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  1. #1

    Thermostat causing short cycles?

    I'm getting pretty short cycles with the A/C running. It usually only runs for 8-15 minutes depending on the temperature (85F-100F) outside. I have the thermostat set to 78F and it turns the A/C on at 79F and off at 78F. Are there any thermostats that allow me to set my own activation and deactivation temperatures? It would be nice to have the A/C turn on at 79F and off at 76F or 77F.

  2. #2
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    This is the Ask Our Pro's forum, and only Pro members that have been vetted by the AOPC may post advise, commentary, or ask questions of the OP here.

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    Last edited by beenthere; 05-02-2013 at 06:03 AM. Reason: Non Pro * Member

  3. #3
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    Short cycles are usually cause by over sized equipment.
    "Hey Lama, hey, how about a little something, you know, for the effort." And he says, "there won't be any money, but when you die, on your deathbed, you will receive total consciousness." So I got that goin' for me, which is nice. - Carl Spackler

  4. #4
    Are there thermostats that can fix my problem so I don't have to spend money on a new A/C? I also doubt the A/C is over sized because it does struggle to stay at 78F on those rare 110F+ days. The house is 1570 square feet built in 1997. I'm not sure about the insulation rating in the walls but it does have double pane windows and the attic insulation is R-38. The A/C is either a 2.5 or 3 ton unit and the furnace is rated at (1000-1500 cfm @ ESP 0.8-0.1).

  5. #5
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    What type of thermostat and what type of equipment. Could be bad thermostat, faulty thermostat/wiring, or wrong settings on the t-stat. Post some more info please.

  6. #6
    The thermostat is a White-Rodger from 1997. I pulled the cover off of it and still don't see a model number. The only number on it is 153-7758 but that isn't the model number. The closet model I can find is the 1F80-51. I'm just looking for a thermostat that lets me select my own on/off temps. The furnace is an Even Temp GB1AAV.

  7. #7
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    ebrage, this is the Ask Our Pro's forum, and only Pro members that have been vetted by the AOPC may post advise, commentary or ask questions of the OP here.

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  8. #8
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    The equipment and thermostat manufacturers have been continuously improving their products for decades trying to eliminate just what you're saying you want to have...... large temperature swings.

    What will probably give you somewhat close to what you're looking for is any thermostat that gives you the ability to modify/change/set the cycles per hour in cooling mode. Look for one that will allow you to set the cycles/hour to .... 2?

    As others have mentioned, if the your equipment is cycling at fairly high outdoor temperatures it is over sized for the home's air conditioning needs. Keep in mind that as contractors we call something over sized when it turns on and off at what is considered "design temperature" for your location. The target we try to hit when sizing equipment is to have that equipment run continuously at design temperature.

    Your description of the unit running 8-15 minute on cycles left out the other part of that equation.... how long does it stay off. A 15 minute on/off cycle ( 4 cycles/hour) when it's not hot enough outside to be considered "design conditions" is getting real close to "normal" operation.
    Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.

  9. #9
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    Quote Originally Posted by skyline97 View Post
    I'm getting pretty short cycles with the A/C running. It usually only runs for 8-15 minutes depending on the temperature (85F-100F) outside. I have the thermostat set to 78F and it turns the A/C on at 79F and off at 78F. Are there any thermostats that allow me to set my own activation and deactivation temperatures? It would be nice to have the A/C turn on at 79F and off at 76F or 77F.
    What's yoru location? The equipment should run at least twice as long at 100F than 85F. I assume this is a hot dry climate (most people in humid climates kee it under 77F). SO 105-110F might be your design temp. If it's not runing continously at 105-110F than it might be oversized and part of your issue. IF your design is les say 105F, then at 85F it should only be runing 25% of the time, or only about 5 minutes at 3 cycles per hour with a 78F indoor temp. In reality, it will run a little longer and overcool the space and run at closer to 2 cycles per hour.

    This actually sounds like it's pretty normal. In hotter climates unless you have 2 stage equipment, in low load conditions, the equipment simply won't run very long.


    Even better thermostats like many of the better honeywell models still won't cycle by more than about 1.5F. You can select 2 or 3 cycles per hours, but if they came on at 79.5, it woudl shut off at 76.5. But it sounds like that's what you want.

    I would try a Honeywell Focuspro 5000 series and set it to 2 cycles per hour and see if you get what your looking for.

  10. #10
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    Why not set your set-point to 77F? if thats where you want to the unit to stop running.
    Philippians 4:13
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    Apostle Paul inspired by GOD.

