Installation practice and reliability of add-on heat pumps

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Installation practice and reliability of add-on heat pumps

Chauhan, R. B.

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BUILDING

PRACTICE

NOTE

INSTALLATION P R A C T I a AND RELIABILITP OF ADD-OM EEAT PUMPS

w

R.B. Chauhan

D i v i s i o n of Rullding Research, National Research Council of Canada

INSTALLATION PRAC'FICE AND RELIAR ILITY OF

ADD-ON

by

R.B. Chauhan

The performance of heat pumps has been evaluated from the Installation

practice and r e l i a b i l i t y viewpoint. The heat pumps are commercially

available units InstaZled by local contractors in homes built in the Ottawa

region during the period between June 1978 and May 1982. Eighteen

installations were monitored i n 1981-82 and 23 i u 1982-83, Over the two year monitoring period, 0.54 corrective actioas p e r unit year were

required.

With the introduction of the Canadian O i l Substitution Program Grants,

the add-on heat pump (to o i l furnaces) becam an attractive o f f - o i l o p t i o n ,

leading to increased heat pump sales. Thus an information update on the f i e l d performance and r e l i a b i l i t y of add-on heat pumps would be of value t o

heat pump manufacturers and installers, homeowners, fuel suppliers and electric u E i Z L t i e s .

A new houslag development in Orleans, Ontario (near O t t a w a ) provided an

opportunity to monitor the performance of several beat pumps added t o gas furnaces. The findings of t h l s study are applicable to add-on heat pumps i n

general

.

Eighteen heat pump systems were monitored during the winter of 1981-82. SevenLeen u n i t s of t h i s group were ad&ons to conventional gas furnaces and

Ode was an a l l - e l e c t r i c heat pump system. For the winter of 1982-83,

one

This r e p o r t comments an the r e l i a b i l i t y of the heat pumps and notes

some instatlation-related problem, During the course of the monitoring program, some problem areas with add-on heat pumps were fdenttfied; these related t o tnetaflatlon, weather conditions, control wiring c i r c u i t and

thermostat control system,

Indoor Fllrnilce Air F l o w

There were a feu cases where a i r flow requirements were not met- In t h e s e cases fan capacity was adequate; i t : merely required a change of drive

motor from 114 HP t o 113 HP and the appropriate pulley, to achieve the

required a i r f lm- The furnace and the s i z e of ductwork i n s t a l l e d d i d l i m i t

The ductwark i n many of the houses had excessive ait leakage a t

connections to furnace and branch runs. Return air capacity was inadequate

in t w o installations where homeowners complained of b e i n g cool in winter and hot tn summer at the second storey l e v e l .

Many of the systems were equipped with by-pass humidifiers, whgch may

have some detrimental effect on the performance of the heat pump, since they cause a reducLion in air flow o v e r the heat pump indoor c o t 1 ( s e e

Figure 1 ) -

L o c a t i o n of Outdoor Unit

One of the most common problems related t o outdoor unit location was

water run-off from the roof (Figure 2). In many cases, eavestroughs had not been installed by the builder and the unit was located under the d r i p line. Under conditions of freezing r a i n andlor wet snow near the freezing point,

dfscharge grilles were completely i c e d over, choking off a i r flow. In other

cases, large portions of the outdoor c o i l would be covered by i c e , r e s u l t i n g in reduced air flow- This problem was noted only on units with vertical

discharge.

Some heat pumps were completely buried in snow (Figure 3 ) . The

homeowners were instructed t o check t h e i r outdoor u n i t occasionally and to

remove any snow build-up. Units located between two houses, where the passageway was o r l e n t e d In the direction of the prevailing winds, were

s u b j e c t to drifting snow problems. Heat pumps located on the leeward side of he house were not: affected. Xn these cases, t h e mounting stand s u p p l i e d by the contractor or manufacturer raised the unit sufficiently, so that drifting snow was n o t a problem. Units with horizontal c o i l s and

draw-through fans were particularly s u s c e p t i b l e to snow build-up and

blockage ( F i g u r e 4 ) .

Control Wiring

Only one u n i t had problems relating to furnace-heat pump i n t e r f a c e

c o n t r o l , Refore the problem was r e c t i f i e d , whenever the thermostat called f o r second stage heat, a 24V transformer fuse would rupture and d i s a b l e the heat pump, On another occasion the transformer in the gas furnace had to be

replaced. T h e problem was eventually traced to faulty wiring of the factory-supplied control box; t h e manufacturer had experienced sfmilar

problems on other units. T h i s problem resulted in a one month shut-down of

the hear pump.

