NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

421.00-20-4212430 19 May 1966 9324

INVESTIGATIONS OF

AIR-TO-AIR HEAT PUMPS FOR MILITARY HOUSING

by

Paul R. Achenbach

NOT FOR PUBLICATION

Presented at the

Annual Meeting

of the

Society of American Military Engineers

IMPORTANT NOTICE NATIONAL BUREAU OF STA

for use within the Government. B and review. For this reason, the whole or in part, is not authoriz Bureau of Standards, Washingtor

the Report has been specifically p

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

accounting documents intended

ubjected to additional evaluation

isting of this Report, either in Office of the Director, National

he Government agency for which

lies for its own use.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
lAworr
INVESTIGATIONS OF

AIR-TO-AIR HEAT PUMPS FOR MILITARY HOUSING

By

Paul R. Achenbach

ABSTRACT

The National Bureau of Standards has performed laboratory and field

studies of electric air-to-air heat pumps as a part of the technical

investigations sponsored by the three agencies of the Department of Defense

These studies have developed information on the coefficient of performance and heating and cooling capacities of typical systems used for

residential applications, design data useful for estimating energy usage

and maximum power demand for similar installations, some information

relative to the unexpectedly high rate of motor-compressor failure, and

comparisons of the annual cost of year-around air conditioning using

heat pumps and gas heating systems combined with conventional air conditioners for cooling. The annual cost of heating and cooling these

military houses with air-to-air heat pumps averaged about $100 for a unit electric energy cost of 0.8?f/Kwh.

Key Words: air-to-air heat pumps, residential air conditioning,

energy usage, annual energy cost.
'
INVESTIGATIONS OF

AIR-TO-AIR HEAT PUMPS FOR MILITARY HOUSING

By

Paul R. Achenbach

Tiie U. S. All Force began using, air-to-air heat pumps for yeararomul air eondit Inning of family housing J n about 1 937. Approximately

10,000 units have been Installed at eight air bases in the southeastern

states, ranging from North Carolina to Florida, and from the east coast

to Arkansas. Most of these houses were of two or three bedroom design,

either detached or in duplex relationship. They were typically of one- story construction with a slab-on-grade concrete floor, frame or brick

veneer sidewalls, with insulation used in the wall and ceiling con- struction. The floor area of most of these houses ranged from 1000 to

1200 sq. ft. with a few as large as 2000 sq. ft.

An air-to-air heat pump contains the same basic refrigeration circuit as the conventional window air conditioner; that is, it has a motor-compressor unit, two finned heat exchangers, and a valve or other

device for controlling the rate of refrigerant flow in the pipe circuit.

The arrangement of the apparatus in an air-to-air heat pump is illustrated

in Fig. 1. During summer operation the unit functions exactly like a conventional air conditioner designed for cooling only. The indoor

heat exchanger absorbs heat from the indoor air and using the refrigerant as a heat transfer medium, this heat is discharged outdoors through

the other heat exchanger. During winter operation the functions of

the two heat exchangers are reversed by means of a four-way valve,

whereupon the outdoor heat exchanger absorbs heat from the outdoor

air and discharges it as warm air from the indoor heat exchanger. All

of the apparatus in an air-to-air heat pump can be packaged in a single unit like the 3- and 3-ton package units commonly used in

restaurants and stores. However, in most of the Air Force installations, the apparatus was divided into two units. In such a split

system the compressor, one heat exchanger, a blower, a flow control

device, and the four-way valve are enclosed in a cabinet mounted on

a base outdoors near the house, whereas the other heat exchanger, a blower, a flow control device, and usually some auxiliary heaters are enclosed in a cabinet, mounted in the attic or a utility closet, and

connected into a duct system for air distribution. The two parts of

the split system are connected by two copper refrigerant lines, 23-feet

long more or less depending on the location of the units, to complete

the refrigerant circuit. The split system was new in 1937, and the

Air Force installations were among the first commercial applications.

1

The National Bureau of Standards has performed a variety of laboratory

and field studies of air-to-air heat pumps as a part of the technical investigations sponsored by the three agencies of the Department of Defense.

