Frost and Ice on Heat Pumps – What’s Normal, What’s Not, and What to Do

29 Dec

Seeing frost or ice on a heat pump can be worrying, but in many cases it is completely normal operation. Heat pumps extract heat from cold outdoor air, which means frost is sometimes unavoidable.

This guide explains why frost forms, how the defrost cycle works, what normal icing looks like, and when ice buildup indicates a fault that needs attention.

Why heat pumps get frost and ice

An outdoor heat pump works by pulling heat from the outside air using a cold heat exchanger (coil). When the coil temperature drops below freezing and there is moisture in the air, frost forms on the fins.

Frost is most common when:

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

This is expected behaviour and is why heat pumps include an automatic defrost function.

What the defrost cycle does

When the heat pump detects frost buildup, it enters a defrost cycle. During defrost, the system temporarily reverses operation to warm the outdoor coil and melt the ice.

During a defrost cycle you may notice:

Steam or vapour rising from the outdoor unit
A change in fan or compressor noise
The outdoor fan stopping briefly
Heating output reducing for a short time

This is normal and usually lasts a few minutes.

What normal frost and ice looks like

Normal frosting:

Forms evenly across the coil
Builds up gradually
Clears completely during defrost
Does not block airflow long-term

After defrost, the coil should be mostly clear, with water dripping from the base of the unit.

What is not normal

Ice buildup may indicate a problem if you see:

Thick solid ice that does not clear
Ice building up day after day
Only part of the coil defrosting
Ice forming into a solid block
Heating performance getting steadily worse

Persistent ice is not normal and should not be ignored.

Common causes of excessive ice buildup

Restricted airflow

Leaves, debris, or snow blocking the coil
Too little clearance around the unit
Ice forming at the base and blocking airflow

Drainage problems

Blocked condensate drain
Ice forming in the drip tray
Water refreezing under the unit

Defrost system faults

Faulty temperature sensors
Incorrect defrost settings
Control or software issues

Flow or system issues

Low water flow through the heat pump
Blocked filters or strainers
Incorrect commissioning

Does defrost mean the heat pump isn’t working?

No. Defrost is a normal part of heat pump operation in cold, damp conditions. A heat pump that never defrosts in winter may actually be malfunctioning.

However, if the heat pump seems to spend more time defrosting than heating, this usually points to a system or airflow problem.

What homeowners can safely check

You can safely check:

The outdoor unit is clear of leaves, snow, and debris
There is space underneath for water to drain away
No objects are blocking airflow around the unit
The heat pump is not sitting in standing water or ice

Do not:

Chip or scrape ice off the coil
Pour boiling water over the unit
Use tools or sharp objects on fins
Disable defrost settings

Frost, ice, and efficiency

Frost reduces heat transfer, which is why defrost cycles are needed. Excessive icing increases energy use because the heat pump must spend more time clearing ice instead of heating the home.

If icing is frequent or severe, addressing the root cause can significantly improve comfort, reliability, and running costs.

When to call an engineer

Contact a qualified heat pump engineer if:

Ice does not clear after defrost
The unit is becoming encased in ice
Heating output is dropping noticeably
Error codes related to defrost or temperature appear
The system has never behaved normally in cold weather

Ongoing icing issues are often linked to airflow, drainage, flow rate, or commissioning problems rather than a failed heat pump.

Key takeaway

Frost on a heat pump is normal. Ice that doesn’t clear is not. Understanding the difference helps you avoid unnecessary worry while spotting real problems early.

If your heat pump is defrosting regularly and returning to normal operation, it is usually doing exactly what it was designed to do.

Jack Mundinano