Bryant T874 Manual

A SERVICE OF

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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

60-2485—828

the thermostat. A set of contacts on the emer

g

enc

y

heat rela

y

is used to b

y

pass each outdoor thermostat so that all electric

heat is under immediate thermostat sta

g

e-two control. So the

emer

g

enc

y

heat rela

y

simpl

y

b

y

passes the outdoor

thermostats. See Fi

g

. 41.

In the

second

method, the emer

g

enc

y

heat relay is cycled

directl

y

b

y

the thermostat mercur

y

bulb. In this hookup, the

rela

y

is controlled b

y

the thermostat

first stage

. The

emer

g

enc

y

heat rela

y

ener

g

izes all of the auxiliar

y

heat rela

y

s

on a call for heat. See Fi

g

. 42.

So the emer

g

enc

y

heat is controlled b

y

the first sta

g

e of the

thermostat. The emer

g

enc

y

heat rela

y

ener

g

izes all of the

electric strip heaters so the heat and the indoor fan come on

to

g

ether as soon as the first sta

g

e calls for heat.

With the first method, the fan remains on

(

bein

g

controlled b

y

the first sta

g

e of the thermostat

)

and the electric heaters c

y

cle

on and off with the second sta

g

e.

In either case, a subbase indicator li

g

ht is turned on whenever

the function switch is in the emer

g

enc

y

heat

(

EM.HT.

)

position.

Fig. 41. Emergency heat relay that bypasses

the outdoor thermostats.

Fig. 42. Emergency heat relay that energizes all of the

auxiliary heat relays on a call for heat.

Crankcase Heat

In an

y

mechanical refri

g

eration s

y

stem, refri

g

erant tends to

mi

g

rate to the coldest part of the e

q

uipment and condense

there. In a heat pump, this coldest spot could be the compressor

because it is usuall

y

outdoors. In addition, the oil in the com-

pressor crankcase is capable of absorbin

g

a hi

g

h concentration

of refri

g

erant. On compressor startup, there are two risks:

(

1

)

the

li

q

uid refri

g

erant, which is noncompressible, can be drawn into

the c

y

linder, and

(

2

)

considerable oil can be entrained in the

refri

g

erant and swept out of the crankcase throu

g

h the

chan

g

eover valve and dispersed in the pipin

g

and coils.

So the mi

g

ration of refri

g

erant to the crankcase is undesirable

because it can interfere with s

y

stem lubrication and because the

li

q

uid refri

g

erant must be kept from

g

oin

g

throu

g

h the compressor.

Addin

g

a crankcase heater creates enou

g

h temperature

differential to prevent this mi

g

ration because the compressor

is no lon

g

er the coldest part of the s

y

stem so refri

g

erant does

not mi

g

rate to it.

Compressor Fault Relay

Another ver

y

simple method of detectin

g

a compressor failure

has been developed. It uses the volta

g

e developed across the

capacitor in a permanent split capacitor compressor motor.

The principle involved is that virtuall

y

all serious problems

reduce the volta

g

e across the motor capacitor. B

y

carefull

y

selectin

g

the correct coil volta

g

e, the normall

y

closed rela

y

does not pull in unless the proper volta

g

e builds up across the

capacitor, which brin

g

s on the SERVICE LED. See Fi

g

. 43.

Here is how it works:

• On a call for coolin

g

, the thermostat makes R to Y, the

contactor coil is powered, and the compressor motor turns on.

• At the same time, the indicator li

g

ht is ener

g

ized.

• As the motor

g

ets up to speed—about one second, the

fault rela

y

sees enou

g

h volta

g

e to pull in. Its normall

y

closed contact opens and the indicator li

g

ht

g

oes out.

• An

y

thin

g

that prevents the motor from runnin

g

prevents the

fault rela

y

from pullin

g

in.

Fig. 43. Compressor fault relay schematic.

M5833

COMPRESSOR

THERMOSTAT

FUNCTION

SWITCH

HEAT 2

HEAT 1

EMERGENCY HEAT

RELAY CONTACT

EMERGENCY

HEAT RELAY

AUXILIARY

HEAT RELAY 1

AUXILIARY

HEAT RELAY 2

OUTDOOR

THERMOSTAT

M5832

COMPRESSOR

THERMOSTAT

FUNCTION

SWITCH

HEAT 2

HEAT 1

EMERGENCY

HEAT RELAY

CONTACT

24 V

EMERGENCY

HEAT RELAY

AUXILIARY

HEAT RELAY

RISE

L1

(HOT)

L2

M5831

COMPRESSOR MOTOR

FAULT RELAY

YR

L

X

LED

CAP.

RUN

START

S

R

C

L1

L3

CONTACTOR

THERMOSTAT-

SUBBASE