GENERAL Assist | AIRSTAGE Technical Information

Understanding Defrost Mode

Why your air conditioner may temporarily stop heating during cold weather
During winter, many reverse-cycle air conditioners will periodically enter a defrost cycle while operating in heating mode. This is a normal part of operation and allows the system to remove ice that can form on the outdoor heat exchanger.

What is a defrost cycle?

In heating mode, the air conditioner extracts heat from the outside air and transfers it indoors. When outdoor conditions are cold, moisture in the air can freeze on the outdoor unit's heat exchanger as air passes across it.

The defrost cycle is the system automatically recognising that ice has formed, or is beginning to form, and removing it so the outdoor heat exchanger can continue operating effectively.

Why does the unit need to defrost?

Ice build-up on the outdoor heat exchanger restricts airflow and reduces heating efficiency. In more severe cases, excessive icing can also place unnecessary stress on the outdoor unit.

Important: Do not turn the system off while it is actively defrosting. Allow the cycle to finish. Restarting the unit shortly afterwards can result in inefficient operation and may contribute to damage.

How can I tell if the unit is defrosting?

During a defrost cycle you may notice the following:

LocationWhat you may observe
Indoor unitHeating output temporarily stops and the indoor fan may stop.
Controller / displayDepending on the model, a defrost icon may appear on the wall controller or LEDs may flash on a high-wall unit.
Outdoor unitThe outdoor fan stops while the compressor continues to operate.

How often will the unit defrost?

The frequency of defrosting is influenced by a number of factors, including:

  • Outdoor temperature and humidity.
  • The amount of heating load the system is trying to deliver.
  • The condition and installation of the air-conditioning system.
Typical timing: The system contains timers and control logic that determine when defrost is required. As a general guide, a unit may need to operate for approximately 35 minutes before its first defrost, with subsequent defrost cycles generally occurring no more frequently than approximately every 35 minutes.

How long does a defrost cycle take?

A defrost cycle will end when the outdoor unit sensors detect that the heat exchanger temperature has risen sufficiently. If the sensors do not end the cycle earlier, the maximum defrost period is generally around 10 minutes.

Frequent defrosting or poor heating - what should I check?

Regular defrosting or insufficient heating can have several causes. The following checks can help determine whether the issue is related to operation, installation, maintenance or a system fault.

1. Has the system behaved this way since installation?

If the issue has been present from the first period of cold weather, check the operating instructions to confirm the system is being used correctly. If correct operation does not resolve the issue, the system may be undersized for the area it is attempting to heat.

If the air conditioner is undersized for the space, this is not a product fault. Correct system selection and sizing are the responsibility of the installing company.

2. Check operating settings

Operating conditions can affect heating performance. Examples include running the unit at a low fan speed or operating all zones at once on a ducted zoned system.

3. Check the outdoor unit location

The outdoor unit's location and airflow can significantly affect how often it enters defrost. Check for:

  • Recirculation of the unit's own cold discharge air.
  • Debris or foliage restricting airflow through the outdoor heat exchanger.
  • Insufficient clearance between the outdoor unit and nearby walls or structures.
  • Installation in a particularly cold location, such as under a veranda or on a shaded side of the building with minimal sunlight.

4. Has the problem developed recently?

A recently developed change in heating performance or defrost frequency may indicate that maintenance is required or that a fault has developed.

Basic maintenance checks include:

  • Clean the indoor unit filters.
  • Ensure the outdoor unit is clear of foliage and debris.
  • Check that the outdoor heat exchanger is not visibly blocked.

If these checks do not resolve the issue, contact the original installer or another reputable air-conditioning service provider.

Can I reduce how often the unit defrosts?

You cannot completely prevent defrosting in cold conditions, but you can reduce unnecessary heating load and help the system operate more effectively.

Maintain the systemKeep indoor filters clean and the outdoor unit clear of airflow restrictions.
Operate it correctlyUse appropriate fan and zone settings and refer to the operating manual where required.
Reduce heat lossKeep doors closed and curtains drawn during cold weather.
Improve insulationImproved ceiling, wall and under-floor insulation can reduce the heating demand on the air conditioner.
Key point: Defrosting is a normal and necessary part of heating operation in cold conditions. Concern is more appropriate when defrosting becomes unusually frequent, heating performance has noticeably changed, or basic maintenance and operating checks do not improve the situation.