A boiler is a heating system that heats water (or produces steam) and distributes it through pipes to radiators, baseboard heaters, or radiant floor tubing throughout a building. Unlike forced-air furnaces, boilers provide hydronic (water-based) heating.
Hot water boilers (the most common modern type) heat water to 140-180°F and circulate it with pumps. Steam boilers heat water to 212°F+, using steam pressure to distribute heat. Boilers are fueled by natural gas, propane, oil, or electricity.
Boiler efficiency is rated by AFUE. High-efficiency condensing boilers achieve 90-98% AFUE. Boiler systems offer advantages including: superior comfort (radiant heat), no air distribution so no duct losses, no forced air so less allergen distribution, and long equipment life (well-maintained boilers can last 25-35 years). They cannot provide cooling, so a separate cooling system is required.