These have included:

(a) Laboratory testiTof stock models of heat pumps to determine

compliance with purchase specifications and to provide better

understanding of the technical characteristics of air-to-air

heat pumps

(b) An analysis-^f electric energy usage and electric power demand

data for 16 houses at Little Hock Air Force Base equipped with

heat pumps, water heaters, cooking ranges, clothes dryers, and

miscellaneous devices, all operated by electricity.

3/

(c) Field studies of the heating performancer~of the air-to-air heat

pumps in five representative houses at each of two air bases.

(d) Field studies of the annual energy usages in five houses at

each of three air bases in which year around air-conditioning

was provided by air-to-air heat pumps, and an equal sampling

of houses and sites where electric air conditioners were used

for summer cooling and gas was used for house heating, water

heating, and cooking.

(e) Laboratory analysis of the causes of compressor failure in the

field and of certain preventative service practices for alleviating this problem.

(f) Drafting of more adequate specifications for air-to-air heat

pumps

Some of the design problems that have been investigated during

these studies are:

(1) Selection of the proper balance point and the correct amount

of supplementary electric resistance heating for a given application,

(2) Calculation of the design heating load for residential buildings.

(3) Maintaining a suitable coefficient of performance in actual use.

(4) Obtaining the proper relation between the surface areas in the

two heat exchangers and in the two capillary tubes.

(5) Providing adequate defrosting.

(6) Selection of controls for defrosting, activation of supplementary heaters, and overload protection.

2
*
Some of the installation problems found during these studies were:

(1) Accessibility of the Indoor units in attic installations.

(2) Air leakage of the duct system.

(3) Location of the Indoor thermostat.

Various operational and technical problems occurred, such

as

(1) Frequency of failure of corapreasor-raotor unit.

(2) Estimation of total energy usage.

(3) Control of maximum electric demand.

(4) Economic comparison of all electric heat pump systems with

gas-electric combinations for year around air conditioning.

The investigations of a few of these problems by the Bureau will be

discussed and some examples given.

A characteristic of a compression-refrigeration system is that the

capacity of the system decreases as the temperature difference between

the indoor and outdoor environments increases. This is approximately

a linear relationship. The design indoor-outdoor temperature difference rarely exceeds 2j 1 during summer operation of an air-toair heat pump, but may be in the range rrom 40”F to 70°F during winter

operation in the geographical areas mentioned above. Hence the heating

capacity of the heat pump is Lower than the cooling capacity at the

respective design conditions and the heating capacity becomes decreaslngly

lower as the heating requirement Increases. The outdoor temperature at

which the heating capacity of a heat: pump exactly equals the heat loss

of the house Is called the "balance point." The balance point for heat

pump Installations In res ldenc.es is usually selected at a temperature

higher than the design winter outdoor temperature, and one or more electric

resistance heater elements are installed In the Indoor unit to provide the

additional heating capacity needed during the short periods of cold

weather

The laboratory data illustrated in Figure 2 shows that the re- lation of heating capacity to outdoor temperature was linear for units

employing expansion valves, but that the capacity drooped at higher

outdoor temperatures for units equipped with a capillary tube as a refrigerant flow control. The heating capacity at an outdoor temperature of 20°F ranged from 55 to 73 percent of that at an outdoor temperature of 50°F, whereas the heating load vould be at least twice

as great at the lower temperature. This graph shows why most airto-air heat pumps incorporate supplementary resistance heaters in

the indoor unit to augment the compression system during the coldest

3

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. des ign and , me of in 4 d Wor^n,anship
\
houeesX^XXlXT T

d throJ«ho« the year m each of f,

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ie

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at

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aumed by the heat pump and bathroom heater fHrOUKhout the /«« was cZ- or water heating, cooking, clothes drvin ’ ^

** Cqual amount was used The energy used for cooling was ahr, , yi g and ml8 ceIlaneous devices heating. A little more thfn" fo^th ° f the total used for

