Bowman

house during the cooling and heating tests were

slightly less than the 2,000 CFM (.56.65 m'Vmin) for which the manufacturer's performance data were

available. The average air flow rate during the

cooling test was approximately 17.50 CFM (49. .56 mVmin), and correction factors supplied by the

manufacturer were used to calculate the steadystate capacity and compressor power consumption.

The average air flow rate during the post-retrofit

heating tests was around 1680 CFM (47.58 m'Vmin)

and the corresponding correction factors employed

in obtaining the steady-state COP from the manufacturer's performance data were obtained by a small extrapolation of the correction factors supplied by the manufacturer. From these correction

factors, it was found that the reduction in air flow

rate from 2000 CFM (49.56 m'Vmin) to around 1680

CFM (47.58) during the post-retrofit heatinv test resulted in an approximate 6 percent decrease in the HEATING COP of the heat pump. The values

presented in sections 4 and 5 for seasonal HEATING COP, EFFECTIVE HEATING COP, SPF and

EFFECTIVE SPF may thus be approximately cor- rected to an air flow rate of 2000 SCFM (49.56 m-'/ min) by multiplying them by 1.06. Since there is no perfect correlation between the

hourly cooling or heating requirement of a house

and the difference between the indoor and outdoor

dry-bulb temperatures, it was necessary to deter- mine the actual cooling and heating load factors in order to properly evaluate the dynamic perfor- mance of the heat pump under test. The equations

used to define these load factors were:

COOLING

LOAD

FACTOR

HEATING

LOAD

FACTOR

COOLING DONE BY THE

HEAT PUMP IN TIME r (MANUFACTURERS

STEADY-STATE COOLING CAPACITY) (r) HEATING DONE BY THE

HEAT PUMP SYSTEM IN

TIME T (MANUFACTURER'S

STEADY-STATE HEATING CAPACITY) (T)

(7a)

(7b)

where r is a time period or cycle over which the cooling or heating COP was measured and it is to be understood that the manufacturer's steady-state

cooling or heating capacities correspond to the

average indoor and outdoor conditions which existed in the time penod t. The numerator in eq

(7b) includes the output from the first step of

supplemental resistance heaters when this step

operated during a defrost period to offset the

coohng effect of the indoor coil during this period.

5
4. Experimental Results

The effect of part-load operation on the cooHng

performance of the Bowman house heat pump is shown in figure 4. The ordinate in this figure is the

ratio of the measured COOLING COP to the

COOLING COP calculated from the manufacturer's steady-state performance data for the aver- age indoor and outdoor conditions existing during

the measurement period. The abscissa is the cool- ing load factor which was defined in eq (7a). The

four steady-state, hour-long cooling tests, corre- sponding to a cooling load factor of LO, had an average ordinate of 0.99, which indicates good

agreement between the measured steady-state effi- ciency of the heat pump and the manufacturer's

data. The data points represented by circles corre- spond to time periods consisting of aU the consecutive hours in each 24-hour period for which a cooling load existed. The triangular data points

represent time periods which were selected for analysis because the heat pump's COP could be

determined for a complete number of ON-OFF

cycles and the cooling load was approximately

constant. As can be seen from figure 4, data were

only obtained for load factors less than 0.35. This was due to unseasonably mild weather which

existed during and after the pre-retrofit coohng test. A straight hue is shown passing through the data in figure 4, with the region where no data exist being

indicated by a broken line. Although the lack of

data over the entire load range precludes certainty

as to the exact shape of the curve in figure 4, this straight line approximation is believed to be correct because similar results were obtained during the

post-retrofit heating test. The HEATING COP of the Bowman house heat

pump, as calculated using the manufacturer's per1.00 -

.90 - ^80-

g .70 -

S 60-

1 .50- a

2 40-

£ .30-

.20 -

10 -

o'—

STEADY STATE TEST • DAILY AVERAGE

SELECTED PERIODS

,20 .30 40 .50 .60 .70 .80 .90 1.00 COOLING LOAD FACTOR

formance data, is presented in figure 5 as a function of outdoor temperature for three different return air temperatures. The data presented is for an indoor air flow rate of 1680 CFM (47.58 m^/min),

since this is the average air flow rate existing

during the heating test. Correction factors for determining the heating capacity and compressor

power consumption at this air flow rate were

extrapolated from the manufacturer's data as dis- cussed in the section entitled Experimental Procedures.

