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  1. #1
    Join Date
    Apr 2012
    Posts
    2

    Default Heating/cooling for small addition

    An addition (15x15) was added to my house in Fairfax, VA by the previous owners, which is not part of the central air/cooling system. As the house is on concrete slap, it appears untenable to add new duct work.

    What is the best option for heating/cooling the room? Is a mini-split heat pump the correct path or will it be overkill (9K BTU seems to large for the room)? Can I get away with an electric baseboard for winter and allow the central air to cool the room in summer?

    Are their better options?

  2. #2
    Join Date
    May 2011
    Posts
    251

    Default Re: Heating/cooling for small addition

    A heat pump is the best option in your climate. A good quality minisplit unit like a Mitsubishi or Samsung is a great option. Mitsubishi units with Hyperheat can generate maximum heating down to something like 0F.

    9000BTU's is the smallest unit they make, but at low speed the delivered output is a little lower and they do great job of removing humidity. So you're dehumidifying as much as cooling.

    With an average of R10 insulation, the heat loss is about 4000 BTU and heat gain around 3000BTU depending on shading.

    SO the unit will be large, but not terrible.

    So AC or heat from yur existing system, you'd need to get about 140CFM to the room. That's a 8" duct with two 4x10" registers or a single 4x14" register.
    1925 Two-Story Stucco Beaux Arts Neoclassical

  3. #3
    Join Date
    Apr 2012
    Posts
    2

    Default Re: Heating/cooling for small addition

    Thank you very much for your thoughtful reply. I'll look into the Mitsubishi and Samsung systems.

  4. #4
    Join Date
    Apr 2012
    Location
    Northern Virginia
    Posts
    140

    Default Re: Heating/cooling for small addition

    My former addition (standalone) was 20 x12 and I went with a mini-spit for cooling/heating.
    My contractor sub the work for about $2,800 (Samsung) - 1 handler; separately, I received bids for about $5,000-$6,000 for 2 air handlers for whole house cooling.
    Surprisingly, I saw a big display at Home Depot (Falls Church 7 Corners which is a hi-volume store that isn't full line) for Mitsubishi mini-split. You still have to be careful of contractors coming through from HD, Lowes, Sears, etc. (unless it's fixed price like $125 for a "basic" toilet installation). A former neighbor received a quote from a HD contractor for $20,000 concrete driveway (2 lanes 30 feet) that should have been quoted at $10,000.
    Mitsubishi is the most expensive mini-split hardware, so I would expect a price higher than $2,800 all-in.
    My Samsung failed just after the 3-year warranty and it cost $800 to repair (recharged coolant, then replaced circuit board) so I'm not a "fan" of Samsung A/C.
    9,5000 BTU turned my 20 x12 room into a meat locker in 45-60 minutes (Silicon Valley, CA weather is among the best in the USA) and it heated well. HOWEVER, I was told that mini-split heating is either ineffective or inoperable below around 37 degrees (although M128 saids good to 0 degrees), and No. Virginia has enough cold nights that you have to check this out as an issue (I'm in Arlington/McLean border). Your cement slab will make the floor feel cold even if the air temperature is warm.
    The mini-split requires 240v for the outdoor unit and 120v for the indoor air handler unit.

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