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

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  1. #1
    Senior Member
    Join Date
    Feb 2006

    Default 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/

  2. #2
    Senior Member
    Join Date
    Jun 2006
    the 'burn


    i suppose you have made sure no windows in spare rooms are open, or anything like that which would allow cold air in, and that the thermostat is set correctly? Anything like that could cause it to be trying to reach an unsubstainable temperature, hence why it is running continuously

    second, be wary if you take the cover off that. if you have kids, don't let them nearby and always replace the cover when you're done. There's live three phase cables in there.

    Third, the unit is nearing thirty years old. If you've been through one compressor in 12 months and think it's about to happen again, I'd be looking at replacing the unit altogether. It's only going to continue to get older, parts will be harder to get and a newer one would likely be more efficient. Also, some refrigerant gases are becoming harder to find, so if it losing gas somewhere, you may not be able to replace it.

    Mix up a bottle of soapy water. not too soapy, just enough to form bubbles. I'd recommend a squirt bottle for this. While it's running, spray it around the pipes and compressor, especially near any joins, where it touches metal or flashing, and if it looks a bit messy/rusty/dirty. Spray it liberally, obviously no where near the wiring, and if it starts to bubble when it contacts the pipe, congratulations, you've got a leak.

    I'd get it fixed straight away if I were you, not neccessarily by the brands technician. At this age, it's going to be out of warranty [unless it's got a damn good warranty] so any refrigeration mechanic should be able to work on it.

    Finally, if it is icing up overnight and everything else is too... it probably just happens due to condensation. If it ices up in the middle of the day, thats definately a problem.

    Getting a new one may not be on the cards right now, but there wouldn't be much cost difference between replacement versus repair if you need to get it repaired every twelve months.

  3. #3
    Heavy Machinery Claw Hama's Avatar
    Join Date
    Mar 2008


    Call your local reputable air conditioning company and have it checked or serviced. Have you cleaned the filter? If it is icing more than normal and not stopping to defrost it is most likely short of refrigerant. If you continue to run it and it is short of refrigerant you can damage that new compressor. Have it checked.
    PS the emergency heat mode is usually electric heater elements and very expensive to run.
    Refrigeration, Air conditioning NSW lic no 32883C
    A good edge takes a little sweat!!

  4. #4
    1K Club Member
    Join Date
    Oct 2007


    Some ice build up is normal but it should automatically defrost regularly (numerous times per day).

    If the compressor is running constantly then apart from the risk of damage, it will be adding some serious $ to your electric bill - probably at least as bad as running the emergency heater elements.

    I'd stop using it and get it checked. Something is wrong if it's building up a lot of ice, running constantly and not defrosting.

  5. #5
    Join Date
    Feb 2007
    south australia


    i agree, if the compressor is running flat out and hasn't before then you have got a fault, could be duct work has come loose and sucking in cold air or something similar, most likely isn't the t/stat as the room would get hot and you would know that..as for the the ice on the outdoor unit it is common in low temps but it should deforst itself around every hour..depends on unit.. so at the very least i would get the defrost checked.. as for refrigerant charge it wouldn't hurt to get it checked out, should only take a qualified person a few mins to check... all in all a few hundred dollars is better than a few thousand..

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