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  1. #1
    Join Date
    Jun 2008
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    63

    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
    Location
    Lancaster PA
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    68,320
    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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    63
    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
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,320
    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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    63
    Thanks again Beenthere.

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

    yeah, we're happy.

  6. #6
    Join Date
    Dec 2007
    Posts
    89
    load calculations?

  7. #7
    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!

  8. #8

    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?

  9. #9
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,320
    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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  10. #10
    Join Date
    Jun 2005
    Location
    SW Wisconsin
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    5,006
    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

  11. #11
    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."

  12. #12
    Join Date
    Jan 2004
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    Although the long run time a 4* offset would create will bring the humidity down while its running. The long off time could allow it to increase more then just 60%.
    At 75*F, and 55%RH, I feel uncomfortable in my house.
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  13. #13
    Join Date
    Jun 2005
    Location
    SW Wisconsin
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    Quote Originally Posted by beenthere View Post
    Although the long run time a 4* offset would create will bring the humidity down while its running. The long off time could allow it to increase more then just 60%.

    At 75*F, and 55%RH, I feel uncomfortable in my house.
    Good point, - beenthere.
    Everyone is different when it comes to the Human Comfort Zone, with air movement I am very comfortable at 76 & 55%RH.

    For my own situation, I would wire a dehumidification stat in parallel with the T-stat, therefore either of their setpoints would trigger 'on or off' cycling.

    Whether the humidity increased to 60% during the off time would depend on the air infiltration rate & the amount of indoor sources of humidity generation.

    If it reaches 50% during the long pull-down runtime at a lower temp, say 74* as the air warms the humidity percentage drops, - unless there is considerable humidity added.

    Everything in the conditioned space gives up its humidity during the long on-cycle pull-down, therefore, there is also a humidity gain lag in that respect because as the air warms it will hold more grains of moisture at even a less percentage of humidity.

    In the beginning of the cooling season, it takes a number of days for everything to give up its moisture, down to the air's %RH level.

    If I set a central system to pull-down to 74 @ 50%RH, how fast or how high it would regain 55 or 60%RH would be mere speculation on my part, as there are countless factors to input to the equation.

    Equipment sizing is still the critical factor in controlling humidity & satisfactory temp levels in humid climates.

    The old units with larger BTUH compressors' to condenser Ratings, when sized right, were great at controlling humidity; though far too many were oversized. - Darrell

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