Why Is My Heat Pump No Longer Defrosting Itself In Winter?

During colder months, an outside heat pump is more likely to develop an excessive coating of frost. Heat pumps are designed to periodically melt the frost away. Owners of air-source heat pumps that are not defrosting adequately may have a faulty defrost control board.

In winter months, a heat pump transfers heat from the outside air to your interior living space. Refrigerant is cycled through a compressor and an evaporator, absorbing heat energy outside and then releasing heat energy inside. As outside temperatures drop to near freezing, humidity in the air becomes more likely to form on the heat pump as frost.

The development of frost in normal. However, your heat pump relies on its ability to transfer heat outside to remove the frost. The heat pump contains a valve that temporarily reverses the direction of the refrigerant flow. If the frost continues to build up, there may be a problem with the defrost control cycle. Most heat pumps contain a component dedicated to operating the defrost cycle, referred to as a defrost control board.

Defrost control cycle

A sensor on your outside heat pump detects when the temperature is low enough to develop frost. When frost conditions are detected, a heat pump system typically undergoes a defrost control cycle lasting from 30 seconds to a few minutes. The need for defrost cycles during colder weather is ongoing, so there should not normally be an excessive build-up of frost.

During the defrost cycle, your auxiliary heat is switched on. Because the flow of refrigerant is reversed, heat is moved to the outside heat pump, When the frost is melted, the auxiliary heat is switched back off, and the flow of refrigerant is returned to the normal direction for winter heating.

You may be able to hear a sound as the reversing valve switches back and forth during the defrost cycle. The sound is normal and possibly provides an indication that the system is operating normally. When conditions for frost are detected, your heat pump may automatically go through continuing defrost cycles on a set periodic basis.

Inefficiency of auxiliary heat

If your heat pump is not removing frost during colder weather, the use of auxiliary heat could be consuming excessive energy. Auxiliary heating relies on the conversion of electricity to heat, rather than simply using electricity to operate an efficient heat pump. However, the increased use of auxiliary heat is normal as temperatures drop even lower. Contact an HVAC contractor for more information on heat pumps or heat pump repair.