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  1. #1
    Join Date
    May 2007
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    DC Metro Area (MD)
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    3,371

    Major short cylcling - why?

    Hi there,

    I've always felt my system short cycled slightly, but today it just seems to be beyond belief. Seriously, every 5 (give or take a minute or two) minutes the system goes on, and every 5 (give or take a minute or two) minutes the system rests. I don't know why it's doing this other than the fact that it's humid outside. Some speculation has gone through my mind that the system is overcharged and it's locking out (although I doubt it) or my thermostat has become inaccurate over time and now is the peak of its inaccuracy. I know enough to know that this is bad for the components in my system. The air coming out of the vents seems very cool and the house is staying cool, though.

    My equipment is as follows:

    -Payne 10 SEER Heat Pump (installed in 2004 I believe)
    -Lennox 13 SEER Air Handler (installed this summer)
    -Bryant Mercury Thermostat (this is from when the house was built around 1986-1987)

    Please advise me on actions you'd recommend I take. I'm tempted to call back the company that installed the air handler and explain the problem; however, this is the same company that installed the Lennox air handler with the Payne system and assured us everything was fine, so I am tempted to choose another company.

    Thanks,

    Ryan

    Edit: After monitoring it for one cycle and one rest, the intervals seem to be approximately:

    6-7 minutes resting
    14-15 minutes running

    Thermostat is set at 71-72.

    Does this sound normal on a humid day outside?

  2. #2
    Join Date
    Aug 2007
    Location
    MN
    Posts
    2,680
    get a new t-stat and go from there, I hate the mercury t-stats with the new electronics, never seem to function properly, a good quality electronic heat pump t-stat and you will probably be good to go. I would suggest using a contractor to replace the t-stat, HP's can be tricky for wiring.
    You can't fix stupid

  3. #3
    Join Date
    Mar 2007
    Location
    Myrtle Beach, SC
    Posts
    2,919
    You have a mix matched system. Those units were not made to work together. What temperature was it outside today? At design conditions it should run constantly or nearly so (If it is the right size). If it is extra humid out, that will ADD to load and make it run LONGER, not short cycle. It may be that your system is too big.

    Get another contractor out to look it over. No contractor should match a system like your first contractor did.

    NOTE: If it was cool outside, short cycling is normal.
    Remember, Air Conditioning begins with AIR.

  4. #4
    Join Date
    Feb 2006
    Location
    FL
    Posts
    566
    it could be a number of things, does your drain line have a float switch. had that call yesterday and they couldn't believe it!

  5. #5
    Join Date
    Sep 2001
    Location
    kalamazoo,mich
    Posts
    2,174
    Quote Originally Posted by Kevin O'Neill View Post
    You have a mix matched system. Those units were not made to work together.

    Kevin, how would his air-handler know his condenser is only 10 SEER? The air-handler probably has a TXV on it, so it doesn't care as long as it gets refrigerant to it.
    Have you hugged the Earth today?
    Donny Baker rules

  6. #6
    Join Date
    Aug 2007
    Posts
    24
    Is your thermostat getting any airflow from a nearby supply register? Is it centrally located on an inside wall near a return air vent? Can you pop the cover off and see the mercury in the bulb shifting when the unit cycles on and off?

  7. #7
    Join Date
    Mar 2007
    Location
    Myrtle Beach, SC
    Posts
    2,919
    Quote Originally Posted by chucko615 View Post
    Kevin, how would his air-handler know his condenser is only 10 SEER? The air-handler probably has a TXV on it, so it doesn't care as long as it gets refrigerant to it.
    The air handler doesn't KNOW anything. These systems should be matched.

    The August 2007 issue of RSES Journal had an excellent article on just this subject. Called !0 + 13 = 8.

    They took a 3 ton system, 13 SEER in & out. Tested it, got 3 tons cooling and 13 SEER.

    Then matched 3 ton, 13 SEER OD unit to 3 ton, 10 SEER (Piston) indoor unit. With factory charge they got 2 tons cooling and about 8 SEER.

    Added charge (8#) to bring the suction pressure up and superheat correct. They got a little better efficiency and capacity, but no where near right.

    Then they put a TXV on the indoor unit. Tested it. They got 2.5 tons and 12 SEER.

