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Estimating energy cost

tim51 Member Posts: 5
We are in the process of converting a 1921 school building (used as a community center) from coal to natural gas. System is two pipe steam, heat loss is 465,000 btuh at 70 degrees TD. Haven't had any good results figuring operating cost from gas co. or boiler manufacturer. Proposed new boiler has 85% thermal efficiency, no listed AFUE. We are near Springfield, IL. Can anyone provide me with a link to this type of calculation?


  • delta T
    delta T Member Posts: 884
    There are a ton of variables to consider here. FIrst of all, when you say heat loss are you referring to the loss of the building on a specific design day? If so what temp was your design day at? Does your heat loss refer to the EDR of the connected radiators? With steam, you have to size the replacement boiler to the radiation, not to the heat loss. Is there insulation on the steam mains? Is the chimney inside the building or is it on an external wall? This being an old school, are there fresh air vents with pin radiators in them? The fresh air system on most old school buildings was pretty neat, but will contribute to heat significantly....
  • Jamie Hall
    Jamie Hall Member Posts: 22,892
    Not really that hard. Also not that accurate... however, the basic principle is that you have the heat loss in BTUh at design day -- a 70 degree differential. Now if you had a design day for 24 hours, you would have 70 degree days, and you would require about 11,000,000 BTU of heat (24 times the 465,000 BTUh). Natural gas has a heat content at 100% efficiency of -- conveniently -- 1,000 BTU per cubic foot; at 85% efficiency you would get about 850 BTU of usable heat from that -- so, it would take somewhere around 13,000 cubic feet of gas to produce the heat you needed on that day.

    But that's on that 70 degree day. A better measure, then would be how much gas is required per degree day? Take that 13,000 and divide it by 70 and you come up with about 200 cubic feet per degree day.

    Now go to the weather records for Springfield, IL. and find the number of heating degree days per year and multiply, and you will have a guess at your gas usage for a year -- then you can ask your gas company what that will cost you.

    Note that this is none too accurate. The heat loss calculation -- even a careful Manual J -- is subject to error; sometimes quite a bit of error -- and is usually conservative. There are other variables which are squirrely, too. But that will get you into the ball park anyway...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Zman
    Zman Member Posts: 7,542
    Like Jamie said :)
    If you are just trying to figure you energy cost difference based on 2 different fuels and efficiencies, this one does that nicely.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • delta T
    delta T Member Posts: 884
    @Zman Love that spreadsheet!! Nice job
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,229
    Hello, Do you have the data from a few years worth of coal purchases? If so, you might be able to calculate (roughly) the BTUs delivered to the building by figuring what yearly percentage (in degree days) was used each month.

    I'd compare that number with what you get from @Jamie Hall and from @Zman 's spreadsheet. With luck, things will line up. B)

    Yours, Larry
  • SeanBeans
    SeanBeans Member Posts: 511
    Very cool table @Zman