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Community and Q&A

Minisplit compressor

Chris King | Posted in Mechanicals on

Hey all, I installed a Fujitsu compact cassette in summer and with the weather getting cold it has started to run its heating cycle. I’ve noticed that the outdoor compressor generates a large amount of water in heat mode. Specifically it is able to fill up a 5 gallon bucket in slightly more then a day. I know during extreme cold it can cause ice to form and then melt out the bottom. But is it normal to constantly have water coming out of the outdoor compressor even during mid 40s weather? Can’t find anything in the manual.

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  1. GBA Editor
    Martin Holladay | | #1

    During the summer, the indoor coil is cold, so that's where the condensation happens.

    During the winter, the outdoor coil is cold, so that's where the condensation happens.

    Sounds normal to me -- but I don't know about the other question ("Is 4 gallons per day of condensate normal?").

  2. Chris King | | #2

    Thanks Martin

  3. Expert Member
    Dana Dorsett | | #3

    At mid-30s to mid-40s weather is usually the peak temperature range for defrost cycling and condensation because the outdoor dew point are still pretty high and there's a real heat load, so it's running quite a bit, unlike when it's in the mid-50s and running shorter /fewer cycles. . At mid-40s outdoor temps there usually lots of moisture in the outdoor air, with dew points in the 30s, or even 40F if it's close to raining outside.

    When it drops below freezing outdoors the dew points drop in to the 20s or lower, the moisture content of the outdoor air is much lower, and even though it's running harder/longer, the amount of moisture it removes from the outdoor air is less.

    The heat of vaporization of water is 970.4 BTU/lb. 4 gallons of water weighs 33.36lbs, so 4 gallons of condensation represents 33.36 x 970.4= 32,373 BTU of heat going into the mini-split. Over 24 hours that's only 1349 BTU/hr average coming from condensing the water. The average heat load of a code-min 2000' house at mid-40s outdoor temps will be an order of magnitude higher than that, so yeah, 4-5 gallons/day would be pretty normal, only about 10% of the heat it's retrieving would be from the condensation.

    But it will vary a lot depending on the outdoor dew points. From the psychrometric tables:

    40F dew point air has about 0.005 lbs of water per lb of air.

    20F dew point air has about 0.002 lbs of water per lb of air (less than half.)

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