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Thread: 100K BTU gas furnace, how many BTUs in a cf of gas??

  1. Default 100K BTU gas furnace, how many BTUs in a cf of gas??

    Well I just got a $521 gas bill today (34 days in the Northeast), yeah, that is one month. Usage is basically the norm, rates through the roof. No big surprise there.

    Anyway, I put an hourmeter on my 100K BTU shop furnace and was keeping a log. Jsut want to know how much it was costing me to run. So if it runs for an hour, how much gas does it use? I recall finding this number before on the web. I'm sure there are efficiency factors to put in, but anyone have any idea? Should I be able (instead)to find some kind of gas consumption rating on the furnace ?


  2. #2


    Southern California Gas company posts the following
    conversion they use for Billing:

    Natural Gas Conversions 1 cubic foot = 10.20 Btu Therm = 100,000 Btu

    Ocassionally, we will test the heat content and find it to be closer to 10.30 (a POCO in Los Angeles). I recommend you use the 10.20 Btu value.

  3. #3
    Join Date
    Dec 2003
    Kent, WA


    Don't forget the efficiency factor either. If you have a 90% furnace, 1 CF of gas makes 9.18 BTU of heat. If you have an 80% furnace, 1 CF of gas makes 8.16 BTU of heat.
    Kent, WA

  4. #4
    Join Date
    Jan 2006
    Western Wisconsin



    What temperature are you running it at and have you confirmed the accuracy of your thermostat's thermometer with a good alcohol one?

    As far as the amount of cubic feet useage goes, I have seen some furnaces that have in their manual how to check the input btu rate by going out and timing the gas meter when it runs, by looking at the fractional foot dial. Then, if it comes out according to what they say, or there abouts, you know that your furnace is consuming the correct amount of gas per the factories specs. (as an example...suppose you timed it and you found the gas meter turned about 1/2 times more than what they said. That would indicate then that your 100,000 btu furnace became more like a 150,000 btu furnace because perhaps the gas pressure at the gas valve was too high.)

  5. #5


    An interesting note... a gas furnace can be "tuned" in much the same way that an oil furnace is tuned with a Bacharrach tester. The test fluid for a gas furnace is green, and checks for oxygen (as opposed to CO2 in oil furnaces). Perhaps one in every 100 HVAC companies owns the "green Bacharrach gas kit", and maybe someone that owns one actually knows how to use it. Almost everyone (HVAC wise) owns and uses the red kit to tune an oil furnace.
    "Nothing is foolproof to a sufficiently talented fool"

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