0*8*/TM th

lnCe the avera8 e cost Of energy at thef T

^

^

tHe Water 0.8,/KWH the annual cost for heating anTX^^^

Indicated an e^rg^age^X/fTthe h and

& h°U" eS at LUtle fi “ck Air Base heating oPeratlon of 2.1 KWH/deeree , hea t pumP during both cooling when the degree-daya were relX tHeX/ °°° ^ ft ° f area? both seasons . The corresponding fL,„r" ° "

temperature of 65-? In o her air bases averaged 2 4 and 17

^ uring heating operation at two

hXeXYsTT1 "8 --g;

4

re

a

q

nulr;me„:r?:r

Ct

hr::

y

o ^eae figures J7

*£

ouses of similar construction ar>A or neat pumP a PPlications in

UonXX tlon,

t0 “^h^XM ZXTlk thXX -Id and 1 tying habits of the occupies! h°U8e ' degrea °f InsulaAt many of the air bases where hear

the

eleatrlc energy depended not o»l! on "“T®

U8ed ' the unlt c °»t the maximum demand of the whole housing area d ?

—thly usage but on dLaXX Perl°d - A detailed analysis of 8lngU ^-minute and ln flve sample houses at Lirt-il d , * th dally pattern of power monthly maximum demand was not dlllctiv T*

A

j

lr

, Ba“ e revealed that the during the coldest hours of the 24 hi

by the hea“ng equipment about by the use of other el.^r^U^n”" t’

^ ^

bro“h t during their dslly activities In

**«U ‘P—•*: by the occults 5-mlnute demand throughout the day for a in ?

lgUre 6 shoWB the average February for a 3-bedroom house aS thl ?

dy pCrlod ln January and demands for any one day of the

and 15-.aln.it. a month fn August under cooling operatio^?

1"^ ? Sh°WB Slmllar data during

i fi ur6 6 showH f-u n f . 1 quickly about 8; 00 a.m. and\elained power demands rose - 3 wTaT: 1— ii « ^VnTgt— izi: -•

m°nth--- “

bTnotd - ^^rss. “* **" ^

average ^or^he^hole^onth . 006

^

^stics similar to the^int^ pattern^

? f°r * 8uimner ®onth has character- emand i n the middle of the afternoon

therC 00 peri°d of low because the high solar load in the Tf,

& opera tion, probab^ operation of the heat pump.

ternoon required fairly steady

5
'
TABLE 1

AVERAGE ANNUAL ENERGY USED IN SAMPLE HOUSES AT

THREE AIR BASES

Component of Load

Total

Energy Used

Percent

of Total

Heat Pump

Cool ing

KWH

5,170 19.7

Heating, Compressor 5,830 22.2

Heating, Supp. Heaters 1 , 440 5.5

Total 1 2, 440 47.4

Bathroom Heater 780 3.0

Water Heater 7,230 27.5

Cooking Range 1 ,1 00 4.2

Clothes Dryer 1 ,200 4.6

Miscellaneous 3,490 13.3

TOTAL 26,240 100.0

6

The maximum electric power demands for the 16 individual houses

studied at Little Rock Air Base averaged about 14 KW in the summer and

about 17.5 KW in the winter, with the heat pump, water heater, and

miscellaneous devices being the principal contributors to these maximum demands. These results indicate that a programming device which

reduced the degree of coincident operation of the various appliances

would lower the maximum demand and thereby the average unit energy

cost. Of the many mechanical -el ectrical devices available for programming a group of component loads in a house, it appeared that a relay which permitted one or more appliances to be energized only if

the load already energized was below some selected value offered the

best possibility for reducing the maximum power demand with least

inconvenience. This type of control is now being installed on a pilot basis in a group of houses at one air base to explore its

effectiveness and the potential inconvenience to the occupants.

Information obtained from the engineering offices at the various

air bases during and since the field studies of heat pump performance

showed that the replacement of motor-compressor units in the air-toair heat pumps was much higher than was anticipated by the manufacturers.

Data for a 6-month period in 1965 showed that the failure rate of motorcompressor units in the heat pumps at eight air bases ranged from 5 to

10 percent of the total number of installations at each air base. Similar records for the motor-compressor units in conventional air conditioners used in U. S. Air Force housing projects ranged from 4 to 10

percent during the same period. Thus it appears that the failure

rate was not significantly different among contemporary heat pumps and

air conditioners.

