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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
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,021

    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

  5. #5
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,540
    Looks like the stat is set to 4 CPH.

    If so, 3 CPH would be better.
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  6. #6
    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.

  7. #7
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,540
    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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  8. #8
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
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    3,304
    >>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.

    Beenthere, that would be great news if they have redesigned the software to do it that way. When shopping for thermostats, I liked everything about the Honeywell IAQ *except* the specs said this is a *random* 35% fan on with "Circ" mode. I made a point of telephoning Honeywell and trying to get an engineer to clarify, to say this was NOT random but a timer which allowed the off-time to elapse before turning on the fan after a cooling call. They reiterated this was a random and not a timed function.

    When I tried to explain why I preferred the fan to be still after a cooling call in the hot-humid south, they told me to go talk to my HVAC contractor. I silently called them "damn Yankees" and politely concluded the call. That answer was a deal breaker for me and I bought another brand.

    Always seemed to me this was a small software issue, all the designers need was have an awareness of the humidity issue and make a minimal effort to do things right. It would take very few bytes of code to employ a counter for on-time and a counter for off-time, and have the off-time counter begin right after an AC call is finished. Aircycler is designed this way, and I believe one White-Rogers model is designed that way.

    If indeed Honeywell has taken this re-design, that is music to my ears!

    Best wishes -- Pstu

  9. #9
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,540
    I think you will find the randomness is based on projected run time compared to the previous run times in teh hour/hours before.

    So as far as I know, they don't come on as ramdomly as Honeywell says.

    I doubt you actually talked to an engineer.

    Honeywell tech reads off a sheet.

    You may or may not have seen where Baldloonie and I disagreed, on how the 8321 treated dual fuel operation.
    He called Honeywell, and they told him he is right.
    Later, another pro member told him he has the Honeywell bulletin that said it works the way I said.

    Yep, its confusing for us too.
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  10. #10
    Thanks beenthere. I started the CPH at 5 and it was way too much, 4 has been much better. I will try 3 next to see if it lets the circ mode work the fan more. One strange thing I found out today is that I could not get the fan to operate in auto mode during the away schedule to reduce wasted energy usage, even though it was programmed correctly in the schedule. I use circ mode for the return, sleep and wake modes, but the unit just stayed in circ mode no matter what. It turns out that I needed to put the unit into Auto mode outside of the schedule mode as it was being overided into the circ mode by the fan mode itself. Anyway I figured it out, but I would think that following the scheduled mode would be the default unless you overrode it, and then it should blink or something to show you it was in an override mode. One last thing that seems dumb is that you can not set the humidity setting based on the schedule as I would love to have the humidity control work when I am home, but be disabled during the away mode.

  11. #11
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    Dang. I really wanted to believe they would keep the fan off immediately following an AC call. You are right, I said it was an engineer but probably it was just someone assigned to deal with technical questions. If I had thought there were another level of support I could have gone to, I would have. From all I have learned, that Honeywell is the best design in just about every other respect.

    Thanks -- Pstu

  12. #12
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    The fan control is a diffulcult thing.
    The normal screen and program can over ride each other.
    Fan on, or circ over ride fan auto in program.
    And fan on or circ over ride fan auto on the screen.

    The ability to set humidity independant for each program would be nice at face value. But detrimental in the long run.

    It would take that much longer to get the humidity(cooling mode) back down when you returned.
    If you have a humidifier, the same thing would happen in teh winter. Long time to get the humidity back up.

    in cooling. As the temp rises in the house, the humidty is easy to keep in check. So it doesn't use much electric to keep the humidity under control.
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  13. #13
    Thanks again for the great explanation, but I must admit I had to read the fan description several times to fully understand. It is great to know that using the A/C to dehumidify is not using much A/C when away and set to 85.

    I purchased the stat on the recommendation of an HVAC acquiantance who installed it for me to solve the comfort problem I created after fixing some other heat loading issues on the upper floor and now being able to raise the setpoints up much higher. With the setpoint up to 75 my old stat did not run the A/C enough to keep the humidity down and it was not comfortable. Now I just moved the setpoint up to 78 and it is more comfortable in the house as the humidity is now 40%. I am really learning that humidity control is the name of the game for comfort, not really as much temperature.

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