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  1. #1
    Join Date
    Jul 2002
    Posts
    79

    HVAC Question for 2nd Story Addition

    I have a 1489 SQ Foot house - Two Story – Pittsburgh, PA - (basement is 744 sq feet) which were adding another 1536 SQ Feet to it – (basement is 768 sq feet). We just finished enclosing it. All new windows – 2x6 walls are highly insulated.

    When we bought the house, I gutted it and had a new Trane XV90 installed – 80K BTU and all new duct work.

    I ran the HVAC Calc for the entire house and addition which came out to 69K Heat Loss. Now I am thinking that the current furnace is way oversized for the existing house, but undersized for the addition.

    I have also noticed that in the summer, the upstairs is significantly hotter than the downstairs even if I shut the downstairs vents.

    Question – would it be more efficient to leave the existing furnace/ac and add a second unit just for the addition, or should I just replace what I have with a bigger unit that will condition the whole house. I’m thinking if I add another system, maybe I could use the existing furnace to condition the basement or 3rd floor attic space so it isn’t so oversized.

    Final question - Is two separate HVAC systems more or less efficient than one system. I really don’t like the idea of two separate systems, but maybe it makes sense.

  2. #2
    Join Date
    Nov 2007
    Location
    St. Louis, Mo
    Posts
    28
    I would install a second unit for the upstairs area. Did it my home and made a world of difference. my 2 cents

  3. #3
    Join Date
    Jul 2002
    Posts
    79
    anyone else have any ideas?

    Thank You,

  4. #4
    Join Date
    Aug 2007
    Location
    MN
    Posts
    2,680
    80k at 92% is 76,600 BTUs, big enough for everything. may want to invest in seperating up and down stairs ducts and zoning, if seperating is not an option, we have installed a pneumatic zone system called an Arzel zone system. works great, dampers available to drop in register boots, or cut into pipes, tubing runs back through the trunkline, making retros to existing duct in finished ceilings possible. Expensive but effective. I didn't do your load calc, not sure of cooling load and size A/C you have, but I believe the 80k should be capable of a 4ton drive.

    http://www.arzelzoning.com/ here's some info on the systems.
    You can't fix stupid

  5. #5
    Join Date
    Aug 2007
    Location
    MN
    Posts
    2,680
    also fyi the basement does not add a significant load to the load calc, but it would be a good idea to make this a seperate zone if possible also.
    You can't fix stupid

  6. #6
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,803
    May be able to zone it.
    You didn't say what the cooling load is, and what size A/C you currently have.
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  7. #7
    Join Date
    Jul 2002
    Posts
    79
    Sorry,
    Heat Gain was 28,187 BTU. Heat Loss was 68,775 BTU

    Not sure what the AC is in Tons, but I'll find out tonight.

    Thank You,

    Jason

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