Check these first, whatever the brand
- Keep it switched on
Don't switch the heat pump off at the mains in freezing weather. Mitsubishi and Vaillant both say frost protection doesn't run while the unit has no power.
- Let defrost finish
Steam and a pause in heating during defrost are normal. Mitsubishi says not to speed defrost up by other means, so don't chip or pour water on the outdoor unit.
- Check the water pressure
Daikin lists low water pressure and flow among the causes. If the gauge reads below about 1 bar, tell your installer.
- Note how often it happens
A one-off in very cold weather can clear by itself. If it keeps coming back, your installer needs to know when.
When it's time for an engineer
If it repeats, your installer will check the water flow and pressure, the system's water volume, the temperature sensors and, if needed, the refrigerant circuit, which needs an F-gas engineer.
Questions people ask
Is it normal for a heat pump to defrost in winter?
Yes. The outdoor unit regularly melts frost off its coil, and you may see steam and a short pause in heating. A freeze protection code is different: it means the system had to protect itself.
Which code is freeze protection on my heat pump?
Mitsubishi Ecodan shows L6 or P6, Daikin Altherma 89-01 and Vaillant aroTHERM plus F.741.