  11. #11
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    Quote Originally Posted by skyline97 View Post
    I'm getting pretty short cycles with the A/C running. It usually only runs for 8-15 minutes depending on the temperature (85F-100F) outside. I have the thermostat set to 78F and it turns the A/C on at 79F and off at 78F. Are there any thermostats that allow me to set my own activation and deactivation temperatures? It would be nice to have the A/C turn on at 79F and off at 76F or 77F. [You can DO THAT.]
    Probably one of the lowest cost & easiest Temp-SWING thermostats you could buy is the ACE ATX 1500 4115176 at some ACE Hardware Stores.
    Call & ask if they sell them; make sure it is a Temp-Swing...

    The other is the LUX PRO PSP511LCa low cost; Sat & Sunday are programmed together, not separate; search for both T-stats...read & save the programming instructions!

    The ACE is programmable weekdays & Sat & Sunday. The swing can be from #1 to #9; by testing real-time, you'll be able to set it for the on & off temps you want. I would never have one without this Swing setting to prevent short cycling & NO dehumidification.

    They used to want, & brag about, the t-stat holding temps close to the setting; that is why you get short cycling; not only that but they had cooling anticipator to bring the A/C on sooner; all that with grossly oversized equipment.

    If a person can't tolerate a little temp-swing; then they'll have to live with a lot of costly short cycling & very little dehumidification...these T-stats can help some on oversized equipment; however, when it's time for new equipment do a load calc & reduce the load with some retro-work & reduce equipment size.
    Last edited by udarrell; 05-02-2013 at 02:19 PM.

  12. #12
    Quote Originally Posted by firecontrol View Post

    Your description of the unit running 8-15 minute on cycles left out the other part of that equation.... how long does it stay off. A 15 minute on/off cycle ( 4 cycles/hour) when it's not hot enough outside to be considered "design conditions" is getting real close to "normal" operation.
    It usually only runs once an hour if the temp outside is only in the mid 80s. It usually starts running twice an hour if the temp outside is above 95F (stays off for 30mins or so).


    Quote Originally Posted by motoguy128 View Post
    What's yoru location? The equipment should run at least twice as long at 100F than 85F. I assume this is a hot dry climate (most people in humid climates kee it under 77F). SO 105-110F might be your design temp. If it's not runing continously at 105-110F than it might be oversized and part of your issue. IF your design is les say 105F, then at 85F it should only be runing 25% of the time, or only about 5 minutes at 3 cycles per hour with a 78F indoor temp. In reality, it will run a little longer and overcool the space and run at closer to 2 cycles per hour.

    This actually sounds like it's pretty normal. In hotter climates unless you have 2 stage equipment, in low load conditions, the equipment simply won't run very long.


    Even better thermostats like many of the better honeywell models still won't cycle by more than about 1.5F. You can select 2 or 3 cycles per hours, but if they came on at 79.5, it woudl shut off at 76.5. But it sounds like that's what you want.

    I would try a Honeywell Focuspro 5000 series and set it to 2 cycles per hour and see if you get what your looking for.
    I live in Sacramento, CA so the temperature can swing quite a lot from April to October. There are usually 20-30 days of 100F+. The A/C does run for about 30mins when the temp outside is reaches 105F and almost constantly when it reaches 110F. I've seen it struggle to stay under 80F the few times it was over 110F.


    Quote Originally Posted by udarrell View Post
    Probably one of the lowest cost & easiest Temp-SWING thermostats you could buy is the ACE ATX 1500 4115176 at some ACE Hardware Stores.
    Call & ask if they sell them; make sure it is a Temp-Swing...

    The other is the LUX PRO PSP511LCa low cost; Sat & Sunday are programmed together, not separate; search for both T-stats...read & save the programming instructions!

    The ACE is programmable weekdays & Sat & Sunday. The swing can be from #1 to #9; by testing real-time, you'll be able to set it for the on & off temps you want. I would never have one without this Swing setting to prevent short cycling & NO dehumidification.

    They used to want, & brag about, the t-stat holding temps close to the setting; that is why you get short cycling; not only that but they had cooling anticipator to bring the A/C on sooner; all that with grossly oversized equipment.

    If a person can't tolerate a little temp-swing; then they'll have to live with a lot of costly short cycling & very little dehumidification...these T-stats can help some on oversized equipment; however, when it's time for new equipment do a load calc & reduce the load with some retro-work & reduce equipment size.
    Thanks, this should work great for me. I only see the ACE ATX1500E, which looks just like the Lux TX1500E. I can get both the Lux TX1500E and PSP511LC for a little over $30 while the ACE is closer to $50. I'll continue to look for more thermostats with the swing option.

  13. #13
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    Once again, Udarrel has the best, easy to understand advice.
    "Hey Lama, hey, how about a little something, you know, for the effort." And he says, "there won't be any money, but when you die, on your deathbed, you will receive total consciousness." So I got that goin' for me, which is nice. - Carl Spackler

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