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  1. #1

    AC Compressor in cold weather?

    I recently installed a new Honeywell programmable thermostat. After installing it I was reviewing the user manual which contains a somewhat cryptic warning along the following lines:

    To avoid possible compressor damage do not use automatic switchover mode when outside temperatures are below 50 degrees.
    Since they give no reasoning and it is a generic thermostat that can work with many kinds of systems I'm not sure if the warning is even relevant to our gas heat + central AC system. This leaves a few questions that I hope you folks can address:

    Does running the AC at low outside temperatures cause potential compressor damage? If so what is the mechanism of that damage?

    If running the AC at low outside temperatures can cause system damage, it seems like there should be an interlock to prevent that. It is a Goodman that is around 10 years old. I can get the exact model number off the unit tonight if that helps you folks know if an interlock is already present (if it isn't already present I guess we better add one because my wife and I really like having the thermostst in auto switchover mode in the spring and fall).

  2. #2
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,338
    In winter, the coldest spot in a refrigeration circuit is the compressor crankcase, sitting outside in the chill. Refrigerant inside the crankcase can condense into a liquid and mingle with the oil. If the compressor is suddenly started with the oil in this state, the refrigerant will either suddenly flash off and take oil with it, leaving critical lubrication surfaces high and dry, or it will enter the compression chamber as a big liquid slug, possibly damaging that chamber since compressors aren't meant to pump liquid.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  3. #3
    Shophound - thank you for the information. Operating without proper lubrication is definitly bad and putting incompressable liquid refigerant into a compressor designed to compress gasses sounds even worse.

    What protections do the compressor units typically have against starting when it is excessivly cold? Do they usually come with the unit or are they added by the installer when called for?

  4. #4
    Join Date
    Jun 2011
    Location
    Houston area
    Posts
    1,493
    Heatpumps or reverse cycle air conditioners operate below 50F all the time. They use a CCH or crankcase heater to boil any liquid refrigerant that might accumulate in the bilge area of the compressor. A pro can easily add a bellyband type heater to your system if it doesn't already have one.

  5. #5
    Join Date
    Jun 2007
    Location
    Round Rock
    Posts
    3,559
    It's really not much of a problem and you're worrying about nothing much, unless you like your house really cold. Like in the 60's. Most people aren't going to run their a/c when it's in the 60's outside, much less in the 50's. And you're a/c probably isn't going to kick on if set in the Auto mode unless you're cooking dinner. If it's something you're worried about. You can have a pro install an outdoor sensor with a cooling lockout.
    I like DIY'ers. They pay better to fix.

  6. #6
    Join Date
    Feb 2006
    Location
    Atlanta,GA.
    Posts
    891
    depending on which model stat you have it may accept outdoor sensor

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