A Carrier's heat pump system relies on its thermostat to operate the condensing unit's reversing valve. A heat pump uses refrigerant during both the heating and the cooling cycles. The reversing valve changes the refrigerant's flow direction to meet the thermostat's demand. The thermostat sends its signal, through thin-gauge wires, to the low-voltage control board in the Carrier air handle inside the heat pump unit. The control board then sends a low-voltage signal to the appropriate relays and circuit boards.

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A Carrier heat-pump system receives information from its thermostat.

Step 1

Turn off the electricity to the Carrier air handler at its circuit breaker. The correct circuit breaker uses a "Furnace" or "Air handler" label. Failure to do this can cause the air handler's 5-amp fuse to blow, or worse, cause an electrical short in its transformer.

Step 2

Remove 1/4 inch of the insulation from each low-voltage wire that exits the wall at the thermostat's location with wire strippers.

Step 3

Bend the end of each low-voltage wire into a hook shape with needle-nose pliers.

Step 4

Slide the red-colored wire's hook around the Carrier thermostat's "R" terminal screw and tighten the screw with a slotted screwdriver. The red-colored wire provides a 24-volt feed to the thermostat.

Step 5

Slide the green-colored wire's hook around the Carrier thermostat's "G" terminal screw and tighten the screw with a slotted screwdriver. The "G" terminal connects to the thermostat's manual fan switch.

Step 6

Slide the yellow-colored wire's hook around the Carrier thermostat's "Y" terminal screw and tighten the screw with a slotted screwdriver. The "Y" terminal operates the cooling stage of the heat pump.

Step 7

Slide the light-blue-colored wire's hook around the Carrier thermostat's "Y2" terminal screw, if equipped, and tighten the screw with a slotted screwdriver. Normally not used, this terminal operates the second compressor in a two-compressor condensing unit.

Step 8

Slide the white-colored wire's hook around the Carrier thermostat's "W" terminal screw and tighten the screw with a slotted screwdriver. The "W" terminal connects to the thermostat's primary heat stage.

Step 9

Slide the brown-colored wire's hook around the Carrier thermostat's "W2" terminal screw and tighten the screw with a slotted screwdriver. The "W2" terminal connects to the heat pump's emergency heat stage. The emergency heat stage uses an electric heat strip as its heat source.

Step 10

Slide the orange-colored wire's hook around the Carrier thermostat's "O" terminal screw and tighten the screw with a slotted screwdriver. The "O" terminal operates the reversing valve.