Hello: Mike134
The rule is usually 40 BTU's per square foot. However, in your climate zone it my have to be increased. This formular is used for forced hot air furnaces in my climate zone and may not apply to any other type of heating unit.
Other methods of suppling space heating {space meaning rooms} variant may be what other heating is supplied by the same unit. An example would be hydronic hot water for heating and hot water.
Since I do not profess to have expertise on all types of heating units or methods, other professionals, replying within this forum, may have alternate opinions and or a better understanding of the question, based upon the various heating systems.
Check back several times.
Regards & Good Luck, Forum Moderator.
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