We have heaters at home, the heaters are gas heaters. I live in an apartment where there are some others apartment, we have a boiler room where all the heaters come together. This type of heat can be good if you use biogas. However, I am not sure what we use.
THis is how it works: The burner burns the gas, the combustion produces heat that is needed to heat the water. A circulation principle allows them to circulate in the heaters.