I’ve been thinking about you folks in Northern Europe with the winter coming and high heating costs. I was just considering my chest freezer and how it might be converted to a heater. I think it can hold around 400 liters of water. As long as you could heat the water(the hotter the better) to at least 10-15 degrees Celsius, you could turn it on and it would be a pretty decent heat pump until the water got too cold. A rocket stove, if you have wood, or solar thermal could do it.