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  1. #1

    Mini split system - condenser in attic?

    I am considering one zone ductless A/C for a 2nd floor bedroom, for example Sanyo (now Panasonic) KS0971 / C0971, 9000BTU/hr. Would like not to have condenser unit in plain view near a main entryway to our circa 1770 house and would much appreciate professional opinion and caution regarding having the condenser unit installed in our attic.

    We are in the Boston, MA area. The attic volume is about 5000 cu. ft. with floor to ridge about 10ft, and two large gable vents with total open area about 18 sq. ft. I would build a rack on which the installer would mount the condenser unit, at the same level as a window in the gable, but about 2 ft. back from the window, allowing space for maintenance / repairs. The window opening is a bit larger than the condenser fan area. I would ask installer to run a short, straight piece of duct (fabric or metal) from the fan outlet side to the window to conduct the hot air right to the outdoors. A small side benefit of this is that the evaporator is only about 8 ft from the condenser.

    Many thanks for your advice!

  2. #2
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    Unless the attic maintains the same temperature as the outdoor temp, you would be creating a hostile working environment. Other than that, I don't see any really negative aspects.
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  3. #3
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    it won't work. the condenser needs to transfer heat to the outdoors. As the condenser rejects heat to the attic and the temp goes to over 100 degrees in the attic, capacity will drop and the unit will operate at high pressures and not last.
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  4. #4
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    We have installed a few condensers in attics IF there are temperature activated powered attic ventilation and we've always been careful in pointing out that we are not responsible for any unit damage created from the failure of this ventilation system. You have to remember that these systems are not designed to run in this atmosphere and problems could come up.

  5. #5
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    one other thing to remember esp, when building your platform. some of the circuit boards for the condensers are accessed via the TOP of the unit.

  6. #6
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    Quote Originally Posted by beshvac View Post
    it won't work. the condenser needs to transfer heat to the outdoors. As the condenser rejects heat to the attic and the temp goes to over 100 degrees in the attic, capacity will drop and the unit will operate at high pressures and not last.
    Even a blind squirrel can find a nut every now and then.

  7. #7
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    Quote Originally Posted by beshvac View Post
    it won't work. the condenser needs to transfer heat to the outdoors. As the condenser rejects heat to the attic and the temp goes to over 100 degrees in the attic, capacity will drop and the unit will operate at high pressures and not last.
    Read the OP original again. He said he's going to duct the discharge out of the attic. My comment on that is that the condenser can only push out as much air as is being let into the attic. Thus you'll need an equal size opening elsewhere in the attic and preferably larger by a magnitude of 2, or double the discharge size opening. One thought that could help all aspects of the project would be to insulate the attic ceiling with spray on dense foam insulation. This would keep the attic temps much lower and improve the overall situation. But a good, steady supply of outdoor ambient temperature air is an absolute must, no matter what else you do.

  8. #8
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    I think you would do better by drawing the air from the window across the condenser then pushing the heated air in the attic to escape out the openings then put a gable fan in one of the openings

  9. #9
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    Quote Originally Posted by catmanacman View Post
    I think you would do better by drawing the air from the window across the condenser then pushing the heated air in the attic to escape out the openings then put a gable fan in one of the openings
    Sounds like a good idea.
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  10. #10
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    klabkebash,

    This is the Ask Our Pro's forum. In order to post advice here, you must have verified qualifications and have been approved by the AOP Committee.

    You can find the rules for posting and qualifications here.

  11. #11
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    Quote Originally Posted by skippedover View Post
    Read the OP original again. He said he's going to duct the discharge out of the attic. My comment on that is that the condenser can only push out as much air as is being let into the attic. Thus you'll need an equal size opening elsewhere in the attic and preferably larger by a magnitude of 2, or double the discharge size opening. One thought that could help all aspects of the project would be to insulate the attic ceiling with spray on dense foam insulation. This would keep the attic temps much lower and improve the overall situation. But a good, steady supply of outdoor ambient temperature air is an absolute must, no matter what else you do.
    A condenser fan motor is not designed to push air through ductwork, and I guess neither is the fan design.
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  12. #12
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    We have pacakge unit at my work in hot factory indoor environments. The compressors don't last long. Even if you ducted cool air, when it's shut down the control boards and compressor will be sitting in a heat bath that could reach 130F depending on where you live and how well vented the attic is.

    I also agree that condenser fans are not designed to push air through a duct. They are designed for very low static pressures. They have a small number of fan blades, and reatlvely large clearances to their cowling.

    I agree that you'd have to mount it near a opening like a gable vent to draw in cool air and have an exhaust fan runing in the attic at all times when it's over about 100F to keep cooler outside air blowing over it.


    For the other pro's, will vibration be an issue? I could see this thing vibrating the entire house depending on how it's mounted.
    Last edited by motoguy128; 11-15-2011 at 10:51 AM. Reason: clarification

  13. #13
    Thanks to everyone who responded. You raise several important points, the most serious concern for me being the ambient temp at the intake to the condenser. The Sanyo unit I mentioned specs the max intake temperature at 115F dry bulb. I have measured the attic temp as high as 107 but it could well go above that, in which case I'd have to add a power vent to limit the temp rise. At least, by directing the condenser exhaust directly to the outside, that heat load would not be added to the attic.

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