RETURN AIR TEMPERATURE

60" F (15 6° CI

70 (°F) OUTDOOR 20 (°C) TEMPERATURE

Figure 4. Variation in performance with load, cooling operation.

Figure 5. COP calculated from MFR's data, heating operation.

Figure 6 is a plot giving the heat pump's capacity

(as obtained from the manufacturer's performance

data) at various outdoor temperatures and three

different curves showing heating load versus out- door temperature. The heating capacity is for a return air temperature of 70 °F (21 °C) and an

indoor air flow rate of 1680 CFM (47.58 mVmin).

Two of the heating load curves were determined for

infiltration rates of 1.0 and 0.5 indoor-air changes

per hour using ASHRAE procedures [5] for calculating the heating requirement at the outdoor design

temperature. The third heating load curve was

obtained by passing a least-square-fit straight Hne

through experimental data and an assumed point.

The experimental data corresponded to 24-hour

averages of the heat delivered to the Bowman house

per hour plotted against the average outdoor temperature existing in each 24-hour period. The data were corrected to an indoor temperature of 70 °F

(21 °C), but were not normahzed to any standard

wind or solar radiation conditions. This lack of normalizing wind and solar effects could be the reason why the heating requirement of the Bowman

house with and without storm windows appears to be the same for the data plotted in figure 6. The

assumed point in figure 6 corresponds to a zero heating requirement at an outdoor temperature of 67.5 °F (19.7 °C). and was arrived at by calculating the outdoor temperature at which the heat gener- ated by the operating interior lights would just

6
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30

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20

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0 — 0

- CALCULATED HEAT REQUIREMENT ASSUMING

1 AIR CHANGE PER HOUR

— CAPACITY OF HEAT PUMP

AT 70°f 121 rci rehjhn air TaiPEFWniRE AND 1680 CfM AS OBTAINED FROM THE MANUFACTURER'S PERFORMANCE DATA

0 5 10 DUTDCOR TMPERATURE

70 CF) 20 (°C)

Figure 6. Heat requirement of house versus outdoor temperature, indoor temperature = 70 °F (21 .1 °C).

cancel the heat loss from the house. vUthough it was originally felt that the infiltration rate would be

at least one air change per hour, actual measure- ment using a tracer gas [2] showed it to be around

0.5 air changes per hour. Even at the correct

infiltration rate, however, the ASHRAE procedure

for determining the heating load at the outdoor

design temperature considerably overestimated the heating requirement of the Bowman house. This is probably due to the fact that the ASHRAE method

is for design conditions with a 15 mph wind and

does not include the effect of sunshine in reducing

the heating load. The points Bj, Bj, and B3, which

are referred to as "balance points," give the

respective outdoor temperatures at which the heat

pump's output (without supplemental resistance

heaters) would just equal the heating requirement if the heating load curve passing through each point

represented the heating requirements of the Bow- man house. Although it appears from the experimental data that B3 (about 23 °F (5 °C)) should be

the correct balance point, the actual balance point

turned out to be in the neighborhood of 30 °F

( — 1.1 °C) which is closer to B2. This was due to the

fact that the heating capacity of the heat pump in this temperature region was considerably reduced

from the value given by the manufacturer due to frost buildup on the outdoor coil and the need for repeated defrost. The heating load factor, as defined by eq (7b), may be estimated from figure 6 by dividing the

measured heating requirement at a given temperature by the manufacturer's steady-state capacity at the same temperature. This has been done in figure

7, where the resulting curve should be considered

as providing only an approximate relationship be- tween heating load factor and outdoor temperature,

since the heating requirement of a house is also a function of the amount of solar radiation received,

wind speed, and the thermal storage capacity of the

structure. These variables could cause the house's

daily heating requirement for particular days to deviate from the straight line passing through B-, in figure 6. In addition, considerable deviation can be

expected from this line for the heating requirement

during periods of time less than 24 hours. It sliould be pointed out that since the definition of heating

load factor involves the manufacturer's steady-state

capacity, which might be considered an ideal

capacity, the heat pump's balance point was actually located somewhere between a load factor of

0.70 and 0.80. Thus the heat pump operated

continuously at a heating load factor considerably

less than 1.00.

