Vaillant [Vapour from the outdoor unit]what to check

This is the single most reported heat pump non-fault, and Vaillant answer it in one sentence: if you observe a cloud of vapour on the product, you do not have to do anything, and the effect may arise during the thawing process. It looks like smoke, it appears suddenly, and it usually happens on the coldest morning of the year, which is why people photograph it and ring somebody.

What is actually going on is worth understanding, because it also explains two other things owners notice. Below about five degrees outside, frost forms on the fins of the outdoor unit as it pulls heat out of damp air. Vaillant detect that automatically and thaw it at intervals by reversing the refrigeration circuit while the heat pump is running — which means, for a few minutes, the outdoor unit is being heated and the heat for doing it is taken from your heating system.

So the cloud of vapour is melted frost evaporating off a coil that has briefly become warm. And the two side effects are entirely normal: your radiators may go cool for a few minutes during a defrost, and you may hear the unit change note as it switches over. None of it is a fault. Persistent ice that never clears, or ice building up in the base tray beneath the unit, is a different matter and is worth a call.

1. 1

   Nothing. Vaillant’s own instruction is that you do not have to do anything.

2. 2

   If your radiators went cool at the same time, that is the same event — the defrost takes its heat from the heating circuit. It comes back within a few minutes.

3. 3

   Do not try to help it along. Never pour water on the unit or pick ice off the fins.

   Daikin publish the general rule for all of these appliances: do not use any means to accelerate defrosting other than those the manufacturer recommends. On an R32 or R290 unit you are working next to a flammable refrigerant circuit with thin aluminium fins in front of it.
4. 4

   Do check that leaves, snow and garden debris are clear of the air path, and that nothing has been stacked against the unit. That is a genuine owner job.

5. 5

   Call your installer only if ice never clears, or if it is building up in the tray under the unit rather than on the fins. That usually means a blocked condensate drain or a failed defrost. Vaillant UK’s one published exception to leaving ice alone is for the low pressure faults F.539 and F.554, where they allow an external heat source to be applied to the rear of the heat pump or to the condensate pipe — that is for a unit that has already locked out with a code, not for a routine defrost.