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  1. #1
    Join Date
    Jun 2008
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    Recommended Cycle Time???

    I have seen 3 cycles per hour (10 min on/10 min off) recommended on several sites. What is the reason for this many cycles per hour over say 1 cycle per hour? It seem the AC is just getting to peak performance before it shuts down.

    The lowest setting on my thermostat is a 1 degree span. This gives me 1 cycle per hour (30 min on/30 min off) during the heat of the day. So, if it takes my house 30 min to warm 1 degree with the AC off, my thermostat would have to read a 1/3 degree difference to get a 10 min off time. Then I guess my AC would run 10 min to cool 1/3 degree. Is there a recommended tstat that reads partial degrees?

    Or is there something else that would need to change to get to the 10 on/10 off recommended cycle time?

    Thanks.

  2. #2
    Join Date
    Jan 2004
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    Lancaster PA
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    There is no fixed run time. Or off time.
    3 CPH is a comfort efficiency mix.
    During our recent heat wave, the one day my A/C ran 6 hours straight.
    My unit doesn't always cycle 3 times per hour on milder days either.

    if your comfortable, and your A/C doesn't seem to be using excess electric, your stat is working fine as far as its cycle times.
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  3. #3
    Join Date
    Jun 2008
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    Thanks Beenthere for the reason for the rule. When rules/targets/goals are cited, and you can't meet them, it causes concern. If this rule is a comfort/efficiency issue, then I'm satisfied with my AC as is and I'll just chill out.

    I'm still trying to understand how you could get to 3 cycles per hour. When my AC runs 10 min in milder weather, it will be off for an hour or so. If my house lost it's cool in 10 min, the AC would stay on for the rest of the day. So would it be a tstat that could read partial degrees that gets you to more cycles per hour?

  4. #4
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    Digital stat that are set by cycles per hour, check temp by tenthes of a degree, but only displat whole degrees.
    At mild temps, they don't do 3 cycles per hour most of the time either.
    And sometimes when they do, the on time may be down to 5 minutes.
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  5. #5
    Join Date
    Jun 2008
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    Thanks again Beenthere.

    "Are we happy Vincent? Vincent, are we happy?"

    yeah, we're happy.

  6. #6
    Join Date
    Mar 2008
    Location
    Jackson, NJ
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    176
    Quote Originally Posted by beenthere View Post
    Digital stat that are set by cycles per hour, check temp by tenthes of a degree, but only displat whole degrees.
    At mild temps, they don't do 3 cycles per hour most of the time either.
    And sometimes when they do, the on time may be down to 5 minutes.
    So stats that are set by CPH, if you go from 3 to 2 on the cooling end of things, does that just widen the spread of .10s of a degree before kicking back on?

  7. #7
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    Basically.
    They just increase or decrease their on off times to meet the CPH setting.
    And if that means overshooting temp, or letting the temp rise higher, thats what it does.
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  8. #8
    Join Date
    Dec 2007
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    89
    load calculations?

  9. #9
    Join Date
    Feb 2003
    Location
    Huntsville,AL
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    4,125
    the general rule of thumb for rotating machines is no more than 4 starts per hour --
    one can check with the mfgr of a given compressor --
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  10. #10

    30 minutes to warm?

    If it takes your home 30 minutes to warm up after a cooling cycle, that is great...means you have good insulation. But if it takes 30 minutes to cool it just one degree I suspect your unit isn't cooling as well as it should. I have a 2800 sq. ft. home that cools from a desd start about 5 degrees in 30 minutes. I would think yours could do better than 1 degree in 30 min. What is the temperature differential during these cycles? Are there some closed registers?

  11. #11
    Join Date
    Jan 2004
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    Lancaster PA
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    Quote Originally Posted by MissouriBound View Post
    I I have a 2800 sq. ft. home that cools from a dead start about 5 degrees in 30 minutes. I would think yours could do better than 1 degree in 30 min.
    And I have mine intentionally set up so it can't do 5 in 30 minutes.

    If you cool too fast you don't get enough moisture out.
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  12. #12
    Join Date
    Jun 2005
    Location
    SW Wisconsin
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    I saw a t-stat that has a 4-F differential, adjustable in half degree increments. You could set it at say 78* & it could be set to cycle off at 74*. That is the extreme setting.

    This temp range is within the Human Comfort Range even at 60% Relative Humidity. Even somewhat oversized equipment ought to achieve 50 or 55% RH with these longer cycles.

    Therefore walls etc., should be cold, & off times should be long.
    Longer operating cycles usually mean longer equipment life & higher SEER performance


    For optimal comfort, you need a 20" fan to keep air circulating, during the temp spread. At reasonable humidity levels those temps are well within the Human Comfort zone, See linked Human Comfort chart.
    http://www.udarrell.com/air-conditio...tent-heat.html

  13. #13
    Join Date
    Jun 2008
    Posts
    63
    Thanks Udarrell,
    There's always good info to be found on your site - too much to absorb in just one reading!

    Since I live in a very high humidity area, I guess I shouldn't try to get my cycle time lower than it is.

    Also from your site, because of the high humidity here, I shouldn't replace my existing system with one with too high an EER?

    "The Supply Air & the Entering Return Air delta-T, - tends towards less & less as the EER goes higher, therefore, dehumidification could become more difficult at the highest EER levels."

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