AVERAGE HEATING _ MEASUREO HEAT REQUIREMENT OF BOWMAN HOUSE

LOAD FACTOR CAPACITY OF HEAT PUMP AS OBTAINED FROM MFR S DATA

70 (°F) OUTXOR

^ TEMPERATURE -10 -5 0 5 10 15 20 (°C)

Figure 7. Average Heating versus outdoor temperature.

Figure 8 shows the effect of cycling on the

heating performance of the Bowman house heat

pump. The data plotted in this figure are of two

kinds. For outdoor temperatures above 40 ° F (4.4

° C), cycles were selected which appeared to be

representative of the heat pump's performance at various temperatures. The heating COP. heating

load factor, and manufacturer's steady-state COP

(as calculated from the manufacturer's performance

data) were determined for each cycle and plotted as shown. For outdoor temperatures below 40 °F

(4.4 °C), the first cycle occurring after a cycle

involving a defrost period was selected, analyzed

and plotted. Thus the data shown in figure 8, do

not show the effect of frost buildup and defrosting on the performance of the heat pump. Since frost buildup does occur and defrosting is necessary,

figure 8 cannot be used for predicting the perfor7
1.00

.90

:.60 -

j -20 -

1.20 -

. WITH STORM WINDOWS

A WITHOUT STORM WINDOWS

A DATA POINT IN THIS FIGURE CORRESPONDS TO THE COP FOR ErfHER;

. A TYPICAL ON.OFF CYCLE OCCURRING AT AN OUTDOOR TEMPERATURE ABOVE 40°F (4.4°C|

- THE FIRST ON-OFF CYCLE OCCURRING AFTER A CYCLE CONTAINING A DEFROST PERIOD AND

HAVING AN OUTDOOR TEMPERATURE BELOW iO°f (4 ^°C|

HEATING LOAD FACTOR

1 » I .20 .30 .40 .50 .70 1.00 30 25 (°F) 20 5 0 .5 CO

OBTAINED FROM AVERAGE LOAD VS OUTDOOR TEMPERATURE CURVE

Figure 8. Effect of cycling on heat pump performance, heating operation.

1,00

.90

^ 80

3 60

£.50 -

| .40

: 30 £ 20

.10 -

20

COPS FOR nPICAL ON OFF CYCLES OCCURRING AT OUTDOOR TEMPERATURES

ABOVE 40"f (44°C|

WITHOUT STORM WINDOWS

WITH STORM WINDOWS

COPS FOR PERIODS OF TIME WHICH STARTS

AT THE END OF AN ON-OFF CYCLE CONTAINING A DEFROST PERIOD AND TERMINATE AT THE END

OF ANOTHER ON-OFF CYCLE CONTAINING A

DEFROST PERIOD THE AVERAGE OUTDOOR TEMPERATURE FOR THESE TIME PERIODS IS BELOW 40°F |44»C|

HEATING LOAD FACTOR

J L A

.90

-+- J_

X

25 (°F)

1- 5 0 -5 CO

OBTAINED FROM AVERAGE LOAD VS OUTDOOR TEMPERATURE CURVE Figure 9. Variation in heat pump performance with load, heating operation.

mance of the Bowman house heat pump in the

defrost region. It does, however, indicate that the

linear approximation used in analyzing the cooling

data in figure 4 is probably correct. Figure 9 shows the heating performance of the

Bowman house heat pump as a function of the

heating load factor. For outdoor temperature above

40 °F (4.4 °C) the data shown in figure 8, are replotted here. For temperatures below 40 °F

