Boiler

A boiler is a closed pressure vessel that heats water (or converts it to steam) for use in space heating systems. Unlike a water heater — which heats potable water for domestic use (showers, sinks) — a boiler heats a closed loop of water that circulates through radiators, baseboard heaters, radiant floor systems, or a steam distribution network to heat living spaces. Boilers and water heaters use similar principles but serve different purposes and are entirely separate systems.

Types of boilers

• Hot water boiler (hydronic): heats water to 140–200°F and circulates it through a closed loop to radiators or baseboard convectors. The water never leaves the system (it's returned to be reheated). Most common residential boiler type in cold climates.
• Steam boiler: heats water to 212°F+ to produce steam, which rises through pipes to radiators and condenses back to water, returning by gravity. Found primarily in pre-1950 homes. Less efficient and more complex to maintain than hot water systems.
• Combi boiler: a single unit that provides both space heating and domestic hot water on demand — common in Europe, increasingly popular in the U.S. for space-efficient installations.

Fuel types

Boilers run on natural gas, propane, oil, or electricity. High-efficiency condensing gas boilers achieve 90–98% AFUE (Annual Fuel Utilization Efficiency) vs. 80–85% for standard boilers. The payback on a high-efficiency boiler replacement is typically 5–10 years in cold climates.

Maintenance

Annual boiler service by a licensed technician is essential: inspect the heat exchanger for cracks, test the pressure relief valve, bleed air from radiators (hydronic systems), check the expansion tank, and clean the burner assembly. An unserviced boiler can develop cracked heat exchangers that allow carbon monoxide to enter the living space — a potentially fatal failure.