Results 1 to 11 of 11
  1. #1
    Join Date
    Apr 2005
    Posts
    29

    Question What would you consider "short cycling"

    I know this is a very subjective question, as it can depend on many variables... but from a general "rule of thumb" perspective during average cooling days, what timing cycle would you consider "short cycling" of the compressor?

    For example, how long would you expect a properly sized compressor to run, stop, then run again?

    Or conversely, how much time would you consider a typically normal run, stop, run cycle to take?

    I'm not asking for early season when the temps are cool, peak season when temps are at their max necessarily.

    I don't really think our system is short cycling, but I'm just trying to get a feel for what is normal for our 5ton Carrier we just put in two summers ago.

    Thanks,
    Al

  2. #2
    Join Date
    Jan 2005
    Location
    USA
    Posts
    1,453
    Where?
    Does it have a crank case heater?
    Does it have low ambient fan control?
    Is the indoor unit variable speed?
    What kind of indoor temp are you trying to maintain?
    Is it a twin single compressor?

    As a general rule of thumb no less than 3 - 5 minutes off time between cycles.
    "It's always controls"

  3. #3
    Join Date
    Apr 2005
    Posts
    29

    Thumbs up

    Quote Originally Posted by skwsproul View Post
    Where?
    Does it have a crank case heater?
    Does it have low ambient fan control?
    Is the indoor unit variable speed?
    What kind of indoor temp are you trying to maintain?
    Is it a twin single compressor?

    As a general rule of thumb no less than 3 - 5 minutes off time between cycles.

    Sanford, NC(Raleigh area)

    I don't know what a crankcase heater is.
    I don't know if it has "low ambient fan control".
    It does use a variable fan on the air handler.
    78 degrees
    Single compressor, 5ton Carrier We have a 5ton Carrier "Performance Series" Air Conditioner(not heat pump), model number: 24APA560A300. It is about 18 months old.


    Right now with temps in the mid 80s to high 80s, the compressor is coming on and only running 3-5 minutes, then stopping for quite a long while(10s of minutes or more). I was just curious what time-frame would be a red-flag for an HVAC guy.

    Thanks,
    Al

  4. #4
    Join Date
    Jan 2005
    Location
    USA
    Posts
    1,453
    Most manufacturers recommend a minimum runtime of 3 minutes and minimum off time of 2 minutes to allow pressure equalization between cycles. This would give a maximum of 12 cycles per hour. I however disagree. That many start stops in an hour is a lot of wear and tear on a system I usually like to see no more than 6 cycles per hour. If your system is off for 10 minutes and on for only 3, I would question the sizing of your application. If your house takes 10 minutes to gain enough heat to cycle your stat and your system removes it in 3 minutes, that just seems to be an oversizing issue to me. Possible a stat setting problem. My system runs for around 15 minutes and is off for around 10 on the average day around 80 outside and 20 minutes on and 5 minutes off at 90 outside keeping the inside humidity around 50% while the outside is 80+%. The stat is a Honeywell Vision Pro and I do not remember what the differential is set at but I am pretty sure it is around 1 degree f. Indoor setting 74 year(Cool) summer and 70(heat) winter.

    Oh I just noticed you are only trying to maintain 78 inside with 80 outside. What is the indoor humidity? That is only 2 degrees from the outdoor, probably not any need for a/c if you are only taking off 2 degrees. I can see that it seems like you may be trying to conserve energy by keeping your house warmer and just trying to take a little humidity out. I do not belive that you are harming your system by doing this but I am sure someone will be able to provide a little more insight on this than me.
    Last edited by skwsproul; 06-04-2008 at 11:44 PM.
    "It's always controls"

  5. #5
    Join Date
    Jan 2005
    Location
    USA
    Posts
    1,453
    Quote Originally Posted by aroethli View Post
    Sanford, NC(Raleigh area)

    I don't know what a crankcase heater is.
    I don't know if it has "low ambient fan control".
    It does use a variable fan on the air handler.
    78 degrees
    Single compressor, 5ton Carrier We have a 5ton Carrier "Performance Series" Air Conditioner(not heat pump), model number: 24APA560A300. It is about 18 months old.


    Right now with temps in the mid 80s to high 80s, the compressor is coming on and only running 3-5 minutes, then stopping for quite a long while(10s of minutes or more). I was just curious what time-frame would be a red-flag for an HVAC guy.

    Thanks,
    Al
    In light of all that. i would recommend that if you are concerned on system performance than you should discuss you concerns with a reputable contractor and than have them do a pm report on your equipment.
    "It's always controls"

  6. #6
    Join Date
    Apr 2005
    Posts
    29
    Sorry, just went and checked, the thermostat is set at 75 degrees, 50% humidity.

    Anyway, along with my two other threads about short cycling and lights dimming, I'll add this info to my to-do list when I chat with them.

    Thanks
    Al

  7. #7
    Join Date
    May 2008
    Location
    Cebu City, Philippines
    Posts
    41
    Short cycling is often cause of oversized a/c. Short cycling increase your electricity bill.

  8. #8
    Join Date
    Feb 2003
    Location
    Huntsville,AL
    Posts
    4,125
    most rotating machinery is not rated for more than 4 starts per hour.

    check with the compressor mfgr
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  9. #9
    Join Date
    Apr 2005
    Posts
    29
    I'm going to go sit outside with a timer tonight or tomorrow to really get a true feel for the cycle durations. I'll let you know what I observe.

    We've got a bit of a heat wave right now, about 100 degrees, so it should be a good test.

    Al

  10. #10
    Join Date
    Feb 2008
    Location
    Dallas & Longview, TX
    Posts
    629
    I would have the stat set for a lower CPH. If you can stand a bit more temp swing, this would allow the unit to run longer which will remove more humidity as the evap coil doesn't reach max effeciency until ~10 min. on the newer more efficient systems. Lower the CPH to 3 or so and see if that helps.

    Regarding the question of short cycling, I would say that depends on the design as each is different. Until it reaches the max. design OD temp. and the setpoint of the stat is at design it will cycle unless you lower the stat set point.

  11. #11
    Join Date
    Jun 2007
    Location
    The Twilight Zone
    Posts
    2,964
    Short cycling: Any adult person under 5 feet tall riding a bicycle.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event