(4.4 °C), the heating COP, heating load factor, and

manufacturer's steady-state COP were determined

for periods of time which started at the end of an ON-OFF cycle containing a defrosting process and

terminated at the end of another ON-OFF cycle

containing a defrosting process. The minimum

period of time analyzed was 3 hours, while the

maximum was almost 24 hours. These time periods

always contained many ON-OFF cycles and a number of periods contained multiple defrosting

processes. Since considerable scatter existed in the data obtained for temperatures below 40 °F (4.4 °C), the ordinates of all the data points were averaged

and found to have a value of 0.74. A straight line parallel to the X axis was then passed through these

data in figure 9 at this average height. In order to determine if the solid lines drawn

through the data in figure 9 were truly representive

of the Bowman house heat pump's performance, all the heat pump data collected after the first and

second stages of retrofit were used to determine an average hourly heating COP for the post-retrofit

heating test. The result was:

8
(Average Hourly Heating COP) =

mt'asurod = ^ I (COP), = 1.71 (8)

where (COP); = COP measured for hour i and A' = the total number of hours of data = 373. The average hourly COP wa-S then calculated for the same period by determining the manufacturer's

steady-state COP for each hour, multiplying by a correction factor obtained from figure 9 and then

averaging. It was found that:

(Average Hourly Heating COP)

calculated

using figure 9

(COP),

is the heat suppHed in temperature bin j

by the supplemental resistance heaters over the entire heating season

is the energy used by the heat pump in dehvering heat OP and is equal to 0"'' / (COP)j

is the average seasonal HEATING COP

of the heat pump for temperature bin j

is the energy required by the supplemental resistance heaters to produce

heat Oj- and is equal to Oj, assuming

jacket losses are negligible.

= — 2, (Mfr's steady-state heating COP),

/V j=i X (Correction factor (Tj)) j = 1.76 (9) where the MFCs steady-state heating COP is the

heating COP obtained from the manufacturer

performance data for hour j and the correction

factor is the ordinate of the curve in figure 9

corresponding to average outdoor temperature, Tj, for hour j. The measured and calculated average

hourly heating COP's were found to agree within 3 percent.

The seasonal performance factor (SPF) of the

Bowman house heat pump was estimated using the

bin method and weather data for Andrews Air

Force Base, which is near Washington, D.C. The

equation used was:

SPF =

Total Energy Delivered Over The Heating

Season

Total Energy Input Over the Heating

Season

I OfP + ^ Of X Ofp + X 05 V /HP + V /| V

or

;io)

j=i (COP), + 1 0|

where

j

M

0]HP

indicates the jth temperature bin

is the total number of temperature bins

used

is the heat supplied in temperature bin j

by the heat pump over the entire heating season

Two calculations were performed to determine

the expected SPF of the Bowman house heat

pump. The first one approximated (COP), by the steady-state heating COP, which was calculated

from the manufacturer's performance data for temperature bin j. The second calculation took

(COP)j = (Manufacturer's steady state HEATING

COP for temperature bin j) (correction

fac tor obtained from figure 9 for temperature bin j). These calculations are shown in table 1. Using

the manufacturer's steady-state data, the SPF was

predicted to be 2.16. Employing the results in figure 9 to correct for the effect of cychng, frost buildup and defrost, the SPF of the Bowman

house heat pump turned out to be 1.74 or some 19 percent lower.

The procedure used in table 1 to estimate the SPF of the Bowman house heat pump yields the same result as the method recommended by the manufacturer of the heat pump for the case where

(COP)_, is calculated from the manufacturer's per- formance data. It is interesting to note, however,

that this method indicates that the Bowman house

heat pump supplied 98 percent of the total energy

required over the heating season, and the supplemental heaters supplied only 2 percent. This is really an underestimation of the supplemental heat

required because the method assumes that the

capacity of the heat pump in the defrost region is the same as that given in the steady-state perfor- mance data. This assumption results in a balance

point at B3 as shown in figure 6. As pointed out

earlier, this is not the case, since the capacity of

the heat pump in the defrost region is affected

adversely by frost buildup and defrosting and this results in the actual balance point being closer to B2. Thus more supplemental resistance heat is required than accounted for in table 1, and the

estimated SPF should actually be slightly less than

1.74. The resulting change would, however, be

small and for simplicity, this effect is not included

in the calculations presented in table 1.
TABLE 1 ESTIMATED SPF OF BOWMAN HOUSE HEAT PUMP

