OP is in Austria where it often gets below freezing.
Air/air heat pumps and ventilation heat exchangers tend to freeze up when it gets too cold. So heat pumps often need to run in reverse (to melt the ice on the condenser) and heat exchangers need to pre-heat air with resistive heating to prevent ice build up.
If he can pre-heat the air with a crypto miner that would be better than either of the above.
Air/air heat pumps and ventilation heat exchangers tend to freeze up when it gets too cold. So heat pumps often need to run in reverse (to melt the ice on the condenser) and heat exchangers need to pre-heat air with resistive heating to prevent ice build up.
If he can pre-heat the air with a crypto miner that would be better than either of the above.