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  1. #1
    Hello,

    I just joined the forum and would like to ask a few
    questions about one of A/C systems in a home I just
    purchased in April of this year.

    The house has two systems. The system for the main part
    of the house was not cooling well in the last couple of
    weeks when the temperature here in Dallas, TX was ~100 degF.
    The best the indoor temp could get during the day was
    about 80deg. I have 8-9 inches of blown insulation in the
    attic and the house was built in 1986. The main room does
    have many windows.

    I called an A/C company. They came out and checked the
    refirgerant (410A) which seemed a little low. The tech.
    said I must have a small leak (most likely in the coil) and
    added <1 lb. of 410A while monitoring the pressure and
    temperature of the line. I had previously changed the
    filter and cleaned the outdoor unit coils. The unit cooled
    a bit better after that, but still only reached ~77 deg. in
    the middle of the day.

    I had the company come back out. This time we looked at
    the upstairs unit. It is a Carrier system and here are the
    model numbers.

    Furance is: Carrier 58CVA090---10116
    Upstairs coil is : Carrier CE3AXA042000ABAA
    Outside unit is: Carrier 38TXA042300

    The second tech. found the following things:

    1) The plenum? (metal box on the outlet of the evaporator
    coil) was built such that ~35-40% of the coil didn't have
    air flow across it. The box inlet is smaller than the
    coil outlet. the guys said it was just a poor install.
    And that makes sense to me.

    2) He said the evaporator coil was only for R-22 and not
    rated for 410A pressures. I've read that some coils can
    be used for either. The coil says R-22 on it and a max
    pressure of 300 PSI. When the first tech added the 410A
    the pressure was ~380PSI and when we were done it was
    ~400PSI. Can anyone tell from the above model numbers
    if my coil can be used with 410A?

    3) My furance is a Carrier Weathermaker 8000 with variable
    speed fan, but the theromstat was a manual mercury
    thermostat (with 4-wires) so I am unable to take
    advantage of the variable speed capability of the fan.

    The second tech recommended that I replace the evaporator
    coil with one that is rated for 410A since my current one
    probably has a leak in it. Have the plenum rebuilt to allow
    better air flow and then install and wire a thermostat that
    can utilize the variable speed fan.

    I need to get the house cooling better in the Texas heat. I
    am looking at adding insulation, but would like to hear
    opinions on what steps I should take to get my system
    cooling better.

    The plenum fix sounds like it needs to be done. I'm not
    sure about the need/benefit of changing the evaporator coil.
    And I've read how the variable speed fan can help cut
    humidity which makes you feel cooler. I'm not sure how that
    works or if it would be of benefit in cooling my home.

    Thanks for the advice/help in advance.

  2. #2
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    1.yes redo the coil box.if tht much air is bypassing there no way to get the refrigerent charge correct.



    2.Most Carrier coils,are fine with either refrigerent.It shows as a match for either in my book,though we don't do many gas furnaces here,so I could be wrong.The metering device,piston or TXV ,needs to be for Puron/R410a,I'd have that checked.


    3.I'd get the variable speed working by adding the required wires.Add a Thermidistat,if it's older,and look at an Infinity control for the CVA furnace.

    I wouldn't replace the coil,until proven to be a leaker.

  3. #3
    Join Date
    Jul 2004
    Posts
    322
    Sometimes they keep the old plenum when they replace the coil or furnace & this can be a problem. Sometimes a tapered transition is needed or a new plenum. I would also check the duct system for leakage including leaks at the boots, plenums, air handler, etc., and make sure you have adequate attic ventilation. There are many good discussions you can find on this forum of these topics.

    A few minor suggestions:
    --Make sure you have one foot of clearance all around the outside condenser units and 8 feet + clearance above them.
    --Flourescent mini-spiral bulbs reduce the heat load
    --shut down computers when not in use
    --keep window blinds/curtains closed, esp. those that get direct sun

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