o

CTl 00

ggS 32- tj ig 23

2: o Z3 1^Q 3 11H a

Oil— ElcCLU puS e M

s: cc: o cjC^ oH M B

A I5 C D = B - C E F G H I J = H X I K = F T H L = F f J

67 19.4 1 27 848 1080 2 50 0.87 2 18 432 495

62 16.7 5 50 761 4190 2 50 0.87 2 18 1680 1920

57 13.9 9 73 701 6820 2.48 0 88 2 18 2750 3130

52 11.1 14 0 670 9380 2 44 0 89 2 17 3840 4320

47 3.3 18 2 649 11800 2 41 0 90 2 17 4900 5440

42 5.6 22 4 744 16700 2 33 0 91 2 12 7170 7880

37 2.8 26 7 720 19200 2 25 0 74 1 67 8530 11500

32 0.0 30 9 639 19700 2 16 0 74 1 60 9120 12300

27 -2.8 35 1 353 12400 2 04 0 74 1 51 6080 8210

22 -5.6 39.4 38.6 0,8 205 7910 164 1 92 0 74 1 42 4120 5570

17 -8.3 43 6 34.3 9.3 111 3810 1030 1 83 0 74 1 35 2080 2820

12 -11.1 47 8 30.1 17.7 47 1410 832 1 65 0 74 1 22 855 1156

7 -13.9 52 0 26.9 25.1 11 296 276 1 54 0 74 1 14 192 260 .

2 16.7 56 3 23.4 32.9 1 23 33 1 .42 0 74 1 05 16 22

SUM OF COLUMNS SHCWN, 114,719 2,335 51,765 65,025

, ^ Jb X E above balance pt. predicted USing Jf ^ _ 114,719 + 2,335 _ , (c X E below balance pt. MFR ' S Data** ?K + Jg 51,765 + 2,335

_ c,^ „f Pol ^ V Predicted Using^^. ^f + 114,719 + 2,335 , X = Sum or Column X n i j. • i- • -J** = = '- '- = 1.74 Kesults in Figure 9 Il + (g 65,025 + 2,335

Table 1

5. Comparison with Present Day

Fossil-Fuel Heating Equipment

In order to compare the energy effectiveness of

the Bowman house heat pump with fossil-fuel

heating equipment, it is necessary to trace the

energy consumption of the heat pump back to the

primary source energy used by the power plant.

This may be done by defining an EFFECTIVE

HEATING COP which is given by:

EFFECTIVE HEATING COP

= (0.29) (heat pump's HEATING COP) (11)

where 0.29 takes into account the fact that, on the average, only about 29 percent of the heat energy

of the fuel burned at the power plant in making

electricity will reach the heat pump [6]. Table 2

lists the EFFECTIVE HEATING COP of the

Bowman house heat pump at various outdoor

temperatures, assuming that the heat pump's

heating COP may be obtained by correcting the manufacturer's steady-state COP using the results presented in figure 9. Equation (11) may also be

used to determine an EFFECTIVE SPF, where

EFFECTIVE SPF = (0.29) (1.74) = 0.50. (12)

10
TABLE 2 EFFECTIVE HEATING COP for Bowman House Heat Pump

OUTDOOR TEMPERATURE

2°F 12°F 22°F 32°F 42°F 52°F 62°F

-I6.7°C -ivrc -5,6°C 0°C 5.6°C ii.rc i6.rc

COP CALCULATED FROM MFR S DATA

1,42 T65 192 2.16 2.33 2.44 250

COP PREDICTED

USING RESULTS PRESENTED

IN FIGURE 9 105 122 142 160 212 2.17 2.18

EFFECTIVE COP = (0.29)

1 COP Predicled USING RESULTS

PRESENTED IN FIGURE 9 I

0.30 0.35 0-41 0.46 061 063 0,63

Results presented are for a CFM of 1680 and a return air temperature of 66 °F (18,9 °C) Table 2 Thus for every unit of energy consumed at the

power plant, only about 0.5 units of heat were

delivered, on the average, to the interior living

space of the Bowman house. Even if this value of the EFFECTIVE SPF were to be corrected to the manufacturer's recommended indoor air flow, the

effect would only be to increase it by approximately 6 percent to a value of 0.53.

