Controls and faults

Defrost cycle

A normal heat-pump routine that briefly reverses operation to melt frost from the outdoor coil. Steam from the outdoor unit, a pause in heating, and a short burst of cool air from indoor units are expected behaviour, not a fault.

Also called defrost, defrost mode

Direct answer

What is a defrost cycle?

A defrost cycle is a heat pump briefly running in reverse to melt frost off its outdoor coil. Steam, a pause in heating, and a few minutes of cool indoor air are all normal during it.

Defrost cycle: how it works

Frost forms because a coil collecting heat has to sit below the air it is collecting from, and moisture in that air freezes onto it. Frost insulates the coil and blocks airflow, so the control reverses the valve, sends hot gas outdoors, and stops the outdoor fan. The frost melts as water and steam, and the system returns to heating. The indoor fan is held off so the room does not get a blast of cold air.

Where you meet the term

  • In a complaint about steam pouring off the outdoor unit, which is melted frost rather than smoke.
  • In a complaint about cool air in heating, when the pause happens to coincide with a call.
  • In a genuine fault, where the coil stays iced across many cycles and defrost is not clearing it.

How to check it

  1. Time the event before treating it as a fault. A cycle takes minutes and ends with heating resuming. A unit that stays iced for hours is a different problem. Owner check
  2. Check the drainage under the outdoor unit. Melt water has to leave. A blocked base pan or a unit sitting in a puddle refreezes the water it just melted. Owner check
  3. Confirm the coil is clear after a cycle finishes. Light frost that clears each time is normal operation. Ice that survives the cycle points at defrost control, the sensor, or the valve. Technician
Read this twice

Defrost cycle: what it is not

  • Steam from the outdoor unit is not smoke. It is melted frost evaporating off a warm coil.
  • Cool indoor air during defrost is not a heating failure. The cycle ends by itself and heating resumes.
  • Pouring hot water on a frozen outdoor unit refreezes at the base and blocks the drainage path.

Defrost cycle questions

How often should a heat pump defrost?

Defrost demand follows frost formation on the coil. Cold, damp weather can create more demand than dry cold, and controls initiate the cycle from time, temperature, or measured coil conditions depending on the design.

Is it normal for the outdoor unit to steam?

Yes. Frost melting off a warm coil produces visible vapour, sometimes a lot of it. It lasts as long as the cycle does.

When is icing on the outdoor unit a real fault?

When the coil stays encased in ice across repeated cycles, when the base is frozen solid so melt water cannot drain, or when the unit never appears to defrost at all. Those point at the defrost control, its sensor, or the reversing valve.

Where this definition comes from

Written from operation manual, manufacturer support article material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.

  • ADUH LUAS1 Operating ManualFujitsu General · operation manualADUH07, ADUH09, ADUH12, ADUH18, and ADUH24 LUAS1 operation, defrost, filter care, and normal behavior.
  • Why Is My Mini-Split Not Working in Heat Mode?Trane Technologies · manufacturer support articleTrane ductless heating checks covering controls, airflow, outdoor clearance, and service conditions.