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  1. #1
    Join Date
    Apr 2008
    Posts
    27

    Is my system short cycling, or is this normal?

    I recently got my thermostat connected to my home automation system, and one of the things I did was log when the AC is turned on and turned off. When I looked at the logs, I was a bit surprised to see how frequently it was cycling. I've attached the past three hours or so from the log, but basically, it looks like it's runs for 10 minutes, then shuts off for 6 minutes, then runs for 10, then off for 6. Is that normal, or should there be longer cycle times? Obviously I'd like the system to run in the most energy efficient manner as well as create as little wear and tear on the system as possible.

    Just for some background, this is my upstairs unit and the thermostat is set for 78 degrees. I'm in central FL and it was about 90 degrees outside during the time this log was taken.

    Each line on the log lists the date and time the AC status changed. A 1 at the end of the line indicates it was turned on and a 0 indicates it was turned off.

    7/22/2008 11:45:00 AM,Air Conditioners Upstairs AC,1
    7/22/2008 12:19:00 PM,Air Conditioners Upstairs AC,0
    7/22/2008 12:25:00 PM,Air Conditioners Upstairs AC,1
    7/22/2008 12:35:00 PM,Air Conditioners Upstairs AC,0
    7/22/2008 12:41:00 PM,Air Conditioners Upstairs AC,1
    7/22/2008 12:53:00 PM,Air Conditioners Upstairs AC,0
    7/22/2008 12:58:00 PM,Air Conditioners Upstairs AC,1
    7/22/2008 1:08:00 PM,Air Conditioners Upstairs AC,0
    7/22/2008 1:14:00 PM,Air Conditioners Upstairs AC,1
    7/22/2008 1:26:00 PM,Air Conditioners Upstairs AC,0
    7/22/2008 1:33:00 PM,Air Conditioners Upstairs AC,1
    7/22/2008 1:43:00 PM,Air Conditioners Upstairs AC,0
    7/22/2008 1:49:00 PM,Air Conditioners Upstairs AC,1
    7/22/2008 1:59:00 PM,Air Conditioners Upstairs AC,0
    7/22/2008 2:05:00 PM,Air Conditioners Upstairs AC,1
    7/22/2008 2:15:00 PM,Air Conditioners Upstairs AC,0
    7/22/2008 2:21:00 PM,Air Conditioners Upstairs AC,1
    7/22/2008 2:33:00 PM,Air Conditioners Upstairs AC,0
    7/22/2008 2:39:00 PM,Air Conditioners Upstairs AC,1
    7/22/2008 2:56:00 PM,Air Conditioners Upstairs AC,0
    7/22/2008 3:02:00 PM,Air Conditioners Upstairs AC,1

    Thanks,
    Brett

  2. #2
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    I am a homeowner too, what kind of automation system did you get? Was it expensive? (I know that pricing rule but cannot help but ask).

    Where you are located, does this day resemble the kind of day the AC system is designed for? My locale is near Houston and the design day is recommended by Manual J as 94F, something like 127 grains moisture. We get day after day of that, but where you live it might not be so common.

    Broadly speaking I would ideally expect 1-2 hour run times without interruption at or above the design temperature. Although the ideal seldom exists in real world houses. I tested one of my two systems and observed very few summer days with more than 75% runtime in any one hour. I consider that proof the system was oversized, although it just may have been less good at humidity removal and slightly less energy efficient. Not a problem worth getting too agitated over. Is "short cycling" a phrase that denotes malfunction, or just less than ideal function?

    Your system is running about 17 minutes on, then 6 off at 3:00pm. Almost 75% duty cycle. Let us see what it does at 5pm, and into the evening.

    Of course these are amateur observations. Let us see what advice our pros give on your observation.

    Best wishes -- Pstu

  3. #3
    Join Date
    Apr 2008
    Posts
    27
    Quote Originally Posted by pstu View Post
    I am a homeowner too, what kind of automation system did you get? Was it expensive? (I know that pricing rule but cannot help but ask).
    I'm definitely more knowledgable about home automation than HVAC and the system I use is based on a software package called HomeSeer. The nice thing about HomeSeer is that it's very extensible and it will interface with almost any technology out there. I use a combination of X-10 and Z-Wave to automate all of the light switches in my house. It also interfaces with my alarm panel and my two thermostats (They are RCS TZ16 thermosats that communicate through Z-Wave).

    Integrating everything together like this allows me to simply arm the alarm when I'm leaving the house. The automation system will see that the alarm has been armed and will make sure all the lights in the house are turned off and set back the temperature on the thermostats. When I come back home and open the door the system will see that the door has been opened and if it's night it will turn on the light in the hall by the door. Once the alarm has been disarmed it will lower the temperature on the thermostats again. I can also send the house an email from my cell phone when I'm on the way home to tell it to lower the temp at that time so it will be cool by the time I arrive home.

    There is a lot more than can be done, and it really is only limited by your imagination. Automating an entire house like this isn't terribly cheap, but it's not outrageous either... especially if you're the type that can do a lot of the work yourself. And the nice thing is that you can start small... maybe only automate a few frequently used lights and your thermostat to see what it can do, and simply expand from there.

    Let me know if you want more information or more details and I'd be happy to send you an email.

    Where you are located, does this day resemble the kind of day the AC system is designed for? My locale is near Houston and the design day is recommended by Manual J as 94F, something like 127 grains moisture. We get day after day of that, but where you live it might not be so common.
    I'm in Central Florida and it's definitely getting to be the peak of summer. I don't know what the Manual J suggests for this area. We definitely get warmer days, but this is still a pretty warm day too.

    Thanks,
    Brett

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,900
    Looks like the stat is set to 4 CPH.

    If so, 3 CPH would be better.
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    How many times must one fix something before it is fixed?

  5. #5
    Can you comment on why 3 CPH is better than 4? I just had an IAQ installed and CPH it is set to 4. I am trying to even out temperature differences in an open foyer house design and I am using the CPH and circ modes to get more air flow. Not to jack this thread, but would just like to know the rationale.

  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,900
    3 CPH gives a slightly longer on time. So rooms will receive conditioned air for just a little longer period of time.
    The unit stays off a little longer.

    In the OPs case.
    3 CPH might increse his average on time by 2 minutes, and increase his average off time by 4 minutes.
    Using less electric.

    In your case.
    3 CPH, may give the harder to cool rooms just enough additional cool air, to bring them down in temp another degree.(not in just one cycle)
    With fan circ eabled.(guessing you mean circ, and not on) the unit has to be off for X amount of time, before it will bring the fan on by itself.
    4 CPH can prevent the circ feature from being able to run the fan.
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  7. #7
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,973

    Experiment, try to lengthen off-time & shorten run-time

    If the register/diffusers throw the cold SA air against the interior walls, it would seem that the off-time ought to be longer.

    The cold walls & furniture absorb the heat-gain & should slow the temp rise.

    I would search for the reasons for only 6 minutes off time, whereas, the runtime is 17 minutes.

    Locating & reducing excessive sources of both latent & sensible heat-gain would also shorten the run-time. (Just an experimental exercise.) - Darrell

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