Control Board: how the HVAC control board works in Dunwoody homes

The control board is the circuit board inside the air handler or furnace that turns thermostat signals into action. It switches the blower, sends the cooling call to the outdoor unit and runs safety checks.

How it works

Thermostat wires land on a terminal strip on the board — R, C, G, Y, W and sometimes more. When the thermostat calls for cooling, the board energizes relays that start the blower and send 24 volts to the outdoor contactor.

Most boards include a small automotive-style fuse, usually 3 or 5 amps, that protects the low-voltage circuit. A shorted thermostat wire or a chewed wire in the crawlspace will blow it.

Many boards have a status LED that blinks a fault code. The code chart is usually printed on the inside of the panel door and is a useful first clue.

Signs of a problem

• Thermostat blank and nothing runs
• Blower runs nonstop and won't shut off
• Blinking LED code on the board
• Burnt smell or discolored spots on the board
• System works intermittently after storms

Simple upkeep

• Use a surge protector on the HVAC circuit if storms are common
• Keep the panel door on — it often has a safety switch
• Keep pests out of the air handler cabinet
• Note the LED blink pattern before calling for service

The control board in Dunwoody homes

Older duct systems in this area often leak 20–30% of their air into attics and crawlspaces during peak summer. In Dunwoody's mix of 1970s and 1980s traditional homes, many with original duct runs and second-generation equipment, we see control board issues most often in homes around Dunwoody Village, Georgetown, Kingsley.

What our technicians check: We read fault codes, check the fuse and test each relay before recommending a new board. A board is replaced only after the cause of the failure — shorted wiring, surge or bad component — is found.

Common questions

See where it sits in the system

Other parts