Heat Pumps
Heat pumps are not combustion-based heating systems. In fact, they are very similar to air conditioners. The difference is that instead of absorbing heat inside the home and releasing it outside, they do the reverse. The outside unit of the heat pump absorbs thermal energy, either from the outside air or from the ground, and sends it inside to be released. Heat pumps can also reverse the flow of refrigerant within their systems, allowing them to act as both heaters and air conditioners.
Boilers
Boilers heat and distribute water throughout the house, using a network of pipes installed in the walls and floor of each room. Heat radiates through the pipes and into the room, keeping it warm without using ducts or air flow. Boiler systems are quite energy efficient, and tend to heat more evenly than more traditional heating systems.
