HVAC Heat Pump Basics
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In the cooling cycle, the liquid line leads into the TXV where the refrigerant pressure rapidly drops and some flash gas is produced. From there, there’s a short expansion phase into the evaporator, where the refrigerant absorbs heat from the space and boils. The refrigerant travels to the compressor via the suction line as a superheated vapor. The compressor rapidly raises the temperature and pressure of the vapor and sends the refrigerant to the condensing coil through the discharge line. The condenser rejects heat from the refrigerant and turns it back into a liquid. From there, the cycle repeats itself.
In a heat pump, you have all of the same basic components. However, the evaporator coil in cooling mode becomes the condenser in heating mode, and the condenser in cooling mode becomes the evaporator in heating mode. In the air handler, you will also have heat strips, which provide electrical backup heat. When the system goes into defrost, the heat strips also turn on to prevent the space from cooling too much. Heat pumps also have two metering devices: one for cooling mode and one for heating mode. TXV systems have check valves that allow the refrigerant to bypass the metering device that is not in use.
At the outdoor equipment, a heat pump has a defrost board, reversing valve, a common suction port. The defrost board is located in the same place as the low and high-voltage wiring underneath a panel, and it has a defrost sensor connected to the outdoor coil. The common suction port is located between the suction and liquid line coming out of the outdoor unit. A 4-way reversing valve has a solenoid that can slide the reversing valve and swap the suction and discharge lines when energized. That process reverses the flow direction and allows the heat pump to switch between heating and cooling mode. There will always be a common suction line and a common discharge line leading into the reversing valve.
There may also be an accumulator on the system, which prevents liquid refrigerant from getting into the compressor. In cooling mode, the refrigerant flows through the suction line and then passes through the reversing valve before the compressor. In heating mode, the refrigerant passes through the compressor before it reaches the reversing valve.
The defrost board depends on a defrost sensor to tell the unit when to defrost. (In this case, it’s a pink wire.) In defrost mode, the defrost board will send a 24v call on the white wire to energize the backup heat inside (heat strips).