Defrost cycle behaviour

Why an air-source heat pump defrosts, what normal behaviour looks like and which signs need investigation.

An air-source heat pump can collect frost on its outdoor heat exchanger when the coil is cold and moisture in the air freezes on it. Frost restricts airflow and heat transfer, so the unit periodically defrosts the coil.

Defrost is normal in suitable weather. A unit that never needs it and a unit that defrosts often can both be normal under different temperature, humidity, load and product conditions. There is no universal correct number of cycles per hour.

How reverse-cycle defrost works

Many air-to-water heat pumps temporarily reverse the refrigerant circuit. Heat is taken from the heating water and sent to the outdoor coil, melting the frost. The outdoor fan normally stops or changes operation while the coil warms.

During the cycle:

normal space-heating delivery is reduced or paused
water leaving the unit can cool temporarily
meltwater drains below the outdoor unit
water vapour can appear as a visible cloud when operation resumes

That vapour is often mistaken for smoke. Burning smell, flame, persistent electrical noise or discoloured smoke is not normal and needs an emergency response.

The unit decides when to defrost

Modern controls use sensor data and operating history rather than a simple household timer. They may consider coil temperature, outdoor condition, refrigerant pressures, run time and loss of heat-transfer performance.

The exact algorithm is manufacturer-specific. Service menus that alter defrost thresholds should not be copied from another product or changed to chase a lower cycle count.

Water volume and flow supply the heat

Reverse-cycle defrost needs accessible heat in the water circuit and sufficient flow through the heat pump. The design must maintain the manufacturer’s minimum active volume and flow in the operating states that can occur during defrost.

Problems arise where thermostatic valves or underfloor actuators close most circuits, a strainer restricts flow, air is trapped or pumps and valves are sequenced incorrectly.

A volumiser can add series water volume. A buffer can add volume or hydraulic separation. Neither should be fitted without calculating the actual requirement and considering mixing and pump energy.

Design for repeated cycles

As at 22 July 2026, MIS 3005-D Issue 3.0 says an air-source heat-pump system should maintain the internal design temperatures across multiple defrost cycles. This is a system requirement, not just an outdoor-unit feature.

The assessment includes:

integrated or declared capacity under frosting conditions
heat available from the water circuit
emitter and building thermal mass
flow during valve and zone changes
supplementary heat control, if fitted
condensate and ice management

Sizing from a single dry-coil output point can miss the effect of repeated defrosts.

Meltwater and ice

Defrost water must drain without creating a slip hazard, blocking an access route or refreezing against the unit. The base, drain and soakaway or drainage route need to suit the product and site.

An ice layer beneath the unit can indicate poor drainage even when the refrigerant defrost is working correctly. Do not chip ice from the coil or pour hot water over the unit. Both can damage fins, coatings, sensors or electrical parts.

Keep the specified air clearances free of leaves, snow and stored items. Do not enclose the unit to keep it warm unless using an approved design that preserves airflow and prevents cold-air recirculation.

Normal signs

Normal defrost can include:

outdoor fan stopping briefly
a change in compressor or refrigerant sound
temporary cooling in the heating circuit
water draining from the base
a short cloud of vapour
the controller showing a defrost state

Radiators should not normally become dramatically cold from every cycle in a well-designed system, although a small temporary change can be detectable.

Signs that need investigation

Contact the installer or service provider if:

the coil remains heavily blocked with ice after a cycle
ice builds around the fan or damages the casing
cycles repeat rapidly for long periods without clearing the coil
the unit trips on flow, pressure or temperature alarms
indoor comfort falls materially in ordinary design weather
direct-electric backup runs unexpectedly for long periods
meltwater creates persistent hazardous ice
the fan strikes ice or makes abnormal mechanical noise

Record outdoor temperature, weather, flow and return temperatures, state, alarms and a safe photograph. A short video can show sequence and sound without approaching the fan or opening covers.

Possible system causes

Abnormal defrost behaviour can result from:

restricted outdoor airflow or cold-air recirculation
dirty or damaged coil
blocked condensate route
low water flow or insufficient active volume
incorrect sensor reading
refrigerant or expansion-control fault
control or firmware problem
an outdoor unit selected without adequate integrated capacity

Only a qualified person should open the refrigerant or electrical system. Adding refrigerant or changing controls without diagnosis can make the fault and legal compliance worse.

Monitoring the effect

One low COP period during defrost does not describe seasonal performance. Review delivered heat and electricity over a longer consistent boundary and correlate cycles with outdoor temperature and humidity.

The useful questions are whether the coil clears, comfort is maintained, backup is controlled and energy use remains consistent with the system design.