Heat Pump Explained

A heat pump provides two services and employs the same basic principles for both. A system consists of an outdoor unit and an indoor unit, with two small refrigerant pipes connecting between. Working like an air conditioner on warm days, the pump will extract heat from inside the home and transfer it outdoors. Conversely, on cold days the pump will flow heat energy into the home from the outside.

In cooling mode, a heat pump operates as an air conditioner…

  1. A refrigerant coil is located inside the indoor air handling unit.
  2. Warm indoor air from the return air duct passes through the coil which is filled with cold liquid refrigerant.
  3. The refrigerant absorbs heat energy from the air and evaporates into warmer gas form, while the now chilled air is delivered back into the house.
  4. The warm gas refrigerant then gets pumped into the compressor located in the outdoor unit which prepares it into a high-pressure, high-temperature state before moving onto the coil in the outdoor unit.
  5. Now the temperature of the refrigerant is much higher than the outdoor air. As the outdoor fan blows cooler air across the coil, it also picks up heat energy from the refrigerant, which then condenses into cold liquid form again before transferring to the indoor unit. And the cycle continues.

In heating mode, a heat pump may, quite simply, be thought of as an air conditioner equipped with a reverse mode.

Unlike water which has a boiling point of 100°C, the liquid refrigerant has a much lower boiling point, even lower than the cold outdoor air. This is why in heating mode, the refrigerant can still evaporate in the outdoor coil at low temperatures and draw heat from surrounding air, making the seemingly impossible heating with cold air possible. The now-heated gas refrigerant flows into the indoor coil and extracts the heat into the indoor air. Despite frigid temperatures the air will, unfailingly, contain enough energy to heat the home, even at -30°C.

Traditionally, heat pumps do not perform well when outdoor temperatures drop below 0°C, even though there is still heat energy out there. These systems need to be matched with a supplementary heating method, which in many cases would be a gas-fired furnace having a much lower efficiency than a heat pump. And since both heat pump and gas furnace cannot operate at the same time, it simply means you are not getting the benefits from a heat pump in winter at all.