    So you see, if you don't have a matched set, you don't know what you are getting.

    Try testing the next mix match you see. You will find out it is a crap shoot.

    What's in YOUR wallet?
    Remember, Air Conditioning begins with AIR.

  8. #8
    Join Date
    May 2007
    Location
    DC Metro Area (MD)
    Posts
    3,371
    As to that article you are mentioning Kevin, that does alarm me a bit. I don't know if the actual model of my air handler, CB28UH-R has a TXV or not. It seems to be cooling fine, though; the short cylcing is what's got me all confused. I think the first step I should take is getting a new thermostat. Any ones out there you guys recommend? I know about the VisionPRO, but sadly, the thermostat cable I have is 20 years old too, and I don't know if it'll work. It was warm and humid out today. I know that partial change outs are not good at all, and I don't think I would ever have done it if my heat pump was only a few years old when I needed a new air handler. That just seemed wasteful to me. Who knows how long this Payne will last, though. One sales person told me 6-8 years.

    One thing that comes to mind, though: for a house that's 2500 sq. ft., wouldn't 3.5 be enough? I know there are many factors, but we have a good amount of electronics/computers/appliances and the home is 20 years old or so, so I would just think this is enough if not too small. Right now it sure seems like it's way too big. I've never run into a contractor (I've had 3 different ones) that have done a load calculation voluntarily; I guess I'll have to ask. Especially keeping in mind what that article said, if my system wasn't running to its full capacity and still short cycling, that's still pretty strange. Any recommendations on thermostats are still appreciated. I was thinking of the Honeywell VisionPRO 8320, but then a Carrier/Bryant thermostat came to mind because the original unit for the house was a Bryant and I have a Payne now, so maybe the wiring is right for it? Who knows; it was 20 years ago. Maybe something will come along and make me feel comfortable replacing these two relatively new units, but I'd like to get a longer lifetime out of them.

  9. #9
    Join Date
    Sep 2001
    Location
    kalamazoo,mich
    Posts
    2,174
    Kevin that's a great point you made, but the OP said they kept the 10 SEER heat pump and put in a 13 SEER air handler. So the RSES article doesn't apply here. He doesn't need a matching system in this case.
    Have you hugged the Earth today?
    Donny Baker rules

  10. #10
    Join Date
    May 2007
    Location
    DC Metro Area (MD)
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    Hmm, not sure what to believe now, . One contractor that I found friendly and knowledgable said I could lose efficiency, but it shouldn't really break it (if I remember what he said correctly). He offered a new Bryant indoor air handler (he also sells Rheem but I guess he offered the Bryant since I had a Payne) or a new Bryant indoor air handler AND heat pump. Both 13 SEER with R410a. I do regret not doing a total change out, but if you put yourself in my shoes, you have a new 3 year old heat pump that seems to be in fine condition.

  11. #11
    Join Date
    Jan 2006
    Location
    Near Atlanta, GA.
    Posts
    14,621
    Quote Originally Posted by chucko615 View Post
    Kevin, how would his air-handler know his condenser is only 10 SEER? The air-handler probably has a TXV on it, so it doesn't care as long as it gets refrigerant to it.
    If they didn't change he orifice, it's probably too large for a ten seer unit. The ten seer will run higher head and over drive the larger orifice.

    Even with a proper orifice or a TXV, the larger coil of the 13 seer air handler will cause the condensing unit to run higher suction and head, possibly enough to overload the compressor.

    Not a good situation either way.

  12. #12
    Join Date
    Sep 2001
    Location
    kalamazoo,mich
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    2,174
    I'm sorry I didn't mean to hi-jack your thread.
    Have you hugged the Earth today?
    Donny Baker rules

  13. #13
    Join Date
    May 2007
    Location
    DC Metro Area (MD)
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    3,371
    Hmm, there's a reference to my earlier speculation that it might be overcharged and locking out. Either way I know something's wrong with the system run times being too short. I think I'll look into a new thermostat (which one? ) and a checkup in the Fall to prepare for heating.

    Chuck, no worries; I don't think you're hijacking my thread. I really appreciate everyone's input in my thread here; you all have more knowledge than I do! I just want to make the right decision here. The question I have now is should I go ahead and try to install a new thermostat. I think I'm up to it; I'm good when it comes to electronics.

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