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  1. #1
    Join Date
    Sep 2003

    Post ****** FURNACE SIZE CALCULATION ******

    How do I manually calculate the number of BTUs to correctly size a furnace for my home? I'm sure I need to know the type and size of window (whether single or double pane), insulation R-value, etc. to determine this number.

    I know computer programs are available (such as the one on this site), but I'm a homeowner and only plan to do this once for this house so I don't see the point in dealing with another program. I know if it can be written into the code of a computer program one can surely figure it out using one's own head.

    I'm asking this because I've had many, many individuals give me bids on a new furnace. I've been told I need a furnace from 50,000 BTUs to 80,000 BTUs. I know undersizing and oversizing a furnace tend to degrade them faster. I live in a 1,000 sq ft ranch style house in a rural area outside of Chicago, IL. I know I have minimal insulation in the attic - maybe 2 to 3 inches. The house has a basement as well as 2 doors to the outside and 8 windows with a large picture window (facing south) with 2 adjacent windows on the sides.

    Would someone please help! I really don't want to buy more or less than I need.

    Thank you!

  2. #2
    Join Date
    Dec 2001
    Ocean Pines, MD

    How Caculate for your furnace.

    May be close to how much the software that sponsors this site could save you. Along with the time and other things I think if your inclined to try it long hand vs this your nuts. Respectfully,

  3. #3
    Join Date
    May 2000


    Stamis is right, get it HVAC-Calc

  4. #4
    Join Date
    Apr 2003
    Northern Virginia, Fairfax County
    I am a homeowner who bought a replacement system last May.

    You can borrow excellent books on HVAC which include a chapter of two on load calcs. Look in any well stocked public library.

    Better and quicker, I think, to do it with software. Also, with software, you can play "what if" much faster. What's the effect on the gain and loss numbers of making enhancements to insulation, windows, etc.

    Either way, most of your time will be in making the measurements of your home and checking the construction features.

  5. #5
    Join Date
    Jun 2001
    Moore, Oklahoma, United States

    Lightbulb Tighten up

    I would go ahead and do the improvements such as adding attic insulation THEN calculate !!

  6. #6
    Join Date
    Jan 2003
    Beaufort, SC
    Tell the contractors if you want the job, heat calc it, we calc every job.

  7. #7
    Join Date
    Feb 2003
    It will take several days to set up a spreadsheet program, after buying and reading the latest ACCA Manual J or ASHREA Fundamentals, either of which costs ~$150. These manuals give the heat transfer values for various types of building components, infiltration values, the design criteria for your locality, conversion values between fuels, etc. The basic heat transfer formulae is trivial, but the resistance value is different for each combination of materials, including surface film, air/ vacuum spaces, etc.

    I do not think you will find either reference in your local library: at least not here in Huntsville AL. No engineering college (UA, Purdue, Ga Tech,nor Redstone Arsenal) library within a 200 mi radius has the current ASHRAE manual.

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