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  1. #1

    300 sqft attic -- 1.5 to 2 tons???

    I'm finishing a space of ~300 sqft in the attic of my 1901 house with a 12/12 pitch slate roof in NE Kansas (Lawrence). I'll have expanding open cell foam blown in to insulate the entire roof, but with ~4 foot knee walls the finished space will be ~300 sqft including a 1/2 bath, 11 x 16 room and tower space.

    I've asked for 2 bids and both are saying 1/5 to 2 ton system required. I think I'll use a Mitsubishi mini split. This seems like a big load calculation.

    I know there is no substitute for a complete calculation, but I can tell that the people on this site have a lot of experience and I'd like to know if this sounds reasonable. Thanks for your help. The rest of the whole house is ~2300 sqft and 4 tons.

    P.S. One more question: Should I insulate the knee walls? Space "behind" the knee walls and 3 gable ends will be unconditioned storage space.

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,474
    I did a 300+ sq ft attic room for a friend. 1 ton with 5kw in the air handler. Did great. Bigger is not better. But the more you an insulated it the better!

  3. #3
    Join Date
    Dec 2007
    Location
    Cedar Grove, Wi-Sheboygan
    Posts
    1,582
    If the rest of your home is insulated like you have your new attic space I would tap off your existing system, or split your existing sytem with say a 2 ton and then 2-1/2 respectively and forget about doing a 3rd system when you currently have 4 ton. although you don't say if the 4 ton is one compelte system of if it is already split amongist the 2300 sq ft. but it's another option for oyu to consider.

  4. #4
    Main house has a 4 ton HP / furnace (American Standard variable speed gas furnace with 16 seer two stage heat pump) for ground and second floor. It is zoned. Unfortunately there is not a way to run a duct up to the attic that would be large enough to supply that space. I'm sure you are right that that would be the best option. As it is, adding a new unit seems to be the only way I can condition that attic space.

  5. #5
    Join Date
    Dec 2007
    Location
    Cedar Grove, Wi-Sheboygan
    Posts
    1,582
    Is this attic space not above a conditioned living space already? If so there shouldn't be too much trouble getting say a 6" oval duck up and over from an adjadcent room or enlarge a duct from the space below to accommodate the attic. I would sooner tear out alittle drywall than install another piece of equipment especially for a mere 300 sq ft. But that's just me.

  6. #6
    Join Date
    Aug 2008
    Location
    Burlington , Mass
    Posts
    470
    Did either do a Manual J load calc ? 1 1/2 to 2 sounds like a guess to me. 2 to 3 cfm per sq ft ? Thats alot in a residence.

    Just a guess here but 9,000 to 12,000 mini split with the invertor compressor, providing you have on high wall to mount the evap on.
    I'll be there when I get there and not a minute later

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