The part(s) or condition(s) listed below for the symptom Heat pump won't turn on are ordered from most likely to least likely to occur. Check or test each item, starting with the items at the top of the page.
The control board sends a low voltage signal of 24 volts to the contactor through the wall thermostat. When the contactor receives the signal from the control board, it closes the circuit, sending voltage to the outside unit components. There should be 24 volts between the common (“C”) and yellow (“Y”), and common ("C") and orange ("O") thermostat wire terminals on the air handler control board when the wall thermostat is in the heating mode. You can check for this voltage with a multimeter. If voltage is not reaching the contactor or reversing valve, it's likely the wall thermostat is defective and will need to be replaced.
The control board sends a low voltage signal of 24 volts to the contactor through the WiFi thermostat. When the contactor receives the signal from the control board, it closes the circuit, sending voltage to the outside unit components. There should be 24 volts between the common (“C”) and yellow (“Y”), and common ("C") and orange ("O") thermostat wire terminals on the air handler control board when the WiFi thermostat is in the heating mode. You can check for this voltage with a multimeter. If voltage is not reaching the contactor or reversing valve, it's likely the WiFi thermostat is defective and will need to be replaced.
If the heat pump unit contactor is not receiving 24 volts or power is not getting to the inside blower motor, a possible cause is a defective heat pump control board. While this is not a common problem, if all other potentially faulty components are working properly, the control board should be replaced.
The central air condensing unit is energized with 240 volts of electricity. This electricity then flows through the disconnect box to the compressor and condenser fan motor. The disconnect box enables the air conditioning unit to be turned off outside. The disconnect box includes a cartridge which may contain fuses. If the compressor and condenser fan motor do not run, the disconnect box fuses may have blown, or the circuit breaker supply power to the disconnect box may have tripped. To determine if the disconnect box fuses have blown, use a multimeter to test the fuses for continuity. If the fuses do not have continuity, replace the disconnect box.
If the furnace or air handler stops working properly, the control board will often display a fault code to help diagnose the problem. The fault codes are displayed by a steady or flashing light on the control board. The light indicates either an error code or the status of the furnace or air handler. Some manufacturers will use red and green lights to indicate specific fault codes. Fault code descriptions are often displayed on the access cover of the unit for easy reference.
The air handler needs to have power reaching it for the heat pump unit to run. If the furnace or air handler control board has an indicator light, confirm the light is illuminated which indicates power is present. If there is no power to the furnace or air handler, check the home's circuit breaker or fuse. Confirm the power switch on the side of the furnace or air handler is turned on.
The control board sends a low voltage signal of 24 volts to the contactor. When the contactor receives the signal from the control board, it closes the circuit, sending voltage to the outside unit components. Some air handlers have a separate transformer for the 24 volt circuit, so if the contactor is not receiving the proper voltage, it's likely this transformer has failed and the component will need to be replaced.
For the heat pump to work properly, the wall thermostat should be set for "Cool" or "Heat" and the fan set for "Auto" or "Fan On". The temperature setting also needs to be set properly. When the thermostat is set properly, both the outdoor unit and the furnace or air handler blower should be running.
Most wall thermostats are either powered by an enclosed battery or by the furnace or air handler transformer. If the thermostat display is dead, you can remove the thermostat cover and look for replaceable batteries. Replace the batteries if needed and check operation.
