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  1. #1
    Join Date
    Dec 2012
    Posts
    8

    Heat Pump Compressor Damage From Cold Start

    I had a Mr. Slim split system heat pump installed in my house. It was working very well until in my never-ending quest to save power, I decided to start turning it off at the breaker during the night and then back on again in the morning. My house is very tight and well insulated so the temperature would only drop from 72 to about 60 while having it off all night.

    Normal start-up procedure was to wait until the sun was hitting the outside unit, then I would turn the breaker on (leaving all indoor units off) and wait 30 minutes before turning indoor units on. The house would then warm up to 72 in about 3 hours.

    One morning after a colder than usual night (got down into the single digits F) - the house had gotten down to about 55 - someone else in the house turned the outside unit on earlier in the morning than usual. The heat pump hasn't worked well since. The lines going to the indoor units don't get as hot during heat mode, and they don't get as cold during defrost as they used to. The heat pump has a hard time holding 72 degrees in the house even running 24 hours/day.

    My guess is that the compressor (or more specifically, the reed valves inside the compressor) have been damaged from starting it up cold.

    Are there any professionals in here that would concur or am I totally off track here?

    Thanks in advance for any input.

    Keywords for future google search: heat pump compressor cold start damage breaker

  2. #2
    Join Date
    Jan 2008
    Location
    Texas
    Posts
    1,666
    I would say it is not comp. Imo 95% of the time it is either locked or shorted to ground when bad. Do you have backup electric strips? I don't want to break the site rules so my advice would be to call out a professional.

  3. #3
    Join Date
    Jul 2009
    Location
    South Carolina
    Posts
    3,092
    If you really want to turn the unit off I would do it at the switch or thermostat but not at the breaker killing all the power to the unit.
    Gary
    -----------
    http://www.oceanhvac.com
    An engineer designs what he would never work on.
    A technician works on what he would never design.

  4. #4
    Join Date
    May 2006
    Location
    Austin, Texas
    Posts
    279
    Call a professional. IN my opinion, probably has a leak and is low on charge. Serviced quite a few and that is majority of issues; leaking flares. Why not just turn unit off with controller? You will never notice the difference on your bills. Good luck.

  5. #5
    Join Date
    May 2009
    Location
    Savannah, Ga/H.H. Island, S.C.
    Posts
    1,403
    Your compressor wasn't harmed from starting up cold. You have another issue going on. Call a professional and get your system looked at.

    And as stated, don't turn your system off at the breaker. Use the remote/thermostat.

  6. #6
    Join Date
    Dec 2012
    Posts
    8
    One person says "either locked or shorted to ground", another person says "probably has a leak and is low on charge" and yet another says "Your compressor wasn't harmed from starting up cold".

    I can definitely say it's not locked. As stated above, it still heats and still defrosts... just not as good.

    I can confidently say it's not low on charge. No, I haven't had it checked, but one thing I didn't state is... I have two identical units and they both exhibit the same problem and they both started after a cold-start. Both worked fine up until the cold start.

    Lots of people, both in this thread and others, say "Don't turn it off at the breaker". In another thread it was stated "If the breaker has been off for an extended period of time, don't start it up until it has had power for several hours". In several threads it talks about compressor heaters to keep them warm when it's cold. I've since learned that my units send a small amount of power to the compressor windings to keep them warm when it's not running.

    So if you can't harm compressors from starting them cold, why have heaters on them? I'm not trying to be facetious, this is a serious question. After reading tons of stuff in this forum and others, there seems to be two contradicting themes:

    1) compressors are tough, you're not going to hurt them - not even starting them up cold.
    2) compressors need heaters to keep them warm so they won't get cold started (and thus damaged?)

    How to reconcile these two themes? Serious replies only please. I'm not trolling, I'm genuinely curios.

    Thanks in advance.

  7. #7
    Join Date
    May 2009
    Location
    Savannah, Ga/H.H. Island, S.C.
    Posts
    1,403
    Please post model and serial numbers. Of all the Mitsubishi systems that I have serviced, I have yet to see one equipped with a crankcase heater. But post the model number and I will confirm this.

    A crankcase heater is a small heater that operates continuously and wraps the base of the compressor to prevent refrigerant migration. Not all systems are equipped with this feature.

    I'm not saying your compressor isn't damaged. I'm saying it is very unlikely that it got damaged from starting up on a cold morning. But we're only going by the info given to us.

  8. #8
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    6,441
    Compressors are generally tough. In very cold weather a heat pump needs a crankcase heater to keep the oil viscosity low enough for it not to have trouble pumping, similar to a Diesel engine having trouble starting on a cold morning, and to prevent refrigerant migration. If the oil is to thick the compressor will generally just not run until it has warmed/losened up. It shouldn't ruin the compressor but possibly could have damaged it after doing this over the course of weeks, months, years. It's odd that both systems have the same symptoms. It could be that the outdoor temp has just reduced the btu capacity of the heat pump to where it is a little bit undersized. The only other think I could think of is maybe the incoming power to the units may be a little lower than acceptable by the compressor (90voltd instead of 115volts etc) it seems if this was the case it would have shown itself in other power consuming devices/electronics in your house. An experienced tech should be able to diagnose the issue if in fact there is a problem. What are the outdoor temps when you are noticing this problem?

  9. #9
    Join Date
    May 2004
    Location
    south louisiana
    Posts
    3,139
    what is/was the point of turning it off @ the breaker?
    just curious
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  10. #10
    Join Date
    Jan 2008
    Location
    Texas
    Posts
    1,666
    You miss read my post. I said it is not compressor failure. I said that 95% that do fail are either shorted to ground or locked.

  11. #11
    Join Date
    Dec 2012
    Posts
    8
    Brent,

    The model # is: MXZ-4B36NA-1
    The SN of the first unit is: 22907021 A
    The SN of the second unit is: 22907023 A

    If you look on page 39 of the service manual in section 10-1, it talks about "PRE-HEAT CONTROL". It tells how to turn it off because it is on by default. It isn't a heater at the base of the compressor, it just energizes the compressor windings with 50W. Not enough to turn it, but enough to keep it warm. I'll quote a small section from the manual:

    "If moisture gets into the refrigerant cycle, or when refrigerant is liquefied and collected in the compressor, it may interfere
    the start-up of the compressor.
    To improve start-up condition, the compressor is energized even while it is not operating.
    This is to generate heat at the winding.
    The compressor uses about 50 W when pre-heat control is turned ON.
    Pre-heat control is ON at initial setting.
    [How to deactivate pre-heat control]
    Turn OFF the power supply for the air conditioner before making the setting.
    Set the "4" of SW2 on the outdoor electronic control P.C. board to ON to deactivate pre-heat control function (MXZ-
    3B24NA, MXZ-3B30NA, MXZ-4B36NA).
    "

    Thanks.

  12. #12
    Join Date
    Dec 2012
    Posts
    8
    jtrammel,

    The outdoor temps the first day it started having problems were about 8F in the morning when it was cold-started and it got up in the low 40s during the day. Before I started cold-starting it, it got down in the single digits several times and it ran just fine and never had a problem.

  13. #13
    Join Date
    Dec 2012
    Posts
    8
    forgot to mention, the voltages are fine, I checked it.

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