If an attempt is made to compare this estimated

seasonal performance of the Bowman house heat

pump with that of fossil-fuel equipment, it is found

that considerable controversy exists concerning the seasonal efficiency of residential gas and oil-lired heating systems. Numerous studies exist which

report seasonal efficiencies from 35 percent to 65

percent for this type of equipment [6]. Many of these studies, however, involved the comparison of houses equipped with fossil-fuel heating equipment

with those using electric resistance heating and

often considerable differences existed between the two types of houses in the amount of installed

insulation, tightness with respect to air infiltration,

and occupant usage. Many other studies calculated

the heating requirements of the house at various

outdoor temperatures, neglecting the heat contributed by lighting, appliances, occupants and solar,

and used monthly fuel bills to determine seasonal

efficiencies. These procedures usually underestimated or overestimated the seasonal efficiency of residential fossil-fuel heating equipment and have

contributed greatly to the controversy.

Recently, another approach to determining the seasonal performance of fossil-fuel heating equipment has been undertaken by the Honeywell

Corporate Research Center with their development

of a computer model for such equipment. The

model is capable of calculating the seasonal losses,

including those due to the presence of H2 in the

fuel; the heat contained in the products of combustion and excess air going up the flue, and off-cycle

drafts passing through the heat exchanger. Figure

10 summarizes the results obtained when this model was used to calculate the seasonal efficiency of a

typical gas-fired furnace operating in Minnesota [7]. A seasonal efficiency of 57.1 percent was obtained,

with off-cycle losses constituting the largest single

loss of 12.7 percent. Honeywell is presently in the process of verifying this model by comparing its

CALCULATED SEASONAL LOSSES

10,3 % - PRESENCE OF HYDROGEN IN THE FUEL

118 % - DRY FLUE GAS LOSS

1,1 % - LOSS DUE TO HEATING COMBUSTION AIR TO ROOM TEMPERATURE

4.0 % - LOSS DUE TO HEATING DRAFT DIVERTER TO ROOM TEMPERATURE

3,0 % - PILOT LIGHT LOSS

12,7 % - OFF-CYCLE LOSSES

42.9 % - TOTAL SEASONAL LOSS

CALCULATED SEASONAL EFFrCIENCY = 57.1 %

h'iGURE 10. Typical seasonal perjormance oj a cas-fired furniue

[7].

output with results obtained from experiments ( (ni- ducted in the laboratory and iieki. Once thi> process is completed, sucli a model could he a powerful tool for determining the average seasonal

efficiency of different types of fossil-fuel li'>aling

equipment.

Recognizing the problems involved uilli all I lie studies which have been conducted on the scaxnial

efficiency of fossil-fuel residential heating e(|uip- ment. it seems safest to conclude that: (al there is likely a wide spread in seasonal efficiencies even among the same types of heating equipment and lb) there probably exists a large number of gas and oil- fired residential furnaces and boilers which have

seasonal efficiencies in the 50 to 60 percent range. Assuming this to be true, the estimated seasonal

performance of the heat pump used in this study was equivalent to the performance of many residential gas- and oil-fired heating systems.

6. Discussion

The effect of part-load operation on the perfor- mance of the Bowman house heat pumi) was found

to be greatest when the heat ()ump oijerated in the cooling mode. At a cooling load factor of 0.30. the

ratio of the heat pump's COOLING COP to the manufacturer's steady-state COOLING COP was

reduced to approximately 82 percent of the measured steady-state value (see fig. 4): at a heating load

factor of 0.30, the ratio of the HEATING COP to the manufacturer's steady-state HEATING COP

was roughly 94 percent of the measured steadystate value (see fig. 9). A large part of the

difference in cooling and heating performance at part load was due to the fact that the heat pump

was located in the basement and. in the heating

mode, the heat contained in the indoor unit would

tend to be transferred to the interior living space b\ natural convection during the OFP^ cycles. In the cooling mode, however, the coolness was trapped in the indoor unit during OFF-cycles and most of it was eventually .transferred to the basement. The

11
situation might have been reversed, with the

HEATING COP being more affected by part-load

operation, if the indoor unit had been located in the

attic. In the heating mode of operation, the largest

adverse effect on the performance of the Bowman

house heat pump was due to frost buildup on the outdoor coil and the resulting requirement for defrosting this coil. For outdoor temperatures below

40 °F (4.4 °C), the average ratio of HEATING COP

to manufacturer's steady-state HEATING COP in figure 9 was 77 percent of the measured steadystate value in figure 8 which does not contain the

effect of frost buildup or defrosting. This result would tend to indicate the need for research to develop improved defrosting methods and to deter- mine the effect of different coil designs upon heat

pumps performance in the high frost region.

