For residential apartments: ensure good heat insulation, you can use portable electric heaters with AC pluggable input or run them on free energy sources. A di

For residential apartments: ensure good heat insulation, you can use portable electric heaters with AC pluggable input or run them on free energy sources. A diy heating type is to burn candles covered by clay pots, which serve as heat conductors. There are reports on youtube that one small candle with 2 clay pots can heat up to 20< m2 apartment for 4< hours or so (the temp in ~1m radius around the pots is ~70 Celsius after ~30 minutes). The drawback is that it consumes oxygen, but for this more oxygen producing plants can be placed indoors.