Air Source Heat Pump Defrost Cycle

Many heat pumps need to go through a "defrost" cycle during operation during the winter months. This is often not explained well before installation, which leads to confusion as to why a heat pump is not working. This article is designed to help users understand the defrost cycle and troubleshoot any issues.

What is a "defrost cycle"?

In heating mode, a heat pump extracts heat from the outdoor air and transfers it indoors to warm the interior. When the outdoor ambient temperature gets very cold (close to 0°C or lower), moisture in the air can freeze on the outdoor unit's heat exchanger as the fan blows air through it. The defrost cycle is simply the system recognizing that ice has formed or is starting to form and automatically fixing this problem.

Why does my unit have to go through a defrost cycle?

Any ice on the outside heat exchanger reduces airflow through it, which affects efficiency, sometimes drastically. In extreme cases, this can also damage the outdoor unit.

How can I tell if my unit is in a defrost cycle?

Indoors, you will notice that the unit will stop heating, the indoor fan will stop, and depending on the model, there will usually be some form of visual indication such as a light on the unit (usually the "Run" light) that will flash continuously. Outdoors, the outdoor fan will also stop and the compressor will run.

How often will my unit go into defrost mode?

There are a number of factors that can affect how often a unit goes into defrost mode. Some of these include:

1. Outdoor temperature and humidity

2. The amount of heating load the unit is trying to deliver

3. The condition of the heat pump system.

There are timers built into the computer controls of the unit that limit how often it defrosts. Typically, the unit must run for at least about 35 minutes after startup to complete the first defrost. Thereafter, defrosts should be no more often than every 35 minutes.

How long does it take my unit to defrost?

Either of two factors can cause the unit to exit a defrost cycle. First, the unit will stop defrosting if a sensor on the outdoor unit detects that its heat exchanger temperature has risen enough. Second, the maximum time the unit can be in a defrost cycle is about 10 minutes if the sensor does not stop the defrost earlier.

It is important not to stop the unit before the defrost cycle is complete, because if the unit is restarted shortly afterwards, it will run very inefficiently and may cause damage to itself.

My unit defrosts/underheats frequently - what could be wrong?

Frequent defrosting or underheating can be caused by a number of factors.

If the unit has been running like this since it was first installed (first cold weather), it is possible that you are operating it incorrectly, or that the unit is not sized correctly for the space it is intended to heat. First, you should consult the owner's manual to make sure you are operating the unit correctly. If this does not resolve the issue, you should consult your installer or another reputable heat pump installer. They can help you ensure correct operation and correct sizing.

If the unit is not sized correctly for the space, it is not a fault. The responsibility for sizing the unit correctly initially rests with the installing company - if the unit is too small, they will need to correct the situation.

Recent problems may indicate a fault or need for maintenance. There are some basic maintenance you can do yourself, such as cleaning the filter on the indoor unit and making sure the outdoor unit is clear of leaves and the heat exchanger is not clogged. If this does not resolve the issue, you should consult your installer or another reputable heat pump installer.

Is there anything you can do to help reduce defrosting?

Of course there is. Keep your equipment well-maintained (as mentioned above) and make sure it's operating correctly. That will go a long way.

Of course, the less load you put on your equipment, the less often it will have to defrost in cold conditions. Ultimately, permanent fixes like installing insulation in the ceiling, walls, and under the floor will help reduce your heating needs (and ultimately save you money). More immediately, keeping doors closed and curtains drawn will also help reduce your heating needs.