Heat Pump Defrost Cycle – What It Is, How It Works, and When to Worry

29 Dec

The defrost cycle is a normal and essential part of how an air source heat pump operates in cold, damp weather. It prevents ice buildup on the outdoor unit so the system can continue extracting heat efficiently.

This guide explains what the defrost cycle is, what you should expect to see and hear, how long it normally lasts, and how to tell the difference between normal defrosting and a genuine fault.

Why heat pumps need a defrost cycle

Air source heat pumps absorb heat from outdoor air using a cold heat exchanger (coil). When the coil temperature drops below freezing and moisture is present, frost forms on the outdoor unit.

Frost reduces airflow and heat transfer. If it isn’t removed, performance drops rapidly. The defrost cycle exists to periodically melt this frost so the heat pump can keep working properly.

What happens during a defrost cycle

During defrost, the heat pump temporarily changes how it operates to warm the outdoor coil. On most systems this is done by reversing the refrigeration cycle.

During a defrost cycle you may notice:

Steam or vapour rising from the outdoor unit
The outdoor fan slowing down or stopping
A change in compressor or fan noise
Heating output reducing briefly
Water dripping from the base of the unit

All of this is normal.

How long does a defrost cycle last?

A typical defrost cycle lasts between 2 and 10 minutes, depending on conditions and the amount of frost present.

Once defrost is complete, the system automatically returns to normal heating operation. The indoor temperature should not drop noticeably during a correctly functioning defrost cycle.

How often should defrost happen?

Defrost frequency depends heavily on weather conditions.

Defrost cycles are most common when:

Outdoor temperatures are between about +5°C and -5°C
Humidity is high (fog, mist, drizzle)
The heat pump has been running continuously

In cold, dry weather, defrosting may be much less frequent. In mild, damp weather, it may occur more often.

What normal defrost behaviour looks like

Normal defrost behaviour includes:

Even frost buildup before defrost
Steam during defrost
Ice fully clearing from the coil
Water draining away afterwards
The system returning to heating automatically

A heat pump that defrosts occasionally and then resumes heating is operating as intended.

Signs defrost may not be working correctly

Defrost issues may be present if you see:

Thick ice that does not clear after defrost
Only part of the coil defrosting
The unit becoming encased in ice
Defrost cycles happening very frequently
The heat pump spending more time defrosting than heating

Persistent ice buildup is not normal and should be investigated.

Common causes of defrost problems

Airflow issues

Leaves, snow, or debris blocking the outdoor coil
Insufficient clearance around the unit
Ice blocking airflow at the base

Drainage problems

Blocked or frozen condensate drain
Water refreezing under the unit
Poor installation height or ground conditions

System and control issues

Faulty temperature or frost sensors
Incorrect defrost settings
Software or control faults

Flow-related problems

Low water flow through the heat pump
Blocked filters or strainers
Pump or balancing issues

Does defrost mean the heat pump stops heating?

Briefly, yes – but this is expected. During defrost, heating output is reduced or paused so the outdoor coil can be cleared.

Well-designed systems use thermal mass in the building, buffer tanks, or smart controls to ensure this short interruption is barely noticeable indoors.

What homeowners should and should not do

You can safely:

Keep the outdoor unit clear of leaves and debris
Ensure water can drain freely away from the unit
Check that airflow around the unit is not blocked

You should not:

Chip or scrape ice from the coil
Pour hot or boiling water over the unit
Disable or override defrost settings
Use tools on the aluminium fins

Defrost cycles and efficiency

Defrost cycles use energy, but they are far more efficient than allowing ice to block airflow. A heat pump that cannot defrost properly will use more electricity and deliver less heat.

If defrosting seems excessive, fixing the underlying airflow, drainage, or flow problem often improves efficiency significantly.

When to call an engineer

Contact a qualified heat pump engineer if:

Ice does not clear after defrost
The outdoor unit repeatedly freezes solid
Heating performance drops in cold weather
Error codes related to defrost or temperature appear
The system has never behaved normally in winter

Ongoing defrost problems are often caused by installation or system issues rather than a failed heat pump.

Key takeaway

The defrost cycle is a normal and necessary part of air source heat pump operation. Steam, noise changes, and brief pauses in heating are expected.

If frost clears and the system returns to heating, it is working correctly. Persistent ice or constant defrosting is a sign something else needs attention.

Jack Mundinano