Bryant T874 Manual

A SERVICE OF

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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

25 60-2485—8

Low Temperature Lockouts

Some manufacturers of heat pumps have previousl

y

controlled

their units so the compressor was locked out below a specified

outdoor temperature. This was done because of the stress

placed on the compressor b

y

the ver

y

cold temperatures, and

the fact that efficienc

y

drops off at low temperatures.

Other manufacturers sa

y

that althou

g

h efficienc

y

is low at cold

temperatures, the problem of startin

g

the cold compressor is

the most critical point. It is more important to keep the

compressor runnin

g

than to shut it off and tr

y

to restart it when

the temperature warms up to +10°F

(

-12°C

)

. Also the

crankcase heater, which can run onl

y

when the compressor is

off, compensates for the lower compressor efficienc

y

. So, it is

e

q

uitable to let the heat pump run even with a coefficient of

performance

(

COP

)

sli

g

htl

y

less than one, rather than to turn it

off and have to run the crankcase heater.

NOTE: To determine the COP of a heat pump, use the fol-

lowin

g

formula:

COP =

Btu Out/Btu we pa

y

for or Btuh Capacit

y

Unit Watta

g

e x 3.413 Btu/Watt

Another factor favorin

g

this control strate

gy

is that newer heat

pump desi

g

ns maintain a level of efficienc

y

even at outdoor

temperatures well below zero. Some brands do not reach

1.0 COP until -25°F

(

-32°C

)

.

Virtuall

y

all manufacturers now let the heat pump run

continuousl

y

rather than lock it out at cold temperatures.

Heat Pumps with Dual Compressors

Sta

g

in

g

residential sized heat pumps is a re

q

uirement that

developed from the need to make heat pumps as efficient as

possible. One of the methods used to improve overall

seasonal efficienc

y

is to reduce the amount of time the

machine is operatin

g

in a transitional mode. Transitional mode

is startin

g

and stoppin

g

in addition to recoverin

g

from defrost.

Under li

g

ht loads, when the heat pump is c

y

clin

g

on and off,

this can be a si

g

nificant amount of time and can result in a

si

g

nificant efficienc

y

reduction. Capacit

y

control is one wa

y

to

reduce the c

y

clin

g

rate and improve the efficienc

y

.

Additional capacit

y

control methods used on other

refri

g

eration s

y

stems

(

unloadin

g

, multiple compressors

)

are

not as likel

y

to be seen on residential heat pumps.

With a sin

g

le-sta

g

e heat pump, the compressor is normall

y

controlled b

y

the first sta

g

e and the auxiliar

y

heat b

y

the

second sta

g

e. The T874G, N and R Thermostats are used for

this purpose.

With a two-sta

g

e compressor, the two thermostat switches

control the individual compressor sta

g

es. Auxiliar

y

heat is

controlled alon

g

with the compressor sta

g

e-two, b

y

the

thermostat second sta

g

e. It comes on with the compressor

hi

g

h speed if an outdoor thermostat—wired in series with it—

is made. See Fi

g

. 35.

Fig. 35. Two-stage heat pump with two-stage thermostat.

The outdoor thermostat is set at the balance point with both

heat pump sta

g

es runnin

g

. So if the heat pump can control the

heatin

g

load down to +20°F

(

-7°C

)

, the auxiliar

y

heat comes

on with the second sta

g

e of the compressor.

In effect, this control strate

gy

chan

g

es the heat input of the

second sta

g

e, dependin

g

on the amount of heat needed.

Additional sta

g

es of auxiliar

y

heat can be added with or

without the control of more outdoor thermostats, See Auxiliar

y

Heat section.

Another wa

y

to control a two sta

g

e heat pump is with a three-

sta

g

e thermostat. The T874W is desi

g

ned for this application

as well as others. See Fi

g

. 36.

The advantage of this system is that no auxiliary heat is used

until there is an actual demand from the controlled space.

Fig. 36. Two-stage heat pump with three-stage

thermostat.

Two speed Heat Pump

One wa

y

to control capacit

y

is to use a two-speed

compressor. The compressor runs at low speed for sta

g

e-one

heatin

g

and coolin

g

. The compressor runs at the low speed

instead of c

y

clin

g

the heat pump on and off under some li

g

ht

load conditions.

The hi

g

h speed is sta

g

e-two coolin

g

. The compressor runs on

hi

g

h speed when loads increase so the low speed can no

lon

g

er handle the coolin

g

load. This reduces c

y

clin

g

rate and

improves capacit

y

.

M5838

THERMOSTAT

HEAT 2

HEAT 1

OUTDOOR

THERMOSTAT

COMP.

STAGE 2

AUXILIARY

HEAT

COMP.

STAGE 1

M5837

COMP.

STAGE 2

THERMOSTAT

HEAT 2

HEAT 1

AUXILIARY

HEAT

HEAT 3

COMP.

STAGE 1