EERE: Energy Basics
Heating Systems
A variety of heating technologies are available today. In addition to heat pumps, which are discussed separately, many homes and buildings use the following approaches:
Furnaces and Boilers 
Furnaces heat air and distribute the heated air through a building using ducts. Boilers heat water, providing either hot water or steam for heating.
Wood and Pellet Heating
Provides a way to heat a building using biomass or waste sources.
Electric Resistance Heating
Can be supplied by centralized electric furnaces or by heaters in each room.
Active Solar Heating
Uses the sun to heat either air or liquid and can serve as a supplemental heat source.
Radiant Heating
Can draw on a number of energy sources, including electricity, boilers, solar energy, and wood and pellet-fuel heating.
Small Space Heaters
Used when the main heating system is inadequate or when central heating is too costly.