In order to investigate the mode of failure of the motor-compressors

in air-to-air heat pumps thirty-five hermetic units that had been re- placed at 7 air bases were cut open and inspected at the National Bureau

of Standards. The 5 compressors from each air base were a group that were received in sequence at the repair shop after receiving the re- quest from the Bureau for units that had failed. No internal inspection

was performed at the air bases. The 35 specimens comprised 8 models

from 4 manufacturers. The following kinds of defects or failures were observed. Some units exhibited more than one defect.

Burned and/or grounded windings 467,

Broken wires and burned control contacts 207. Bearings, connecting rods, cylinders worn,

scored or seized 347,

Other mechanical defects 97. Undetermined defect 37.

7

This summary indicates that: 66% of the specimens revealed

electrical detects and 43% revealed mechanical defects. However,

these data do not establish the basic cause of failure. About half

of the units with burned or grounded windings revealed general

charring oi the electrical insulation whereas the other half showed

only localized overheating.

Other laboratory work that has been carried out relative to

correction of the motor-compressor failure problem include an investigation of the properties of the compressor oil as an indicator

of impending failure, the development of reliable field methods for

removal of moisture and air from the refrigerant circuit during installation and repair, and the use of a liquid accumulator at the

compressor inlet to prevent liquid refrigerant from entering the

cylinders. The acceptance tests of commercial models of heat pumps

employing capillary tubes as refrigerant flow controls revealed

that liquid refrigerant consistently entered the compressor

during heating operation at outdoor temperatures of 20°F or lower.

The study of the causes of compressor failure is continuing at the

Bureau and extensive consultations with manufacturers are in progress

to provide the basis for revising and strengthening the specification

requirements for the system design of heat pumps. The manufacturers

of air-to-afr heat pumps are now fully aware of the high incidence of

motor-compressor failure and are seeking causes and cures for this

problem.

In the course of implementing the air defense of the country , the U. S. Air Force has installed air-to-air heat pumps at 8 air

bases and a combination of a gas heating system and an electric airconditioning unit at a number of other sites to provide year around

air conditioning of family housing. The choice of system has been

based on economic factors of first cost, operating cost, and maintenance cost even though it is generally recognized that sufficient

reliable data do not exist for satisfactory decision-making.

Against this background the National Bureau of Standards was asked to make field measurements of the annual energy usage in fivehouses at each of six air liases, half of which employed all -electric

air-to-air heat pumps and the other half gas heating and electric

cooling systems. Figure 8 is a comparison of the actual energy used

for heating, cooling, water heating, cooking, clothes drying, and

miscellaneous purposes at each of the six air liases. The energy

is expressed in both kilowatt hours and therms and both vertical

energy scales apply to all the bar graphs. This figure shows that

the energy used for heating with a gas furnace is much higher than

that used by an electric heat pump. LLkewise, the energy used by

the gas water heaters and gas ranges exceeded that used by the

corresponding electric appliances by an appreciable anount. On the

other hand the energy used for cooling, for clothes drying and for

miscellaneous purposes was about equal in the two groups oi houses.

The legend of Figure 8 shows that there were some differences in

floor area, and in cooling and heating degree-days at the various

air bases, and a considerable variation in the unit costs of gas

and electric energy.

In order to make the best economic comparison possible with

the available data the actual energy costs at the six air bases were adjusted for a floor area, and heating and cooling degree-days equal

to the average of all sites in direct proportion. Likewise, the

unit energy cost for the all-electric houses was adjusted to an average value for the three bases using heat pumps, and for the

other- group of houses, to the average cost of gas and electricity,

respectively, at the three bases using the combination system.

A comparison of these adjusted values is shown in Figure 9. The

average adjusted annual energy cost for the 15 sample houses em- ploying gas-electric combination systems was 90 percent of that

for the 15 sample houses employing air-to-air heat pumps. Figure 9

also shows that the cost of water heating was significantly less

with gas than with electricity and the cost for cooling and miscellaneous devices was appreciably higher at the sites employing

gas furnaces simply because the unit cost of the electric energy

was higher at these sites where it was used only for cooking,

lighting, and clothes drying.

