ice on outdoor unit of ducted heat pump (R/C air con in heat mode)

Recently I noticed that our old ducted heating (which is a reverse cycle air conditioner) has been running continuously. Normally it cycles on and off even in the most extreme temperatures.

We had the compressor replaced nearly twelve months ago (which cost about $2200). If interested, thread about that here http://www.renovateforum.com/showthread.php?t=51195
I believe the whole system was installed in the late 70's (brochure is copyright 1977).

The tech said the compressor failed because it leaked out all the gas and thus all the oil leaked out of the compressor as well. I asked how would I know in future if the gas has leaked out and he said the compressor runs all the time and there will be ice on the outside unit.

So checked the outside unit last night, and I found there were ice crystals over substantial portions of the outdoor coil (the radiator type thing) and the suction accumulator and pipes inside the unit, plus a small part of the compressor. See photos of the ice at http://www.flickr.com/photos/totoblu...540300/detail/

The layer of ice crystals seemed to be about 5mm thick on the pipes inside the unit and probably about the same on about half of the outdoor coil fins (under the fan, where I couldn't easily see)..

It has been a bit colder than normal here overnight (down to 0-1 degrees which is 6-7 degrees below average min for this time of year). At the time of the photos, it was about 6 degrees. It was a dry sunny day before the photos were taken (around midnight). Other weather stats included in the photos.

N.B. We don't leave our heating on overnight - it is turned off manually at around 12 midnight - 2am.

Is this ice normal? According to the manual and what I read on the web, ice on the unit is not unexpected under some circumstances but the unit should pause every hour or so and defrost, but I don't recall it turning off in recent nights. If it gets too cold for the heat pump, the unit is supposed to switch to emergency heat (which just uses radiant heat coils to supply the heat). The unit seems to be North American and the manual talks about snow etc, so it should handle the cold here with no trouble.

After I saw the ice on the unit, I turned it onto emergency heat. It reached the set temperature in about 5 minutes and thereafter cycled on and off say 5 mins on, 15 mins off, which is what the heat pump mode normally does.

FYI emergency heat uses about 18kW and in air con cooling mode it uses about 6kW (I never measured it in heat mode but I presume it's the same), so it costs a fair bit more to run on emergency heat.

Would this amount of ice prevent the unit from functioning effectively? I'm wondering if the defrost circuitry is broken and whether this could have caused the original compressor to seize. I vaguely remember the compressor running most or all of the time last year (but I didn't take much notice of it before I got the $2200 bill).

I felt the top and bottom of the compressor and the bottom was warm but not hot (probably 20 degrees or so) so it seems like it still has oil in it.

By the way, which way should the fan blow? It is blowing upwards in heating mode (i.e. the fan is blowing away from the outdoor coil) which doesn't seem right to me if it is trying to extract heat from the air.

I don't want to call a tech if there is nothing wrong - that means $100 or so plus not using the heat pump for weeks until the tech comes. However I don't want to blow another compressor. Comments?

Photos from before the compressor replaced http://www.flickr.com/photos/totoblu...952409/detail/