I would be a little cautious using data from a single day to make this calculation, though I notice many others have much cleaner straight line relationships between indoor - outdoor temp and space heat requirement than I seem to get…
Here’s my average daily space heating only heat output vs indoor - outdoor temperature. The spread of values has always surprised me but probably reflect the high thermal mass of our solid stone mid-terrace house?
All in all this plot for every day in this heating season so far suggests a space heating demand of 3.3 kW ± 0.5kW? at 23 DT for our house:
