What Would Cause Me Not to Have High Voltage to My Air HVAC Unit?

Thermostat

• A thermostat that is set in the "off" position or is malfunctioning won't send a signal to the contactor. If the contactor does not receive a signal, it can't close its contacts and send the high voltage current to the HVAC air unit. Some thermostats have a built-in feature that prevents the air unit compressor from cycling on and off too often. The first step in troubleshooting no high voltage to the HVAC air unit is to ensure the thermostat is set to "cool" and is applying voltage to the contactor wire.

Contactor

• The contactor uses voltage from a transformer to open and close a switch. When current is received from the thermostat, the contactor closes the switch that allows the 240-volt, high-voltage current to flow to the HVAC air unit. The voltage from the transformer usually measures 24 volts and provides the low voltage that operates the contactor. A malfunctioning transformer, weak coil or worn contacts prevent the contactor from sending high-voltage current to the air unit.

Safeties

• The HVAC air unit has various safeties that protect the unit from severe damage under certain operating conditions. Safeties include high or low coolant pressure, overheating sensors, or ambient temperatures that are too high or too low. The safeties act like switches to prevent the signal from the thermostat from reaching the contactor. Since the signal from the thermostat does not reach the contactor, the high voltage is not supplied to the air unit.

Power Off

• Circuit breakers and fusable disconnects control power to the HVAC air unit. In the circuit breaker or fuse panel, a tripped or shut off circuit breaker or blown fuse can prevent high voltage current from reaching the contactor and the unit. A disconnect switch located near the air unit may also have fuses which often blow before the circuit breaker in the panel will blow. Broken or disconnected wires cause the same problem.