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  1. #1
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    modulating gas furnace combustion efficiency

    Been doing some reading on modulating gas furnaces.

    Noticed a few posts here about the lack of efficient combustion at lower fire rates.

    Seems like all I see are manufacturers saying how great they are.....nothing about a loss of efficient combustion. does anyone have a technical info on this??....BTU output etc.??

  2. #2
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    Quote Originally Posted by 71CHOPS View Post
    Been doing some reading on modulating gas furnaces.

    Noticed a few posts here about the lack of efficient combustion at lower fire rates.

    Seems like all I see are manufacturers saying how great they are.....nothing about a loss of efficient combustion. does anyone have a technical info on this??....BTU output etc.??
    The overall efficiency at low stage is slightly
    less but the overall comfort level is much higher.

    It is hard to modulate a fixed size burner chamber and adjust the gas flow and air mixture to match for high combustion efficiency over a broad range. But they work well for there intended purpose of comfort.

  3. #3
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    I think it still gets factored somewhat into the the AFUE. It's partially offset by efficency gains due to fewer cycles. There's a lot of electricity and heat wasted cycling a furnace on and off each time. Having a unit more closely matched ot the heat loss, and also running the blower at a lower speed more of the time, will save energy and make-up for any actual direct efficiency. Also cnsider that you use less eenrgy as well when you can maintain a indoor temp lets say within 0.5F rather than within 1.5F. It means you can set the thermostat 0.5F lower and still be jsut as comfortable, and that you aren't overheating the space by 1F every time it cycles.

    Aslo a 60k modulating furnace can stage all the way down to 20k BTU but still have 60kBTU available for setback recovery... , while a 2 stage 40k BTU furnace can only stage down to 25k BTU. So you get a "smaller" furnace with a 60k mod compared to a 40k 2 stage. That why the mfg's don't both making a 40k mod furnace. Not enough demand for such a small furnace. Althohg these newer ultra insulated homes might change that. But most of those are going geothermal.

  4. #4
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    Thread Starter

    Hmm

    I'm aware of the "general efficiency" gained by modulating the BTU output, cycling etc.

    what I'm really interested in is some technical data regarding the amount of BTU's per, let's say 1k cubic foot of gas, at the lower fire rate as apposed to the high fire rate.

    Of course BTU ratings change from supplier to supplier, but there has got to be some numbers out there somewhere.

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