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What Is A Heat Pump & How Does It Work?


In temperate climates like Oregon and Washington, a heat pump is the most common type of air conditioning system. This type of system is popular in moderate temperatures because it’s more energy efficient than a furnace or air conditioner. Heat pumps are also environmentally friendly because they don’t burn any fossil fuels to produce heat. But what is a heat pump exactly? And how do heat pumps work?

What is a heat pump?

A heat pump is part of your home or office’s central heating and cooling system. It uses outside air outside air to both heat a home in winter and cool it in summer. A heat pump isn’t a single element, it’s a system comprised of the following main components:

  • Compressor
  • Reversing valve
  • Indoor heat exchanger
  • Expansion valves with non-return valves
  • Filter drier
  • Sight glass
  • Outdoor heat exchanger
  • Controller
  • Pressure sensors around the system

There are many different types of heat pumps including:

  • Air to air systems
  • Water to air systems
  • Ground source/geothermal systems
  • Water source systems

The most common type of heat pump is the air to air system. Just like split unit air conditioning systems, air to air heat pumps have one unit inside and one outside. If the heat pump has a reversing valve, the heat pump can both heat and cool a space. However, without a reversing valve, the system can only provide heating.

Either gas or electricity powers heat pumps, which can be installed as a split system, package systems, or as a ductless or ‘mini-split’ system.

How do heat pumps work?

All air conditioning systems, including heat pumps, use the relationship between pressure and temperature of the refrigerant to heat and cool a space. When a refrigerant boils (evaporates), it absorbs heat at a very high rate. Refrigerant has a very low boiling point, so room temperature air and pressure can be used to manipulate the indoor temperatures.

Heat pumps don’t create heat. They simply move warm air from one place to another with the help of refrigerant. The unique properties of refrigerant allow the heat pump to extract outside heat from cold outdoor air to heat indoor spaces and remove heat from inside spaces to cool your home or office.

The heart of the heat pump is the compressor. The compressor is where both the heating and cooling cycles begin and end.

How do heat pumps work in the winter?

During the winter, when you set the indoor temperature higher, the refrigerant leaves the compressor as a high-pressure, high-temperature vapor. It passes through the reversing valve and heads to the indoor unit.

In the indoor unit, cool air is blown over the indoor unit’s heat exchanger to remove some of the thermal energy from the refrigerant, heating to the indoor space. Then, the refrigerant leaves the indoor unit as a high-pressure, slightly cooler liquid.

As it passes through two non-return valves, the filter drier, and the sight glass, it turns into a cold, part liquid, part vapor mixture. It then travels outside to the outdoor unit where cool outdoor air is blown over the outdoor heat exchanger.

The refrigerant picks up the thermal energy from the outside air and leaves the outdoor heat exchanger as a low pressure, low temperature, slightly superheated vapor. It makes its way through the reversing valve and back to the compressor where the cycle repeats.

How do heat pumps work in the summer?

The same heat pump system that heats indoor spaces during winter months, cools your home or office during summer months. Like the heating cycle, the cooling cycle starts and ends with the compressor.

When you turn the thermostat down to cool your home or office, instead of the compressor forcing the high-pressure, high-temperature vapor refrigerant into the indoor unit, the refrigerant passes through the outdoor heat exchanger first. The outdoor unit’s fan blows air across the heat exchanger, which carries the thermal energy of the refrigerant away. As the refrigerant condenses, it loses its thermal energy. It leaves the outdoor unit as a high-pressure, slightly cooler liquid. The liquid refrigerant then passes through a non-return valve, the sight glass, the filter drier, and the expansion valve, before entering the indoor heat exchanger.

As the liquid refrigerant passes through the expansion valve, it turns into a part liquid, part vapor mixture, which causes it to drop in pressure and temperature. The cool refrigerant enters the indoor heat exchanger where a fan blows warm indoor air over the coil, causing the heat to transfer from the air into the refrigerant. The cooled air is returned to the indoor space and the low-pressure, low-temperature, slightly superheated refrigerant returns to the compressor to repeat the cycle.

Repair or install a new heat pump

If your heat pump system is overdue for maintenance or you need to install a new one, schedule an appointment with a trusted HVAC professional today.