Why Defrost Cycles Occur
When an ASHP operates in cold, humid conditions, moisture freezes on the evaporator coil. The heat pump detects this and initiates a defrost cycle to clear the ice.
What Happens During Defrost
The heat pump briefly reverses refrigerant flow to pass hot refrigerant through the outdoor coil, melting the ice. Duration: 5–15 minutes. You may notice: reduced heating output, steam from the outdoor unit, dripping water, fan stopping temporarily.
How Often Is Normal?
In mild weather, defrost may not be needed. In cold damp conditions (0–5°C), every 1–3 hours is normal.
When Is It a Fault?
A fault is indicated when the unit ices completely without defrosting, defrost cycles occur every 15–20 minutes, or heating output is severely reduced for extended periods.
Sources: independent field trials and the MCS scheme.
