- The thermostat, set to cooling mode, calls for cooling. The switch inside the thermostat closes. This energizes the “Y” and the “G” circuit in the thermostat or the compressor contactor and the fan circuit.
- The compressor contactor closes and the condenser fan motor and the compressor turns on. This happens simultaneously as the compressor contactor controls both the compressor and the condenser fan motor. This is important because if the compressor contactor pulls in or engages and the fan doesn’t start but the compressor does then the technician knows there is a problem with the condenser fan motor and vice versa. If neither starts but the contactor is pulled in then the technician knows there is either a problem with the contactor passing voltage across the contacts or a power problem. If the contactor does not pull-in then the tech knows there is a control problem.
- At the same time the compressor contactor is doing its job the blower fan relay closes (from the “G” contact in the thermostat) and energizes the blower fan. While the compressor gets the refrigerant moving through the refrigeration system the fan begins moving air through the duct system and across the evaporator coil.
- When the thermostat satisfies the “Y” and the “G” contacts in the thermostat open de-energizing the compressor contactor and the fan relay. The compressor and condenser fan motor de-energize and stop operation. The fan, in heating and cooling mode (no matter what type of heat you have) will remain on because of a time delay in the air handler.This is to dissipate any cool or hot air left in the duct system. It adds efficiency to the system and makes use of any residual energy left in the system. In heating mode it also allows for a cooling down of the heat left in the system.
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Air Conditioner Sequence of Operation | The Basics