Hamilton
Home
Products
Canal
Winchester,
OH
43110
61MH0028A
Page
14
If you have separate furnace and air conditioner with separate
transformers and your thermostat does not have the “RC” and
“RH” terminals it is recommended that you purchase a new
thermostat. If the furnace and air conditioner are both
connected to the thermostat “R” terminal it can cause
transformer burnout or it can cause either the furnace or air
conditioner control system to go into lockout.
Separate Heating and Cooling Units, Separate
Thermostats
If the heating/cooling system in your house is a central heating
and cooling system using the furnace transformer but the
furnace and the cooling unit are controlled by separate
thermostats then the use of a thermostat interlock switch is
required in order to prevent the furnace and the air conditioner
from operating at the same time. Refer to Figures 16 and 17.
Common
Transformer
R
Transformer
24 VAC
Air Handler
Switch
Interlock
Thermostat
COOL
HEAT
COM
C
W
G
R
Y
C
Thermostat
Cooling
Thermostat
Heating
Y
G
R
W
G
Condenser
Outdoor Unit
R
Figure 16: Separate Thermostats; Separate Air
handler and Cooling Unit
G
R
W
R
G
Y
Heating
Thermostat
Cooling
Thermostat
C
Y
R
G
W
C
COM
HEAT
COOL
Thermostat
Interlock
Switch
Air Handler
24 VAC
Transformer
Transformer
Common
Condenser
Outdoor Unit
R
HEAT
COOL
Figure 17: Separate Thermostats; Separate
Transformers, Separate Air handler and Cooling
Unit
RC
RH
G
Thermostat
R
G
W
C
Air Handler
24 VAC
Transformer
Transformer
Common
Condenser
Outdoor Unit
W/E
C
W2
Y
O
Y
RC
RH
G
W/E
T
C
W2
Y
O
Add Jumper if Outdoor Stat is not used
24 VAC
Transformer
Transformer
Common
Optional Outdoor
Temperature Switch
Figure 18: Same Thermostat; Separate Air handler
and Heat Pump Unit with separate transformers
RH
RC
G
Thermostat
R
G
W
C
Air Handler
24 VAC
Transformer
Transformer
Common
Condenser
Outdoor Unit
W
C
Y
R
C
Y
24 VAC
Transformer
Transformer
Common
Figure 19: Same Thermostat; Separate Air handler
and Cooling Unit with separate transformers
When using separate thermostat, a thermostat interlock
system must be provided to prevent simultaneous operation
of the air handler and air conditioner. Simultaneous
operation can result in equipment overheating, equipment
damage, and wasted energy.
Do Not
connect the Yellow wire to the thermostat unless an
outdoor unit is installed.
