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  1. #1

    AC is not cooling enough...

    I have 3.5t Lennox AC unit which is supposed to be more then adequate to cool down a house of 3200 sq.ft. With temperature outside hitting +32C, my AC having hard time to bring inside temperature to 25C. It is working non stop and still cannot do better then 25.5C.

    I suspect that something is wrong, but not sure what it might be.
    I would appreciate an advice on what I should check to troubleshoot.

    thanks

  2. #2
    Join Date
    May 2007
    Location
    DC Metro Area (MD)
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    3,371
    Was a Manual J load calculation done to size the system? I'm not second-guessing your size because my house has around the same square footage (incl. conditioned basement) with 3.5 tons doing just fine. I think it's time to have your system looked at. It could be undersized, or it could be another problem (leaking ductwork, wrong refrigerant charge, etc.).

  3. #3
    Join Date
    Sep 2002
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    I don't know
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    2,909
    Is this a new problem?

    If the ductwork is exposed and in the conditioned space, seal all leaks.

    Change the filter, ensure that the blower wheel is clean, and make sure that all vents are clear. If you have a humdifier, make sure that the bypass damper is closed.

    Also inspect the condenser coil for heavy dirt/dust builtup.

    If problem persists, get it checked.
    General public's attitude towards our energy predicament: "I reject the reality of finite resource depletion and substitute it with my own; energy is infinite, we just need an alternative storage medium to run the cars on. The economy can grow indefinitely - we just need to "green" everything! Technology is energy! Peak what?"

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,603
    Thats only a 13°F difference.

    You didn't give enough info to help you though.

    Has it always had this problem.
    New construction, or old.
    Original A.C system or replacement.
    Was a load calc done.
    What was the total and sensible load the salc same out to.
    What are your design conditions.
    Where are you located at.
    Do you have a fresh air vent. Or a HRV/ERV.
    How often do you change the air filter.
    What type of air filter do you use.
    What did your service company say is wrong, or why it isn't cooling better.
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  5. #5
    Join Date
    Jul 2008
    Posts
    7
    Quote Originally Posted by beenthere View Post
    Thats only a 13°F difference.

    You didn't give enough info to help you though.

    Has it always had this problem.
    New construction, or old.
    Original A.C system or replacement.
    Was a load calc done.
    What was the total and sensible load the salc same out to.
    What are your design conditions.
    Where are you located at.
    Do you have a fresh air vent. Or a HRV/ERV.
    How often do you change the air filter.
    What type of air filter do you use.
    What did your service company say is wrong, or why it isn't cooling better.
    Add to that:
    Is the direct sunlight entering through windows?
    Do you have enough insulation (not alway's do they put in the amount you pay for.)

    There is alot that can cause a house to cool poorly,aside from just plain udersized. My advice: Pay for a service call from a reputable company.
    but check your filter first. (Thats my #1 service call)

  6. #6
    Join Date
    Dec 2007
    Location
    East central Indiana
    Posts
    477
    One thing you can do is to check the indoor temp (prefferably as close to the return vent as possible), and then check the temp of the air coming out of the supply registers. If the difference in the temperatures is around 14 F, then the unit is probably functioning properly. But if, in spite of a proper difference of temps, the unit is still running all the time and not keeping up, then it may well be undersized.

  7. #7
    Here is some extra info.

    Has it always had this problem. - yes
    New construction, or old. - new construction
    Original A.C system or replacement. - original
    Was a load calc done. - not sure what this is, so probably it wasn't
    What are your design conditions. - 2 floors detached
    Where are you located at. - toronto canada
    Do you have a fresh air vent. Or a HRV/ERV. - no fresh air vent
    How often do you change the air filter. - in 3 moths period
    What type of air filter do you use. - a regular filter
    What did your service company say is wrong, or why it isn't cooling better - did not give a meaningfull answer, just said this is the way it should be.

    I think the air flow is not as strong as it should be. What should I check in this case?

    thanks

  8. #8
    Join Date
    Jul 2008
    Location
    North Dallas
    Posts
    18
    Off hand it would easily be suspected that the unit is undersized. Get an other "good" company out there to reevaluate. Operating pressures, superheat and subcooling would almost confirm or shoot down that theory. It will probably answer any ductwork conditions too for the most part. Tell them you want those readings.

  9. #9
    Join Date
    Apr 2007
    Posts
    623
    I am hoping some of that 3200 square ft is basement,

  10. #10
    Join Date
    Mar 2001
    Location
    North Carolina
    Posts
    428
    Yep somethings wrong. You shouldn't trouble shoot it call you hvac guy out to trouble shoot it thats his job and he knows what to look for. If not call his competitor and I am sure he will do his best to find the problem to 1 up him.

  11. #11
    Join Date
    Sep 2002
    Location
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    I'm outside of toronto (same climate), and with typical construction a 3.5 ton unit should be okay for 3200 sq ft. (It should run a lot to hold 24-25) If you have a lot of windows on the south/west sides, it might be undersized.

    I used to be in a slightly larger house than that (+200-400 sq ft.) and a 3 ton unit ran continuously (easily from 10am-9pm) to hold 24 in that kind of weather. Do what you can to reduce heat gain (shading, solar screens, CFLs, blinds, attic insulation) and have another company check it out prior to making any changes.
    General public's attitude towards our energy predicament: "I reject the reality of finite resource depletion and substitute it with my own; energy is infinite, we just need an alternative storage medium to run the cars on. The economy can grow indefinitely - we just need to "green" everything! Technology is energy! Peak what?"

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