But if the sun heats stronger than the fireplace, something must be wrong too? Where does all the energy from the wood go with you? Straight out the chimney?
I would say the energy input from the sun is greater than from the fireplace :) But I can't prove it with a formula. I quickly found nothing on Google regarding energy input from the sun. Only energy transmittance (g-value) of the windows... It would be interesting what, in theory, should work faster:
16 sqm pure glass surface in the south at a favorable angle of incidence in autumn/winter vs.
3-4 kg firewood to heat 260 m³ of air (with about 2.5-3 kg/l water contained in it) by 1°C. But in practice, we are very satisfied. If there is too much heat from the sun, the blinds go down. And the fireplace can run for hours without creating a sauna-like environment. Heating only with the fireplace will probably not work (but maybe the fireplace increases the room temperature more significantly if it runs around the clock, we have not tested that and do not intend to). Even if we could heat with the fireplace (temperature-wise), it would still not be economically attractive for us because we do not get the wood cheaply and gas is ultimately significantly cheaper.