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wow, it's true that it's cold over there. in my list of DJU, I besancon: 2900DJU, mulhouse 3000DJU and colmar 3000DJU
I would say that being at 650m altitude, the severity of the climate must be at least as much as Colmar or more. I took 3300 for my calculation.
2200L it's about 22000kwh and 12steres it's about 19800kwh. 0.7 has a medium yield (logs + fuel + radiator circuit), and an intermittency at 0.6, it gives us a loss of your house of about 1.75W / m3 ° C which is not great.
for info of the new isolated correctly gives in the 0.7
of the former isolated moderately gives in the 0.9 to 1.3 and the old evil or not isolated gives in 2 to 3.5
to see if my calculation has the big ladle is correct but it seems that you can do better in terms of isolation. I remind you that this calculation takes into account the rigor of the climate and is therefore comparable everywhere.
Now, with these existing losses if you still want to have 19 ° C by -20 ° C outside, you need 24KW.
it would take a boiler 25KW but with 30 and a big buffer tank, it must go. the ideal would be to isolate to go down the losses and pass with an 20Kw