General Assist Technical Reference

Air Conditioner Defrost Function

Frequently asked questions about defrost operation in heating mode
Questions are often asked about why an air conditioner needs to defrost. This guide answers the most common questions in non-technical terms. It does not cover the specific control logic of individual models, so minor differences may apply between products.

What is the defrost function?

Defrost is a normal operating function used by reverse-cycle air conditioners to remove ice build-up from the outdoor unit heat exchanger during heating operation. Ice can form because the outdoor unit is extracting heat from the outside air and moisture in that air can freeze on the outdoor coil.

How can I tell if my unit is defrosting?

Most units will indicate defrost operation by flashing LED indicators on the indoor unit. Depending on the model, this may be a red or green LED. Systems fitted with a wall controller may instead display a defrost symbol or icon.

Important: The exact indication varies by model. A flashing light or defrost icon during heating is not necessarily a fault.

Why does defrost need to happen?

If the outdoor heat exchanger becomes covered in ice, airflow is restricted and the efficiency of the heat exchanger is reduced. The ice also acts as an insulating barrier and limits heat absorption. Defrosting removes the ice and restores heat-transfer performance.

How often does it need to happen?

Defrost frequency depends on several factors, including:

  • How cold and damp the air is around the outdoor unit.
  • How hard the system is required to work, including room insulation, thermostat settings and attempts to heat adjacent areas.
  • Whether there are any mechanical issues with the unit.
  • Whether the unit has been sized correctly for the area.
  • Whether the indoor filters and outdoor heat exchanger are clean.
  • Whether the outdoor unit is recirculating its own discharge air.

Assuming there are no mechanical defects, how often would my unit be expected to defrost?

Most units will not check for a defrost requirement until at least approximately 45 minutes have elapsed from initial start-up. This period should normally allow time for the system to begin bringing the room toward the selected temperature, assuming the temperature setting is realistic and achievable.

Typical initial check periodApproximately 45 minutes from start-up on many models
Typical defrost durationDefrost can terminate within approximately 10 minutes, depending on conditions and model logic

After defrost has started, how long should it take to complete?

On most units, defrost ends when either a set period of time has elapsed or the outdoor heat exchanger temperature has risen sufficiently to indicate that the ice has been removed. Defrost termination can occur within approximately 10 minutes.

If my unit is frequently defrosting, what could be wrong?

Regular or unsuccessful attempts to defrost can indicate that the system is working harder than expected. Possible causes include:

  • The system is undersized for the area being heated.
  • The thermostat is set to an unrealistically high temperature.
  • Refrigerant loss or another mechanical defect.
  • Dirty indoor or outdoor heat exchangers.
  • Restricted airflow caused by foliage or another obstruction around the outdoor unit.
  • Blocked indoor air filters.
  • The indoor fan has been fixed at a low fan speed.

What should I check first?

  1. Clean the indoor air filters. Restricted airflow can reduce heating performance and may increase defrost frequency.
  2. Check the outdoor unit. Make sure the coil and airflow path are not blocked by dirt, foliage or other obstructions.
  3. Set the fan to AUTO. A fixed low fan speed can prevent the system from delivering its available heating capacity.
  4. Use a realistic temperature setting. Excessively high thermostat settings make the system work harder.
  5. In very cold conditions, start heating earlier. Leaving the system operating can help maintain room temperature instead of requiring a large recovery load.
Service may be required: If cleaning the filters, checking airflow and adjusting the operating settings do not improve the situation, an air conditioning specialist should inspect the system for sizing, refrigerant or mechanical issues.

If my unit is undersized, what can I do?

Selection and sizing of the air conditioning system is the responsibility of the installation contractor. If the equipment has been incorrectly sized for the application, the issue should be discussed with the installing contractor. The equipment itself is not considered faulty simply because it has been selected below the heating load required for the space.

Is there anything I can do to decrease defrosting?

An air conditioner will generally need to defrost less often and use less energy when the heating load on the room is reduced. Helpful measures include:

  • Keep doors and curtains closed where practical.
  • Ensure floors and ceilings are adequately insulated.
  • Close off unused zones where the system design allows this.
  • Keep filters and the outdoor unit clean and unobstructed.
  • Use AUTO fan speed rather than a fixed low fan speed.
  • Avoid unnecessarily high thermostat settings.
Recommended heating setpoint: As a general guide, a maximum setting of around 21°C is usually sufficient for comfort. Higher settings can increase the heating load, increase energy use and may result in more frequent defrost operation.

Summary

Defrost is a normal and necessary part of heating operation. Its frequency will vary with outdoor temperature, humidity, system load, installation conditions and maintenance. Frequent defrosting does not automatically indicate a fault, but persistent or unsuccessful defrost cycles should be investigated if basic airflow and maintenance checks do not resolve the issue.