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Community and Q&A

Heating system sizing.

Randy Williams | Posted in Mechanicals on

Is it possible to calculate the size of a heating system based on past energy usage? I’m working with a customer that I think their heating system is much larger than it should be. The average hourly BTU usage of the home was 28,000 BTU’s for the time period of December 27th thru January 26. He has an electric boiler rated at 27 KW, producing around 92,000 BTU’s. Weather data for this time period was 1810 heating degree days, average mean temp of 6 degrees. The location is zone 7, northern Minnesota. I believe the design temp for my area is -19.

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Replies

  1. User avatar
    Dana Dorsett | | #1

    "Is it possible to calculate the size of a heating system based on past energy usage?"

    Yep! But it's not about the average hourly use, it's about what it needs at the coldest hours, using a linear approximation.

    See:

    https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/out-old-new

  2. User avatar
    Dana Dorsett | | #2

    Using 28,000 BTU/hr at a mean temp of +6F, which is (65F-6F=) 59F below the presumptive base 65F balance point is a linear constant of (28,000 / 59=) ~ 475 BTU/degree-hour, give or take.

    -19F (is this near Hibbing?) would be (65F- -19F =) 84F heating-degrees...

    ...x 475 BTU/degree-hour is an implied heat load of 39,900 BTU/hr @ -19F.

    Heat load isn't EXACTLY linear with temperature- window losses are extremely non-linear, but that would be the right range.

    So that's less than half the capacity of his electric boiler. ASHRAE recommends 1.4x oversizing as the compromise (that gets you through Polar Vortex coolth without suffering, but not ridiculously oversized), which for a ~40K load would be 56,000 BTU/hr, or 16kw.

    That's the gist of it, but run your kwh/HDD numbers carefully, and use both base 60F and base 65F linear approximations to bracket it.

    Is the electric boiler sub-metered, or was that the entire power use on the premises, including outdoor lights, hot water that ended up going down the drain, etc?

  3. Randy Williams | | #3

    Thanks Dana, that was exactly what I was looking for.

  4. Randy Williams | | #4

    The boiler is sub-metered at a rate of $.0765 per KW. Total usage for the time period was 5890 KW. A high bill complaint brought me to his house. He usually heats with an outdoor wood boiler. Your close on the location, about a half hour west of Hibbing.

  5. User avatar
    Dana Dorsett | | #5

    A smaller electric boiler isn't going to be cheaper to run than an oversized electric boiler- the effieincy is the same either way.

    If using this data to size some other heating equipment it has to be a period where auxiliary heat from the wood boiler wasn't in use.

  6. Randy Williams | | #6

    Got it, thanks again.

  7. Jon R | | #7

    > Heat load isn't EXACTLY linear with temperature

    It can can get fairly in-exact when wind variation is factored in.

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