The seasonal performance factor or SPF of the

Bowman house heat pump was estimated to be 1.74 using the results presented in figure 9 and weather

data which were representative of the Washington,

D.C. area. This turned out to be approximately 19 percent lower than the SPF calculated using the

manufacturer's performance data. This difference

in seasonal performance was due to the effect of part-load operation, frost buildup and defrosting.

When the estimated seasonal heating perfor- mance of the Bowman house heat pump was traced

back to the power plant, it was found that only

about a half - unit of heat energy was actually

delivered to the interior living space for every unit of energy consumed at the power plant. This is comparable to the probable performance of many

gas- and oil-fired residential heating units. If this estimated seasonal performance of the Bowman

house heat pump is representative of residential

heat pumps in areas having heating seasons similar

to Washington, D.C, there does not appear to be

any clear-cut advantage to choosing either a heat

pump or a fossil-fuel heating system in these areas

for the purpose of saving primary source energy.

7. References

[1] Newman. Dorothy K. and Day, Dawn, The American Energy Consumer (BaUinger Pubhshing Co., Cambridge, 1975).

[2] Burch, D. et.al.. Experimental Study of Retrofitting Existing

Residences, (to be published).

[3] ASHRAE Handbook of Fundamentals. 1973 (ASHRAE,

N.Y., 1972), Page 440.

[4] Method of Testing for Rating Unitary Air Conditioning and

Heat Pump Equipment. ASHRAE Standard 37-69, (Ameri- can Society of Heating, Refrigeration and Air-Conditioning

Engineers, New York, 1969).

[5] ASHRAE Handbook of Fundamentals, Chapter 21, 1973 (American Society of Heating, Refrigeration and Air- Conditioning Engineers, New York 1972).

[6] Hise, E.C., Seasonal Fuel Utilization Efficiencies of Residential Heating Systems, Oak Ridge National Laboratory

Report ORNL-NSF-EP-82.

[7] Bonne, U., and Johnson, A.E., Thermal Efficiency In NonModulating Combustion Systems, Proceedings and Adden- dum to the Proceedings of the Conference on Improved

Efficiency in HVAC Equipment and Components for Residential and Small Commercial Buildings, Purdue University, October 7-8, 1974.

12
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBS BSS-93

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Dynamic Performance of a Residential Air—to—Air Heat Pump

5. Publication Date March 1977

6. Performing Organization Code

7. AUTHOR(S)

George E. Kelly and John Bean

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Worlc Unit No. 462 4150

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Federal Energy Administration (FEA)

Washington, D. C. 20k6l

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Final Report

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15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 76-50050

16. ABSTRACT (A 200-word or less factual summary of most si^ificant information. If document includes a significant

bibliography or literature survey, mention it here.)

Information is presented on the dynamic performance of a 5-ton air-to-air heat pump,

which was installed in a residence in the Washington, D.C. area. The effect of

part-load operation on the heat pump's COOLING and HEATING coefficients of performance

(COP) was determined. When the pump operated in the heating mode at outdoor

temperatures below 40°F (4.4°C), a considerable discrepancy was found to exist

between the measured performance and the performance data supplied by the

manufacturers. This discrepancy is apparently due to the adverse effects of

frost buildup and defrosting of the outdoor coil. The seasonal performance factor

(SPF) of the heat pump was estimated and then traced back to the power plant

to obtain an "EFFECTIVE SPF" which is then compared with the performance which

might be expected from fossil-fuel heating equipment.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Cooling and Heating Coefficients of Performance; Effective Seasonal Heating COP;

Heat Pumps; Heat Pumps and Energy Conservation; Part-Load Performance; Seasonal

Performance Factor.

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