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  1. #1

    Optimal AC Cycling Times?

    I think this is highly subjective, but I would like to know what's the general consensus of what should an optimal AC cycling times be at?

    If you're putting in a new AC today, obviously you don't want a short cycle time like 5 to 10 minutes. What cycle time would you prefer? 30, 45, 60 minutes?

  2. #2
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    There is no one optimal time.
    An A/C will run longer on a 90° day, then a 80° day.
    If you keep your stat at 72, and its 75 outside, it would not be out of line for it to only run 5 minutes.

    If you keep your stat at 72 and its 90 outside, and it only runs 5 minutes, its most likely over sized.
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  3. #3
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    When it's 98* outside my upstairs unit will sometimes run 10 hours straight to maintain 75* indoor...but the RH is 40%

  4. #4
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    This past Sunday, I recorded 104 degrees from my outdoor temperature/humidity sensor. My attic reached 135. My indoor setpoint was 75. The longest run time I recorded that afternoon was a tad over thirty minutes, with six minute off periods between cycles. Leading up to that long thirty minute run, run times had increased from a measured low of 12 minutes on, 8 minutes on around lunchtime to the thirty on/six off just past recorded peak attic heating at around 4:15 PM. I had to leave shortly after that so did not record diminising run times till almost two hours later, and it was then down to around twenty on/seven off. During peak heating, the house held my desired indoor conditions, which is ~75^F and < 50&#37; RH (observed 42-46% RH). I do not consider my system oversized, I consider my structure a little better than average in regard to thermal performance. Still a ways to go, but better than what we inherited when we bought the place last year.

  5. #5
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    I should add that my downstairs unit runs about 1/2 as much as upstairs...SF is about the same 1650 each....have a 2 ton down and 3 ton up....Power bills are very good too... Systems are RUUD high efficiency....

  6. #6
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    Quote Originally Posted by toohothere View Post
    I think this is highly subjective, but I would like to know what's the general consensus of what should an optimal AC cycling times be at?

    If you're putting in a new AC today, obviously you don't want a short cycle time like 5 to 10 minutes. What cycle time would you prefer? 30, 45, 60 minutes?
    Cycle time is more dependent on the structure you are trying to cool than the HVAC equipment.

    One of the extremes would be a PAHS (Passive Annual Heat Storage) house which needs no HVAC to cool or heat the house - most of them do require a dehumidifier though.

    A little less extreme would be a house / hvac system where you only run the AC once per day, during the early morning hours, to cool off the house, and the house has enough thermal mass and insulation to keep it cool all day.

    Slightly less extreme would be my house where the AC comes on no more than once per hour and the house never varies from 74 degrees.

    All of those ideas have their proponents that think they are the optimal answer.

  7. #7
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    Jackson, NJ
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    I think this thread has other sister threads dealing with the same unanswered question:

    From the pros point of view, if its x degrees outside and my system only runs for y amount of time, do I have a problem or is this okay?

    I've been struggling w/ this for weeks, and I'm not 100% sure I even have a problem. Somebody earlier said most units are engineered for 3 starts per hour. So, if in 100 degrees of heat my system runs for 10 minutes on, 10 minutes off, is it really misconfigured?

  8. #8
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    In PA, if your system cycles 3 times an hour when its 100&#176; outside, and your stat is set under 80&#176;. Its over sized.

    If your system only runs 30 minutes when it 100 out, it is over sized, for PA.
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  9. #9
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    Quote Originally Posted by beenthere View Post
    In PA, if your system cycles 3 times an hour when its 100 outside, and your stat is set under 80. Its over sized.

    If your system only runs 30 minutes when it 100 out, it is over sized, for PA.
    *groan*, that's what I thought.

    Well, it would be far too costly to replace a brand new system, so when it goes belly up I hope you'll still be on this forum. Lancaster is commutable and I apparently need somebody who knows what they are doing.

    *GROAN*

    So, for me I guess its time to think about having another duct dump into the basement. I guess on the bright side if I finish the basement I might actually be okay.

  10. #10
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    Just focus on the a/c supply temp on this graph. Look at the cooling cycle on the hotter day with temp setup when unoccupied on a warmer day. We have a 75^F setting verses a 78^F setting lowering the tstat seting to 76^F. We have five cycles verses two.

    The dead band of the tstat also reduces the cycles. Regards TB
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    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  11. #11
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    Whoops, wrong thread. Sorry.

  12. #12
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    Feb 2008
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    Lucas, TX
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    Paul42, what are the specs of your home that you get 1 run per hour ?

    Where in DFW ?

    Thx.

  13. #13
    Join Date
    Jun 2005
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    A few degrees, - big difference in number of CPH

    Quote Originally Posted by teddy bear View Post
    Just focus on the a/c supply temp on this graph. Look at the cooling cycle on the hotter day with temp setup when unoccupied on a warmer day. We have a 75^F setting verses a 78^F setting lowering the t-stat setting to 76^F. We have five cycles verses two.

    The dead band of the t-stat also reduces the cycles. Regards TB
    A few degrees difference made a lot of difference in number of Cycles Per Hour.
    I would want a t-stat setpoint with a 3 or 4-degree total differential, with half-degree incremental adjustments.

    On a sized right system, that would permit very long High SEER run times that effectively controls humidity levels, & reduces the numbers of startup wear-n-tear on components.

    Low conditioned space humidities permit, wide degree spread, human comfort levels.

    With the basement you would have 4,000 sf.
    Is the basement mostly below ground level, or what portion is above ground level?

    Wow Teddy, you are good at graphs. Though maybe two graphs would be easier to read & follow.

    I appreciate your HVAC expertise & graphic data illustrations. - Darrell - WI

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