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Thread: AC runtimes

  1. #1
    Join Date
    Jul 2005
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    First off..Thanks to all who have replied to my previous postings. As a home owner who has no concept of all that is involved in Heating/Air, I find this site very educational!

    From my readings here, I have learned the importance of the Manual J for sizing. Lucky for me the company that put in my new Furnace/AC 2 weeks ago did too. My question has to do with how much my AC should run at design temp. Am I correct in my understanding of some of the posts Ive read here that if it is 95 degrees outside, and the system was sized to do 75 degrees cooling, that the unit will run constantly to maintain the 75 deg inside temp? Today was a good test with outside temp at 98. House kept to 75 degrees and ran for about 20 min every hour from 4pm till now. Just kinda wondering if this may mean something was off on the J, or if all of the extra cellulose and new windows are working like they should.

    Thanks for any input!

  2. #2
    Join Date
    Oct 2003
    west chester pa
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    I would guess the new windows are saving you money as we speak!!!!!!

  3. #3
    Join Date
    Aug 2004
    Tyler TX
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    If its running 20 min of every hr at or above design temp then it is over sized. Many contractors add a little fudge factor to thier calc to cover ther rear.

    Did they do a manual calc or did they use software? Also did they use the region design temps or did they use thier own? many I have seen use 68 deg inside with 102 outside but the design temps are actually 75 inside and 97 outdoor for my area. Ask for a copy of the man j load calc to see what they did. That should tell you why its not running longer
    HVAC Contractor, Tyler Texas.

  4. #4
    Join Date
    Jan 2004
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    The 20 minute run times would indicate an over sized unit.

    If its keeping the humidity in check when the temps are in the low 70's then it shouldn't be a problem.
    If your getting high humidity when it 78 outside, your going to have alot of trouble with mold.
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