Heat Pump Sequence of Operation - Every electrical-mechanical piece of equipment has a sequence of operation. While some manufacturers of heat pumps vary slightly with their heat pump sequence of operation the overall sequence of operation for a heat pump is the same for most manufacturers. So unless you have a special type of heat pump (they are out there – the Acadia heat pump is designed for extreme cold weather temperatures up North like Canada) the sequence of operation for your heating equipment should be very similar to what is described here. This is the basic operation of a heat pump like you will find in conventional heat pumps.
Heat Pump Sequence of Operation
In automatic operation on a call for heat, the thermostat switch closes. This energizes the compressor contactor and the blower motor in the air handler. The compressor contactor closes and the compressor turns on 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. The 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.
Heat Pump Sequence of Operation - Industry Differences
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.
Heating the Air
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. This process heats the air providing heat into your home or business.
The thermostat is located near the return. 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.
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. Regular readers who read the Heat Pump pages here at High Performance HVAC will know the defrost cycle method is typically on a timer. When the heat pump runs 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 - Defrost and Backup Heating Methods
When the defrost cycle energizes, the reversing valve switches mode to the air-conditioning mode. This switches the whole heat pump system into an air conditioner in the cooling mode. This is necessary to defrost the condenser coils. It is counteracted through heating strips by the defrost control. The defrost control energizes the backup electric 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. Everything switches back to normal heat pump mode.
There are also heat pumps that have gas or oil furnaces for the backup heat. It works the same as with the heat strips described above except instead of electric heat strips providing the heat the gas or oil furnace provides the heat. These systems are referred to in the HVAc field as dual fuel systems and are considered more efficient than heat pumps with electric heat strips for back up heat.
Heat Pump Troubleshooting
If you are troubleshooting heat pump problems you will want to break things down into groups. Is it a control problem or is it a mechanical problem. Mechanical problems include refrigerations issues, airflow issues, and mechanical parts. Control problems include relays, the heat pump thermostat, and anything in the control circuitry of the system. Understanding the heat pump sequence of operation will help you identify the issue quicker, therefore solving the problem faster.
More information about Heat Pumps can be found here.
Heat Pump Sequence of Operation