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  1. #1
    Join Date
    Jun 2010
    Location
    S. Texas
    Posts
    63

    Unit cools well, but occassionally freezes evaporator on mild nights-mid 70s

    Long post, just wanted to anticipate required information.

    Occasionally freezes evap overnight when temp drops to mid 70's. Otherwise Cools well, humidity good , pulls down fine. Drains checked and flushed .

    29 degree split on evap, plenum penetration above unit reads 43.5 deg typically. nearest duct 48-49 degree
    system superheat 4 -5 degrees which is really close on generic s/h chart (piston unit)
    liquid line 88
    95 ODB condenser output air 115
    88 OWB
    IDB 78.4
    IWB 63.7 ( humidity gauge reads 43% in kitchen)

    South Texas, 10 SEER 4 ton ICP, fixed orifice /piston 1900 square ft, brick, slab, 8 ft ceilings. Attic well ventilated, typically max 112 at 98 ambient, doors seal well, duct boots mastic treated at ceiling, single glaze windows. Extra R13 laid over duct trunk in attic 60 %. No holes in ducts in attic. Attic hatch insulated & sealed. Normal lineset length, accross hall and 8x5 bath to outside.Unit has 20 x 20 filter space, not used, have 4 - 20 x 25 filters stacked across entire closet doorway treated with filter spray. Plenum penetration to attic sealed. Top 2 are fiberglass, bottom two are pleated. Done to keep dust and cat hair out. Doesn't restrict airflow much either.

    Replaced leaking f/d last fall and recharged. Used a new 5 ton f/d. Charged to superheat for the day. Did not put gauges on today, I am assuming it hasn't leaked since recharge.
    Cleaned coil and blower last year, used nubrite on coil several times until no more dirt rinsed out.

    Can keep unit from freezing by blocking condenser 10-15 % overnight, keeps piston feeding enough I think. Evap has some coil fin damage, not really sure if it is enough to cause 29 degree split. I realize this should be closer to 20 degrees. Runs typically 20-25 minutes, off for 15 to 20 minutes even hottest days, probably one ton oversized.

    Any comments appreciated
    Thank you

  2. #2
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    Low airflow through the unit would be the most likely culprit.
    If more government is the answer, then it's a really stupid question.

  3. #3
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,986
    Probably inadequate airflow heatload going through the indoor coil.

    If outdoor temp gets too low pressures will drop too much with a piston metering device can starve indoor coil &, indoor coil pressures can drop below freezing point, similar to low charge.

    Replacing piston with TXV could be helpful, plus increasing indoor airflow through coil & not setting thermostat too low.

    A two speed condenser blower motor that drops the CFM when temps go below a certain point...

  4. #4
    Join Date
    Jun 2010
    Location
    S. Texas
    Posts
    63
    I missed a detail when I posted, I've tried increasing fan speed to high from medium, but it doesn't change the split, just noisier. Also the LL temp was 92 not 88. The unit has done this occasionally for several years. I read Udarrell's site . I think the inside unit is not original to the condenser unit. Can't really straighten the fins. Agree with responses, just wondered what I might have missed.

  5. #5
    Join Date
    May 2006
    Location
    Austin, Texas
    Posts
    281
    Not sure how you are stacking 4 filters in doorway, is this a mobile home? Even with plenty of filtration and return air you may be short on supply side ductwork causing low airflow.

  6. #6
    Join Date
    Jun 2010
    Location
    S. Texas
    Posts
    63
    I have 4 filters across entire doorway, all in parallel, if the lower one is dirty the air can go up and over and still get filtered. There is no return duct, just the hallway. The bottom filter is taped to the floor to catch dirt and hair. I guess it's the fin damage causing the high split. My instructor said the 20 x20 single filter was too restrictive for 4 tons, when the filter got dusty it would get distorted by the airflow.

    At 4 AM last night it seemed to be freezing up again so I just shut it off until now.

    Was thinking of putting a window unit tstat on the suction line and connecting to the wire to the outside contactor coil.

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