In general t h e add-on heat pump requires more knowledge on the part of

the installer, with respect to control wiring, than conventional

a l l - e l e c t r i c heat pumps.

Outdoor thermostats

Add-on heat pumps, in many cases, are equipped with outdoor thermostats

t o p r e v e n t heat pump operation below a particular outdoor temperature. Nine

OF the nine system, three were equipped with balance p o i n t * cut-off c o n t r o l s , while t h e others were intended t o allow some combined operation of furnace and heat pump below the balance point.

In most cases, the contractor's original settfngs tended to terminate heat pump operatLon at a higher outside temperature than was necessary. _In

the case of the balance p o i n t cut-off system, heat pump operation was

terminated at an outdoor temperature 4 ro 10 K higher than the c a l c u l a t e d balance point. DurZng the 1981-82 heating season, about 33% of the heating

hours were in the outdoor temperature range between -7°C and -2°C. A lower cue-off setting could no doubt have increased heat pump "on" t i m e and

reduced gas consumpti on.

In a d d i t i o n , durfng the 1981-82 heating season, a number of systems

operated a t temperatures significantly below the set point on the control

(in fact below t h e 1982-83 settings). Thermostats installed on south-facing

walls were more prone t o this problem because of the influence of s o l a r radiation. Although it is perhaps d~fficult i n some cases t o locate

thermostats on north o r north-west facing walls, b e t t e r s h i e l d i n g should reduce the solar influence on the thermostat s e t point.

Plenum Thermostat Cwrrol Settings

The most popular add-on heat pump control in rhe Orleans project was the plenum control. As summarlzed

In

of

Manufacturers B, C and E u t i l i z e d t h e same approach, i . e . , a senstng bulb and capillary (Manufacturers B and C) and a bimetallic T i d t type of control

(Manufacturer E) to turn

off

30 to 35OC, with a differential of approdmately 2 K.

Wo problems related to improperly set plenum controls were encountered in the study, but it 1s essential to'set the controls as instructed by the

manufacturer. I n one series of t e s t s the sensor (sensing bulb and

capillary) lagged considerably behind a i r temperature, up to 20 K (due to

the mass of t h e b u l b ) , One manufacturer pointed out the importance of measurtng the a i r temperature a t

the

temperature entering coil below a c e r t a l n value, Thfe ensures that t h e

presslrre in Lhe refrigeration system does not exceed s pre-set maximum

value, which could lead to campressor problears or nuisance t r i p s of the high pressure s a f e t y limit.

I n one case, the second stage anticipator (controllfng the gas furnace)

was s e t at a much higher value than recommended (i.e., anticipator action

was nonexistent), r e s u l t i n g in wide fluctuations of indoor temperatures durfng furnace operatton. This reduces the hear pump r u n t i m e because the

furnace operating temperature range overlaps with the heat pump operating

range. Thus the contribution t o load the heat pump would normally make is

reduced,

*Balance p o i n t is the outdoor temperature above which the heat pump can

COMPONENT FAILURES AND OTHER CORRECTIVE ACTIONS

From the standpbint of reliability, the results of the current study

indicate a marked improvement over the situation reported from an Ontario

Hydro field evaluation. l r The component failures observed over the

monitoring period arc:

1) Compressor bum-out; replacement compressor had d e f e c t i v e internal relief valve (House fl3);

2) Outdoor fan motor (bad bearfngs) (Wse #7);

3 ) Soli&state control board (l3ouse f7);

4 ) Furnace gas valve failure caused by short in control wiring

(House #8);

5 ) Furnace gas valve (Eouse

81);

6 ) Outdoor fan blade (HOUSE #4).

Table 2 summrlzes the t o t a l correcttve actions taken over the course

of the two heating seasons. fn the two-year period there were 22 corrective a c t i o n s taken by the installing contractor or by NRCC technicians. This

represents 0.54 corrective actions per unit year (conpare with 1.33

corrective actions per unft year from the Ontario Hydro f i e l d evalua tion2

1.

The problem resulting in corrective action =re categorized as follows (frequency of occurrence is shown in brackets):

1 ) installation-related (3)

(locatf on, control wiring, etc. ) 2) design-related ( 13)

(component Eaf lure, factory wiring, weather, etc, )

3 ) omer education ( 5 )

4 ) furnace (1).

