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W
R
G
C
Y
W
R
G
Y
Standard Furnace Interlock Wiring
THERMOSTAT
TERMINALS
FURNACE
24-VOLT
TERMINAL BLOCK
FOUR
WIRE
TWO WIRE
heating only
TWO
WIRE
COOLING SYSTEM
W
R
G
C
Y
W
R
G
Y
Alternate Furnace Interlock Wiring
THERMOSTAT
TERMINALS
FURNACE
24-VOLT
TERMINAL BLOCK
FOUR
WIRE
TWO WIRE
heating only
TWO
WIRE
COOLING SYSTEM
WIRE JOINT
WIRING DIAGRAM
(CONT'D)
WIRING DIAGRAM TO
FURNACE
FOR A FURNACE
CONNECTION TO
A COOLING SYSTEM:
On some newer furnaces and older
thermostats, energizing the R and
G terminal at the furnace has the
effect of energizing the Y at the
thermostat and thereby turning on
the cooling system. If you identify this
type of thermostat, you must use the
“Alternate Furnace Interlock Wiring”
Standard Accessory Control Contact
Alternative Accessory Control Contact
As per building codes and installation requirements for combustion appliances:
Air return ducts, or openings for air return, should not be placed in enclosed spaces containing combustion
appliances that are subject to spillage.
