Daikin [Ice on the outdoor unit]what to check

Frost on the fins of an outdoor unit in cold damp weather is expected, and every manufacturer in this category says so. The evaporator has to sit colder than the outside air to take heat from it, so in the right conditions the moisture in that air freezes onto it. The unit detects the frost and clears it by briefly reversing the refrigeration circuit, which is why you sometimes see a cloud of vapour rising off it, hear the note change, and find the radiators going cool for a few minutes mid-cycle. All four of those are the same thing happening. The part that matters, and the reason this is a caution rather than an information page, is what you must not do about it.

Daikin publish the instruction in their own safety wording: do not use cleaning materials or means to accelerate the defrosting process other than those recommended by the manufacturer. Concretely that rules out the four things people reach for — a kettle of hot water, a heat gun, de-icer spray, and a screwdriver or scraper. They also publish the reason it is not merely bad practice: do not pierce or burn refrigerant cycle parts. The fins have refrigerant pipework running through them, the newer Altherma monoblocs are charged with R32 which Daikin label as mildly flammable, and a scraper that goes through a fin can reach it.

Daikin add a detail that changes how you would notice a leak: the refrigerant is odourless, and does not contain an added odour the way domestic gas does. So the smell test that works on a boiler does not work here. What is legitimately yours is the space around the unit. Keeping snow, leaves and vegetation away from the coil and the airflow path is published owner maintenance, as is making sure condensate and rainwater can drain away freely — because meltwater that cannot escape refreezes under the unit and builds up over a cold week into something that genuinely does block it.

1. 1

   Watch it for a full cycle before doing anything. A unit that frosts and then clears itself, with vapour coming off it as it does, is working exactly as designed.

2. 2

   Clear snow, leaves and anything growing around the unit, keeping well away from the fins themselves. Air has to reach the coil and get away from it.

3. 3

   Check that meltwater can drain away from underneath. A blocked drain path turns one night of frost into a week of accumulating ice.

4. 4

   Do not pour hot water on it, use de-icer, aim a heat gun at it, or scrape the fins.

   Daikin publish this as a safety instruction, not a preference: do not use means to accelerate the defrosting process other than those recommended by the manufacturer, and do not pierce or burn refrigerant cycle parts. The fins carry refrigerant pipework, newer units are charged with R32 which Daikin label mildly flammable, and the refrigerant is odourless so a leak gives you no warning smell.
5. 5

   If the ice builds up day after day instead of clearing each cycle, call your installer. Progressive icing is a fault, and it is one that gets worse rather than better.