T h e _{design-related category was by far the largest source of problems} l e a d i n g to corrective action. The repeated compressor replacements i n

House $13 added three corrective actions and most ' l i k e l y represents an exceptional case. Two of the factorp-supplied replacement compressors were d e f e c t i v e and did not fail as a result of f i e l d application. The problem

seems t o have been corrected following the burn-out of the original

compressor, as no problem were experienced in 1982-83.

The majority of design-related problems

were

outdoor fan motor failure, the cracked fan blade and

t h e

t o turn, discharge g r i l l e covered with ice) are probably unavoidable w i t h an

"operrtop" fan disch'arge design, Even if due consideration is given to location of the outdoor unit away from drip-lines or eaves, under adverse weather conditions thie d e s i g n d l 1 be subject to faa-mator aseembly problems possibly leading t o component failure.

In the owner education category, the most frequent complaint leading t o

corrective action was that of "cold" a i r discharging from the supply air

g r i l l e s . This, no doubt, is due t o the relatively large volumes of air

these conditions heat pump supply a i r temperatures are

fa

from the heat pump, This s i t u a t i o n is aggravated by thermostat droop* (by as much as 1.5 K lowef). The lower telllperature in the house perhaps makes

the homeowner more aware o f the relatively low discharge temperatures. Apparently no one had adequately explained this inherent characteristic of heat pump operation t o the homeowners concerned. Various solutions w e r e

proposed by the installing contractors.

For

emergency heat when it is cold outside; raise the thermostat setting to compensate f o r droop; or modify the solid-state control circuit board to

raise the heat pump cut-off temperature from -23% to -12OC (an irreversible

corrective aetlon I n aame earlier madels of Manufacturer B).

Manufacturers A and

E

compensating therdstors to combat droop, As the outdoor temperature f a l l s ,

the resistance of the thermistor (located in the outdoor u n i t ) increases,

thus reducing the current flow through the room thermostat control circuit.

The amount of heat dissipated by a re'sistur in the thermostat is reduced

under t h i s candition, e f f e c t i v e l y r a i s i n g the control p o i n t or extending t h e

'.on" tfme of the heat pump.

New electronic heat pump thermostats introduced recently should

eliminate the problem of droop and go a long way toward eliminating this frequent complaint from heat pump owners.

CONCLUSION

This p r o j e c t has covered the performance of add-on heat pumps in nearly

new houses in a suburb of O t t a w a . The performance- and reliabiity of the

heat pumps as a heatin system have improved significantly compared to the On tarto Hydra studies. 8 , 2

Qver the two-year monitaring period a total of

0-54

raken during the Ontario Hydro field eval~atfon.~ The f i n d i n g s of this

study suggest that installation and component: r e l i a b i l i t y have Improved significantly over the p a s t few years.

Of the 22 corrective actions, 13 were in the design-relared category.

Component fa-llures, factory w i r i n g and weather-related problems are the matn

areas rovered by this category. The outdoor fan motor failure, the cracked

f a n blade and the numerous problems encountered due to operation under

freeztng rain condftions, only e x i s t with an "open top" fan discharge desfgn. More a t t e n t i o n needs t o be p a i d to the desfgn of the outdoor unit

to a v o i d weather-related operational dtfflculties.

Haay of the homeowners demonstrated a lack of understanding of the

normal operational charactetistics of heat pumps, such as low supply a i r

temperatures, thermostat droop (soon to be eliminated with electronic h e a t pump t h e m s t a t s ) and when t o employ thermostat set-back. The industry as a

whole needs t o explain, as simply as possible, the do's and don'ts to each

heat pump c u s t o ~ r . A number of the operational difficulties could have

-

been avoided i f the owner better understood the characteristics of heat pump system.

Finally, proper heat pump Installation requires a higher degree of technical sophistication and knowledge than the i n s t a r l l a ~ l ~ n of other

heating systems. A heat pump is less forgiving of instqllatfon mfstakes Lhan other comEort conditioning equipment. A mechani~m for ensuring that a

heat pump i n s t a i l a t i o n conforms t o good practice exists in the form of a

CSA Standard "Installation Requirements for AirtrAir Heat Pumps". 3. It is imperative that a l l heat pump installations meet the requireeats of t h i s standard,

REFERENCES

I. E l m s , W.P., Residential Heat Pump Evaluation: Phase I:

-

December 1977.

