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Thread: Short cycle?

  1. #1
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    Short cycle?

    As a new student to the HVAC world I was wondering why short cycling is bad? I'm trying to find info on this in my school books but no luck. It is in the book I'm sure.... But I just can't find it. So I guess I'll just ask why is short cycling bad for a ac or heating unit.

  2. #2
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    Tell us why you think it would be bad? Start with simple air conditioning please.

  3. #3
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    I would say it would take more energy to startup the system then to run it. I would also say that the moisture in the air wouldn't change. Humidification and dehumidifying of the air would take some run time.

  4. #4
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    Quote Originally Posted by Motard View Post
    I would say it would take more energy to startup the system then to run it. I would also say that the moisture in the air wouldn't change. Humidification and dehumidifying of the air would take some run time.
    Exactly right. And it also takes a few minutes for any HAC equipment to hit its peak efficiency range.

    How about a gas furnace?

  5. #5
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    on the ac side single phase with no time delay will trip a breaker or internal overload (pressures being unequal)...when motors/compressor (3 phase and or single phase) start they see Locked Rotor Amps and the contactor is taking that hit on the contactors....along with the windings of the motors taking more starts then deigned for... ending up with burn outs
    "when in doubt...jump it out" http://www.youtube.com/watch?v=Px5YcOeQB4I

  6. #6
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    I'm going to have to think about a gas furnace. Other then the power of start up and the humidification. I will get back to you on this one

  7. #7
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    This question came to me when talking about why we have so many different size furnaces. It would cut down on storage and production at the manufacture. If every home could just have a 3 or 5 ton unit. I said if it's was to big it would short cycle. Then I was asked why it was bad to short cycle. My class mate said it's just not running as much.

  8. #8
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    I don't see much commentary on this, so I'll throw my two cents in...


    Sit in the driveway, in your car and do the following:

    Start it and let it run.

    Now, start it, shut it off. Repeat this process.


    Which process do you think will lead to longer engine life?

  9. #9
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    I would Believe that the inrush current to the compressor will slowly destroy itself from over heating causing a short and then a acid burn out.

  10. #10
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    Quote Originally Posted by Davekoz View Post
    I would Believe that the inrush current to the compressor will slowly destroy itself from over heating causing a short and then a acid burn out.
    We are discussung short cycling due to improper HAC application. What you are referring, I'd call a malfunction.

  11. #11
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    Moose Jaw, Saskatchewan, Canada Occupation:Interprovincial Plumber, Commercial Gasfitter Interests:
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    Think of driving on the highway.
    0-60 you burn a lot of gas. Once you are cruising at 60, you burn less gas. 0-60-0 with no cruising time is short-cycling

  12. #12
    If the short cycling is caused by over sized equipment, the space will be less comfortable. For example, if the furnace is too large it will dump a bunch of hot air into the space satisfying the call. The unit has a time requirement to meet before it is able to enable heat again. The homeowner will experience burst of heat rather than a system that runs longer maintaining a constant supply of warm air bringing the space up to temperature over a longer period of time. The short bursts due to short cycling create a hot-cold-hot environment that is less comfortable. As mentioned above, the short cycling then adds unnecessary wear on the unit leading to premature failure. The equipment (motors, compressors, valves) are engineered to run. Think of it like a light bulb... which will give you longer life? Turning the light on and leaving it or flipping the light on-off-on-off-on-off?

  13. #13
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    Quote Originally Posted by HeatherIGP View Post
    If the short cycling is caused by over sized equipment, the space will be less comfortable. For example, if the furnace is too large it will dump a bunch of hot air into the space satisfying the call. The unit has a time requirement to meet before it is able to enable heat again. The homeowner will experience burst of heat rather than a system that runs longer maintaining a constant supply of warm air bringing the space up to temperature over a longer period of time. The short bursts due to short cycling create a hot-cold-hot environment that is less comfortable. As mentioned above, the short cycling then adds unnecessary wear on the unit leading to premature failure. The equipment (motors, compressors, valves) are engineered to run. Think of it like a light bulb... which will give you longer life? Turning the light on and leaving it or flipping the light on-off-on-off-on-off?
    in reguards to the furnace; what else happens as a result of short cycling

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