We have an add on cooling unit to our central heating. It's a Pioneer, 3 phase, 19kw unit. The house is approx. 32 squares. The heating and cooling is zoned separtely upstairs and downstairs. There is a return air vent both upstairs and downstairs. The whole system, and house, are only 3 years old. The thermostat is a Brivis Networker.

The problem is, the larger of the two copper refrigerant pipes from the indoor coils to the outdoor unit that is covered with foam insulation(I think it's the suction line) becomes frozen because the refrigerant is being forced back to the outdoor compressor. When this happens, the copper pipe freezes up like a balloon, and the indoor temperature keeps rising. The whole air conditioner stops cooling, even though the fans and compressor keep running, and the whole system just wastes electricity.

If the air conditioner has not been used much (ie, if there haven't been many hot days during summer), it fires up and works perfectly. However, the pipe seems to freeze whenever it's been working hard during the day, and there's a drop in outdoor temperature (such as a cool change, common in Melbourne Australia during the summer). The outside temperature will drop, inside temperature in the house will rise from about 23 degrees C to 25 or even 26 degrees C. The compressor will continue to work all night, with the pipe frozen, and the temperature will not drop unless it's significantly colder outside (yeah, I know this wastes money, but it's uncomfortably hot to sleep at night).

Furthermore, if the unit has been working all night with the pipe frozen, and hot weather has been forecast for the remaining few days, the a/c will continue to struggle, and the pipe will remain frozen (I guess because the unit hasn't been off long enough for the refrigerant to unfreeze).

Now for the interesting part - the a/c company who installed the system cheated on us and installed non-genuine Pioneer coils in the roof, without our knowledge or permission . We only found out when another a/c repair company inspected the coils in the roof. And of course, that same a/c company has gone bankrupt (I wonder why), no longer exist, and of course we now have no warranty .

Some other information -

- The first repairman was "not so smart", and reversed the polarity on the compressor, and over-filled the unit with freon, so the compressor would pop and bang when switched on, and not run at all.

- The second repair company, sent by the builder, fixed the above, and drained and re-filled the unit again with freon. But ever since he did this, the pipe has been freezing even worse, and the cooling problems became worse. The cooling capacity was always suspect, and the pipe would always freeze to some extent, but not as badly as it has done last summer and this summer.

- The second repairman fitted a thermostatic expansion valve (TXV) near the compressor on the outdoor unit. We both thought this would fix the problem, but when he hooked up his guages to the unit, it still did weird things, like working perfectly for 5 minutes, before sending refigerant back to the compressor again, and freezing the pipe, and then unfreezing and working perfectly again after a few more minutes.

I have been reading the following website to try and diagnose the problem, but my knowledge of a/c units is limited :

In summary, considering all of the above, I can only conclude :

a) The unit is overcharged with refrigerant. Perhaps both repairmen are incompetent, and are not sure what the correct amount of refrigerant the unit requires.

b) There are not enough return air grills in the house for the unit. Less likely, imo, as the unit cools adequately unless the pipe is frozen, as described above.

c) The non-genuine indoor coils are incompatible with the Pioneer outside unit.

d) The whole compressor and outdoor unit are screwed, and need to be taken out the ocean and attached to a anchor and chain

Any advice would be appreciated