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woody_007

What does Bonnet Cap mean for a furnace.

woody_007
16 years ago

Does anyone know what the term "Bonnet Cap" means for a furnace?

I looked inside my 70's furnace and it says: Input 110 MBTU, Bonnet Cap 88 MBTU.

For a finding a replacement furnace do I go off the Input BTU or the Bonnet Cap BTU.

Comments (3)

  • daddo
    16 years ago

    The bonnet is the actual btu's you'll get after the loss from the flue and ambient loss etc. I find it puzzling that furnace has a 80% difference.
    When calculating a heat loss load in a home, it's best to use this bonnet to decide the size heater needed.
    *Much like using the sensible capacity in an a/c instead of the total btu capacity.

  • woody_007
    Original Author
    16 years ago

    Thank you for the response.

    If I'm not mistaken doesn't the heat loss work out to be 20% as opposed to 80% as was stated in the previous post.
    110 BTUH in, 22 BTUH loss, and 88 BTUH left over to heat home. The loss is 22/110 * 100 = 20%.

    If I understood correctly, I should use the Bonnet Cap BTUH to guide me on the size of the replacement furnace.
    88,000 BTUH * 60% AFUE(70's furnace) = 49,800 BTUH. My new furnace should have an ouput of no less than 49,800 BTUH. Is my thinking correct here?

  • Humphrey Benfufay
    8 years ago

    Woody,

    Coming a bit late to answer, but if someone is looking for an answer...
    Bonnet Capacity is an old way of describing the net output.

    Furnaces made before 1980 had a AFUE of 60 to 70%, after 1980 minimun of 80%

    The input capacity is "With Loss", the "bonnet capacity" is about the net output, in BTU/HR

    Nowadays, efficiency range between 80 to 98% (Nat. Gas furnace) depending on the model.

    If you are in the South, 80% should do, if you are in a cold climate region, look into 90% +

    95% efficiency roughly mean that 0.95 cents on the dollar are used to heat your home and 5% are lost.

    Depending on your bill, that's where you should do your homework, as per savings and payback period.

    A good rule of thumb (for the North East) is 45 to 50 BTU per square foot on old houses, and 35 for well insulated houses. It will also depend if you are in VT or CT.

    i.e. 1000 sqft * 47.5 = 47,500 BTU Hour (net output)

    Hence a 50,000 BTU/HR furnace @ 95% efficiency will do a net 50,000 * 0.95 = 47,500

    Please understand that the above stated are rules of thumb: An old drafty house will require more, as well as other factors such as number of windows , etc.

    You can find online "Load Calculators" that would be more precise than this rule.

    For those willing to spend some time: http://www.loadcalc.net/

    In any case, do not oversize or undersize too much. There is no benefits, and might even create issues (Temp swing, Short cycling, premature wear and tear)

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