In automatic operation on a call for heat the thermostat switch closes and energizes the compressor contactor and the blower motor in the air handler. The compressor contactor closes and the compressor energizes along with the condenser fan motor. The refrigerant inside the refrigeration circuit begins to flow. The refrigerant in the condenser absorbs heat from the outside air and this refrigerant is transferred indoors to the evaporator coil where the blower is energized. The fan speed is typically slower for heating than it is for air conditioning so the blower inside the air handler most likely has a multi-speed blower or an ECM blower motor and is controlled to a slower speed than when the air conditioning is on.
There is only a slight difference in the operation if you own a Rheem or a Ruud heat pump system. As the blower motor fan and the compressor contactor are energized so is the reversing valve. All other manufacturers energize the reversing valve in the cooling mode. The reason most manufacturers energize the reversing valve in the cooling mode is because if the valve fails they want the valve to fail to the heating cycle as heating is more important than cooling from the air conditioner.
The refrigerant passes through the evaporator coil while the blower is blowing air across the coil where the heat is transferred from the refrigerant and coil and into the air. The air flows through the duct work and is delivered into the spaces via the diffusers. The air recirculated and pulled back to the air handler through the return where the filter is located. The thermostat is located near the return so when the thermostat senses the temperature is at the set point it breaks the switch inside the thermostat to turn off the heat. The compressor contactor inside the condensing unit de-energizes and the compressor shuts down along with the condenser fan motor. The blower continues to run on a time delay and when the time expires the blower shuts down. That is the operation without including the defrost cycle.
Heat Pump Sequence of Operation with the Defrost Cycle
On colder nights when the heat pump runs for an extended period of time the heat pump will collect frost simply because it typically operates below the dew point and so the humidity will accumulate in the form of frost or ice on the condenser coils. This ice needs to be defrosted and regular readers who read the Heat Pump pages here at High Performance HVAC will know the defrost cycle method is typically on a timer so when the heat pump running for a specified time the defrost cycle will kick in and thaw out or defrost the frost or ice on the condenser coils. The time interval in the defrost cycle is manually set by the HVAC installation technician when the unit is installed and will vary from region to region.
Heat Pump Sequence of Operation - Conclusion
When the defrost cycle energizes the reversing valve switches mode to the air conditioning mode switching the whole heat pump system into an air conditioner in the cooling mode. This is necessary to defrost the condenser coils but it is counteracted through the defrost control by the defrost control. The defrost control energizes the back-up heat so the heat pump continues to provide heat. This timed cycle ends after a certain amount of time as determined by the manufacturer of the heat pump and everything switches back to normal heat pump mode.
Heat Pump Sequence of Operation