Ok the customer has a ducted unit in the house and they complain of the outdoor unit icing up when they have the unit on heating and the temperature outside is below 5 degree celcius..........i have seen photos of the iced up coil and it seems it's only some sections of the coil icing up mainly the top half.

I have checked charge and also relocted the thermistor to a better location due to it being placed very low on the coil and quite close to the compressor which may be affectng it's reading.........

I couldn't get the unit to icing up at all when i was there but the ambient temp was 20 degree celcius, even tried blocking the coil but still no ice.

I did witness the unit go into a defrost once while i had it running for 45min, when it goes into defrost i hear the Reversing valve go and the outdoor fans slow right down for 1 minute.

Oh and it's doens't have its own defrost board.............the defrost is built in to the units outdoor pcb, i can't adjust any defrost setting as the unit doesn't allow it. The only thing i can change is the thermistor.

Anyone got any suggestions??????