It should be pointed out that the comparisons made in Figure

9 do not account for variations in degree of insulation, amount

of window area, infiltration rates, solar radiation, wind velocity,

and living habits that may have existed for the two groups of houses

that are being compared.

Not all of the significant investigations and useful resul ts on air-to-air heat pumps that have been developed during this program could be described in this brief review. Additional information is available in the published papers referenced. There is

little doubt that the U. S. Air Force has contributed significantly

to the development of fully air-conditioned military housing of

good quality In which the total annual utility cost is moderately

low and that the National Bureau of Standards has assisted the

defense agencies in becoming informed purchasers. This program

has also served to challenge the heating and air conditioning industry to meet its responsibilities in design, manufacturing, and

installation practices in residential construction.

9

REFERENCES

1/ Dnv I s , J. 0. and Achenbach , P. II. Performance Characteristi cs of Split-Type Residential Air-to-Air Heat Pumps, Suppl . Bui. HR

Annexe 1962-1, 349-355 (1962).

2 ! Achenbach, P. R. , Davis, J. C. and Smith, W. T., An Analysis of

Electric Energy Usage in Air Force Houses Equipped with Air-toAir Heat Pumps, NBS Monograph 51 (1962).

3/ Achenbach, P. R. , Phillips, C. W. , and Smith, W. T., Heating

Performance of Air-to-Air Heat Pumps at Two Air Force Housing

Projects, Trans. ASHRAE 70, 390 (1964).

10

INDOOR

UNIT

OUTDOOR

UNIT

FLOW

DIAGRAM

FOR

HEAT

PUMP

TESTS
OJ

c>

u.

fa

4-J O o o

r-~ u m t4 o o

u a) o xi

QJ

M-l X

<4-1 *H W W

3mVA 3DN3d333d / AllOVdVD 9NI1V3H Q3Aa3S90

OUTDOOR

TEMPERATURE
lN3Da3d‘ 3WI1 9NI±Vd3dO dWfld 1V3H

Fig. 3 Operating time and balance point for the heat pump in a 3-bedroom

house at Seymour Johnson Air Base.

OUTDOOR

TEMPERATURE

,
in

<r e>

Ll.

o

<uu

3

4-1 4J

to C

u «

UJ u

&> mG

O

o

(Uu

QJ JSH

> J-4 4-1 O

O

i-i "d o G

<4-1 v4

30NVI/\IU03d3d 30 -LN3I01 33300

OUTDOOR

TEMPERATURE
30NVWy0dH3d 30 1N3IDIJ3300 dWfld 1V3H

‘ dOlOVd 30NVWd03d3d IN31SAS

Fig. 5 Heat pump coefficient of performance and system performance

factor for 2-bedroom house at Seymour Johnson Air Base for

a range of outdoor temperature.

OUTDOOR

TEMPERATURE
O®«)5£lO<S0'lD ,*'t'J O

Fig.

6

Daily

pattern

of

power

demand

in

a

3-bedroom

house

at

Little

Rock

Air

Base

for

the

period

Jan.

8

to

Feb.

8,

1960
Fig.

7

Daily

pattern

of

power

demand

in

a

3-bedroom

house

at

Little

Rock

Air

B

a

se

for

the

period

Aug.

7

to

Sept.

8,

1959.

ENERGY

USAGE

MAY,

I960

THROUGH

APRIL,

1961

AVERAGE

OF

FIVE

HOUSES

EACH

AT

SIX

AIR

BASES l|s

Ul *

CO 2• oc «oi-l

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UJ la. <¥

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10

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cm cm cm cm £2 cm

to CM CM <2 10

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£EO

CL< <S2OC0!Il

TOTAL

HEATING

WATER

COOLING

MISC.

RANGE

DRYER

HEATER

FlfL 8

ADJUSTED

ENERGY

COST

MAY,

I960

THROUGH

APRIL,

1961

AVERAGE

OF

FIVE

HOUSES

EACH

AT

SIX

AIR

BASES

o O O o o o

o m O in o m

ro C\J CM Sdvinoa Nl IS 00

o

TOTAL

HEATING

WATER

COOLING

MISC

RANGF

DRYER

HEATER

FIG.*

- *