2. Elms, W.F., Residential Heat Pump Evaluation: Phase IK - Existtng

Homes. Ontario Hydro, Energy Conservation Division, ECD Report

No. 79-3, June 1979,

'3. CSA Standard C273.5-1980, Tnstallatfon Requirements for Air-to-Air Heat Pumps, Canadian Standards Association, Sexdale, 1980.

h n~ n h n n n n A hl-3

y

.

.

.

Table 2

Type of Corrective

Date House NO. Problem Corrective Action Problem Action By

Oct. 8181 L i U n i t not k s e t Hi PR-switch Owner NRCC

operating Power off for 24 hwrs education

Unit aff on Hi Press when

power restored

-

O c t . 2 3 1 8 1 19 Fan blade &moved ice blockage I n s t a l l a t i o n NRCC

tced-f rozen- (location)

unable t o turn

Uov, 13/01 8 Furnace gag

valve burnt -control wiring

Dec. -3181 10 Control wiring

-furnace appears

t o have provided

most a£ heating through fall

Replace gas valve Installation I n s t a l l i n g

-remove factory jumper (control contractor

be tween thermos tat wiring) & WRCC

power terminals

Disconnect w i r e Ins t a l l a t t o n Installing

-not used in add-on (control CORE ra c t or

applicaron wiring)

- - - - - - -

Dec. 10181 11 Outdoor fan Remove ice Design Installing

b l a d e s hitting buildup (ueather contractor

ice buildup related)

Dec. 10181 17 Cold air None M er Installing

discharge at education contractor

g r i l l e s

Dec. 21/81 7 C~mpressor Replace solid-s r a t e Design Installing prevented from control board (component contractor

running by f allure 1

d e f e c t i v e

p rf n te d ciscui t

board

DCC, 21/81 13 Compressor burn- R e p l a c e compressor out ( d e f e c t i v e ( i n s t a l l e d Jan. 29/82) start r e l a y )

Jan. 4/82 8 Service l i g h t h e r turned o f f

on s y s t e m

-

-control wiring contractor

-problem in factory wf ring Design Installing (componenr S??;%&g??Z- f a i l u r e l Deslgn Installing (factory contractor wlr-tng of interface control)

Jan. 4 / 8 2 17 Cold air

discharge a t g r l l l e s Concractoc told owner t o switch to cnergency heat -MR%C told owner t o

leave heat pump on

Owner educar ion

Installing contractor

- . -- - - - - --

Feb, 5/82 1 Gae p i l o t valve Gas p l l o t valve

f a i l u r e replaced

- - -

Furnace Ottawa component Gas

Table 2 (Cont'd)

Correctf ve Actions (1981-82) Cont 'd.

Type of C o r r e c t i v e

Date House No. Problem Corrective Action Problem Action By

Mar. 1/82 7 Outdoor fan Replace outdoor Dea ign Installing

motor failure fan motor (component contractor

- b a r i n g f a i l u r e )

f a i l u r e

Wr. 15/82 13 Replacement Replace compressor Design Installing

compressor had (component contractor

d e f e c t i v e relief failure 1

valve

Mr. 22/82 13 Replacement Order new Design Installing

compressor compressor ( d e f e c t i v e contractor

could nor be component)

started on bench

mr.

complains about temperatures are educat Lon

l o w discharge normal a i r tempera-

ture at grilles

Corrective Actions ( 1982-83)

Nov. 15/82 8 Compressor o f f Reset high pressure Desf gn NRCC

on high switch

-

thermstar replaced

l a t e r )

- - -

-N O ~ . 19/82 4 O.D. fan b l a d e Owner switches to Design Owi~es

cracked emergency heat (defective

-

Jan. 12/83 20 Low discharge Owner instructed contractor Owner I n s t a l l i n g a l r teaperature t a cut resistor an circuit education contractor at g r i l l e board to prevent heat pump

operation below -12'C

Feb. 4 / 8 3 i 5 Discharge grille Remove fce Design NRCC

20 on vutdonr unirs (weather

25 covered with ice r e l a t e d )

following freez-

ing rain storm

Peb. 14/83 25 Owner di8satis- I n s t a l l control

fiad with system t o allow operation performance below balance point (high gas usage)

Design RRCC &

Ine t a l l i n g contractor

Figure 3 Heat pump outdoor unit burled in snow