Honeywell T874 Crankcase Heat, Compressor Fault Relay

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

60-2485—8 28

the thermostat. A set of contacts on the emergency heat relay

is used to bypass each outdoor thermostat so that all electric

heat is under immediate thermostat stage-two control. So the

emergency heat relay simply bypasses the outdoor

thermostats. See Fig. 41.

In the second method, the emergency heat relay is cycled

directly by the thermostat mercury bulb. In this hookup, the

relay is controlled by the thermostat first stage. The

emergency heat relay energizes all of the auxiliary heat relays

on a call for heat. See Fig. 42.

So the emergency heat is controlled by the first stage of the

thermostat. The emergency heat relay energizes all of the

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

together as soon as the first stage calls for heat.

With the first method, the fan remains on (being controlled by

the first stage of the thermostat) and the electric heaters cycle

on and off with the second stage.

In either case, a subbase indicator light is turned on whenever

the function switch is in the emergency 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 any mechanical refrigeration system, refrigerant tends to

migrate to the coldest part of the equipment and condense

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

because it is usually outdoors. In addition, the oil in the com-

pressor crankcase is capable of absorbing a high concentration

of refrigerant. On compressor startup, there are two risks: (1) the

liquid refrigerant, which is noncompressible, can be drawn into

the cylinder, and (2) considerable oil can be entrained in the

refrigerant and swept out of the crankcase through the

changeover valve and dispersed in the piping and coils.

So the migration of refrigerant to the crankcase is undesirable

because it can interfere with system lubrication and because the

liquid refrigerant must be kept from going through the compressor.

Adding a crankcase heater creates enough temperature

differential to prevent this migration because the compressor

is no longer the coldest part of the system so refrigerant does

not migrate to it.

Compressor Fault Relay

Another very simple method of detecting a compressor failure

has been developed. It uses the voltage developed across the

capacitor in a permanent split capacitor compressor motor.

The principle involved is that virtually all serious problems

reduce the voltage across the motor capacitor. By carefully

selecting the correct coil voltage, the normally closed relay

does not pull in unless the proper voltage builds up across the

capacitor, which brings on the SERVICE LED. See Fig. 43.

Here is how it works:

• On a call for cooling, the thermostat makes R to Y, the

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

• At the same time, the indicator light is energized.

• As the motor gets up to speed—about one second, the

fault relay sees enough voltage to pull in. Its normally

closed contact opens and the indicator light goes out.

• Anything that prevents the motor from running prevents the

fault relay from pulling 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

M583

COMPRESSOR

THERMOSTAT

FUNCTION

SWITCH HEAT 2 HEAT 1

EMERGENCY

HEAT RELAY

CONTACT

24 V

EMERGENCY

HEAT RELAY

AUXILIARY

HEAT RELAY

RISE

(HOT

)

M5831

COMPRESSOR MOTOR FAULT RELAY

YRLX

LED

CAP.

RUN

START S

ONTACTOR THERMOSTAT-

SUBBASE