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If a home uses a supplemental heat source such as electric baseboard or hydronic panel radiators to raise the room temperature by 10 degrees, how do you calculate the BTUs required in a given room?

I have a 1985 built ranch style home in southern Minnesota with 2x6 walls and triple pane windows. The central furnace is rated at 70k BTUs and the fireplace is rated at 75k BTUs. I want to be be able to heat the home with wood plus supplemental heat, and avoid use of the central propane furnace except as needed on below average days (say below 10 degrees which is the average low here in January). I believe the fireplace produces enough BTU to heat the home, but I have no way to transfer the heat from that room to the bedrooms at the other end of the house.

I tested this out a few days ago by turning the central heat off on a 10 degree day. On the ends of the house, I was able to maintain 62 degrees over a 12 hour span. 67 degrees is comfortable in those rooms. For one of the rooms that I want to heat, at a design temperature of -20, the heat loss calculation results in 1800 BTU/hour of loss. If I use a design temperature of 10 degrees (the target I mentioned above), that heat loss is reduced to 1200 BTU/hour.

Is using an adjusted design temperature for the heat loss calculation (1200 BTU/hr in the result above) the right way to go about this? Or is there another method I should be using?

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    The calculations are only as accurate as their inputs. But a clear result will be obtained by noting that if you divide by the difference between inside and outside temperature you are evaluating, you get a figure that is "So many BTU/hr per degree F" and you just multiply that by how much you want to raise the temperature (5 degrees, evidently)
    – Ecnerwal
    Commented Dec 9, 2021 at 23:42

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