18
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
HRV
ELECTRONIC BOARD
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
HRV
ELECTRONIC BOARD
WIRE JOINT
ELECTRICAL CONNECTIONS (CONT'D)
Caution:
• Never connect a 120 volt AC
circuit to the terminals of the
Accessory Control Contacts.
Only use the low voltage
class 2 circuit of the furnace
blower control.
For a Furnace Connected to a
Cooling System:
• On some newer furnaces and
older thermostats, energiz-
ing the R and G terminals at
the furnace has the effect of
energizing the Y at the ther-
mostat and thereby turning
on the cooling system. If you
identify this type of thermo-
stat, you must use the
“Alternate Furnace
Interlock Wiring”.
PRACTICAL TIPS
ELECTRICAL CONNEC-
TION TO A FURNACE
Standard Accessory Control Contact
Alternative Accessory Control Contact
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