There are several renewable energy heat sources that can supply some or all of the space-heating load of residential and light commercial buildings when coupled with suitable hydronic distribution systems. Those heat sources include solar thermal collectors, geothermal water-to-water heat pumps, air-to-water heat pumps, as well as pellet-fired, chip-fired, and cordwood-fired biomass boilers.
Optimal performance of these heat sources requires them to be coupled with properly-designed low-temperature hydronic distribution systems.