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  1. #1
    Join Date
    Mar 2008
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    Confused newbie remodeling question re: attic in furnace

    We are looking to add onto the house. Our present furnace ~60K Btu is inside the house of ~1200sf
    We are looking to add ~800sf to our house.

    a) How do I decide what is the best size for our new furnace or whether we can use the present one. (our present furnace is about 1 yr old). The contractors that came in just did a rule of thumb and said I need ~90K Btu. I have read you can do a J calculation to find the correct size but noone is willing to do that. Does it take a long time to do so?
    b) Some of the contractors say you can put a 90K in the same space as we have but others say I will have to go the attic. I am afraid to put it in the attic because if something breaks what do I do? Also how do I replace filter? But then I don't know how I'll fit in the same space.
    c) As an added bonus question, we do have a 3tonne A/C. We hardly use the A/C but everyone was saying I have to go to 4tonne A/C since I am upgrading the furnace so the condensor coil needs to be upgraded as well.

    Sorry for the long email but I'm a newbie

  2. #2
    Join Date
    Mar 2008
    Central Maryland
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    I just went through the same thing. The bidding process was revealing. First, everyone except one bid the same size furnace, the odd one out bid 10K BTU more. All but one (different one) wanted to replace the coil, one insisted on replacing the entire HP/AC system.

    Fortunately, I got good advice here- replace the coil when I replace the rest of the Heat pump/A/C system, and not before. So that's question c.

    In my case, the winner bid just what I wanted- a furnace, not a new HP or pieces of it.


  3. #3
    Join Date
    Jun 2003
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    Whoa......slow down!!!!

    a) The way to size your furnace needs is with a Manual J load can do it yourself off of this site if you know what materials your house is made of( insulation, etc.). Look at the top of this site for Hvac-Calc. Or PAY a pro to do one for you.

    b) This all hinges off of a) and the physical size of the selected furnace.

    c) the load calc will also tell you what size a/c you don't just go Bigger Furnace,Bigger A/C.

    So everything depends on the load calculation....the beginning of your project.
    If everything was always done "by the book"....the book would never change.

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