Heat pumps

Heat pump icing up

Warning Do not attempt any electrical or gas work yourself. Always use a qualified, MCS-certified, or Gas Safe registered professional for repairs.

What you’re seeing (symptoms)

White frost or ice is covering the metal fins on the back or sides of your outdoor heat pump unit. The system may be making a louder-than-usual humming or “steaming” noise periodically.

What’s normal vs not normal

Normal:

  • A thin, even layer of white frost when the outside temperature is below 5°C.
  • The unit periodically stopping the fan and making a “whooshing” or steaming sound (this is a defrost cycle).
  • Water dripping or pooling beneath the unit during and after a defrost cycle.

Not normal:

  • Solid, thick blocks of blue or clear ice covering the entire unit.
  • Ice forming only in one specific patch or corner of the coils.
  • The fan blades hitting ice and making a clicking or grinding noise.
  • Ice that remains on the unit for several hours even in temperatures above 7°C.

Likely causes (ranked)

  1. Restricted airflow: Dead leaves, litter, or snow have built up behind or underneath the unit, preventing air from moving through the coils.
  2. Faulty defrost sensor: The sensor that tells the heat pump when to start a defrost cycle is loose or faulty.
  3. Low refrigerant: A gas leak can cause the coils to get much colder than they should, leading to rapid ice buildup.
  4. Poor drainage: Meltwater from previous defrosts hasn’t drained away and has frozen into a solid block at the base.
  5. Fan motor failure: If the fan isn’t spinning, the unit will ice up almost immediately.

Quick checks you can do safely

  • Clear debris: Gently brush away any leaves or snow from the back and sides of the unit.
  • Check the base: Ensure the drain holes underneath the unit are clear of ice and dirt.
  • Observe a defrost: Wait near the unit for 30-60 minutes; does it eventually stop the fan, get warm, and melt the ice?
  • Check the setting: Ensure your controller isn’t set to an extreme “Quiet Mode” which can sometimes reduce fan speed too much in cold weather.

When to call an installer

  • If the unit is a solid block of ice and the “Defrost” light or icon on your controller has been on for hours.
  • If you see the fan is physically stuck against ice.
  • If there is a visible oily residue on the coils (a sign of a refrigerant leak).

Questions to ask your installer

  • “Is the defrost thermistor positioned correctly on the coil?”
  • “Is the condensate drain pipe heated (trace heating) to prevent freezing in winter?”
  • “Can you check the refrigerant pressures to ensure there isn’t a leak?”

Related NetZeroNow tools & guides

FAQs

Why does it ice up anyway? As the unit extracts heat from the air, the coils become very cold. Moisture in the air naturally freezes onto these cold surfaces, just like in a freezer. Can I pour hot water on it? No. This can cause the metal to contract too quickly or create more ice once it cools down. Let the system’s own defrost cycle do the work. How often should it defrost? In damp, cold weather (around 2°C), a heat pump might defrost every 30 to 90 minutes. This is perfectly normal. Will it still heat my house while defrosting? Temporarily, no. It actually steals a little heat from your house (or a buffer tank) to melt the ice on the outdoor unit. You shouldn’t notice this inside. What if it’s steaming? That’s a good sign! It means the defrost cycle is working and the ice is turning into water vapour.\n

Written by NetZeroNow | Last updated on 2026-03-02