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  1. #1
    Join Date
    Apr 2011
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    Performance of my A/C system

    Guys I have a problem with my A/C unit atleast I think it is a problem.

    I have a RHeem 3.5 ton A/C unit atleast 19 yrs old and I have noticed that its cooling performance drops significantly during hot days. If the outside temp is 90 and above and if I set the temp at 75 deg F it seems to run for long periods of time before cooling to the set temp, sometimes like 5 to 6 hrs.

    If the temp outside is below 80 then the unit has no problem cooling down to even 70. So hot days works very very hard, nights works fine

    The A/C tech checked my freon levels and said it is OK so what gives is it the age of the unit or is my house leaking a lot. Please help.

    My home is 1600 sq ft single story and built in 92. Thanks.

  2. #2
    Join Date
    Feb 2010
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    Cooling capacity declines when:
    A. System is run below 80 indoors
    B. Outdoor temp is above 95
    C. Outdoor coil is fouled(age)
    D. Ducts leak cold air and hot air is introduced to replace whats lost

    Duct leakage can have a big negative impact

    At 75inside, 95 outside, and 20% leakage rate, you have about 2.5 tons of capacity(still paying for 3.5 tons of effort to the power co).
    A load calc will determine what your capacity needs are.

  3. #3
    Join Date
    Oct 2010
    Houston, Tx
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    On another thread you asked about dirt in filters and if dirt sucked into your system will be spread throughout the dwelling. That answer was yes but it should also be mentioned that if your return air leak, if you have one, is in the attic where the air is ambient (as hot if not much hotter in Houston attics than outside air temperature) then the hotter air from the attic that would be being sucked into your system is negating your system's ability to properly cool. You also could very well have supply leaks as well. Now if dirt was being passed through the system than as I mentioned in that other thread, the evaporator coil could very well be blocked which would decrease your system's performance, noticabely and especially on hot days.

    The only way to properly check refrigerant levels is to make certain both coils (condensing and evaporator) are clean, no if ands or buts about it so unless that tech verified that the coils are clean and that you have proper cfm he has no right to talk about any "freon levels being okay". He should have told you what the sub cool or superheat was but again, you must have clean coils along with verification of clean coils to check and/or charge to sc or sh and you have dirt flying through your system and a system that is not performing up to par. I'm taking it he did not check the evaporator coil.

    Begin with the simple maintenance and checklist for wear and tear of the system as a whole as that system has been working for the majority of those 19 years under these same conditions. Unless of course no one has said anything until now of it's incapabilities.

    A heat load analysis is not mandatory, not yet.

  4. #4
    Join Date
    Oct 2004
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    Also if you are manually turning the thermostat down 5 or 6 degrees when its 90 degrees, it's going to take the air conditioner longer to cool the house down than when it's 80 outside and your house doesn't have as large a cooling load.
    If you leave it at 75 all the time, does it keep the house at 75?

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