Never connect a 120 volt AC circuit to the terminals of the furnace
interlock (Standard Wiring).
Only use the low voltage class 2 circuit.
/fan-coil/heat pump
W A R N I N G
!
!
On some older thermostats, energizing the R and G terminals at the furnace has the effect of energizing Y at the
thermostat and thereby turning on the cooling system. If you identify this type of thermostat, you must use the
“Alternate Interlock Wiring”.
For a furnace connected to a cooling system:
Wiring Diagram
(cont’d)
5
Furnace / Fan-Coil / Heat Pump Interlock:
Standard Interlock Wiring
INTERLOCKING
RELAY
W
R
G
Y
W
R
G
Y
C
FOUR WIRE
TWO WIRE
heating only
Cooling
System
FURNACE / FAN-COIL / HEAT PUMP
24-VOLT TERMINAL BLOCK
THERMOSTAT TERMINALS
TWO WIRE