Furnace

Furnace is a device that generates heat for two main purposes: heating spaces and processing materials at high temperatures. In residential and commercial buildings, a furnace is a central heating appliance that warms air for distribution through ductwork. Most modern units are combustion-based and burn natural gas or oil, though electric resistance furnaces also exist. A typical gas or oil furnace includes a combustion chamber, a heat exchanger, a blower, and controls. Combustion heats the exchanger; a fan circulates warm air through ducts while exhaust gases exit via a flue or vent. Electric furnaces generate heat directly in resistance coils and do not produce combustion gases.

Furnaces vary by fuel and venting: gas and oil furnaces are common in colder climates; electric furnaces provide clean operation and can be installed where emissions are a concern. Condensing (high-efficiency) furnaces recover latent heat from exhaust by cooling it below the dew point, increasing efficiency compared with non-condensing units. Efficiency is measured by AFUE (Annual Fuel Utilization Efficiency). Safety features include flame sensors, limit switches, and carbon monoxide detectors; regular maintenance—filter changes, heat exchanger inspection, and vent cleaning—helps ensure safe operation.

Industrial furnaces refer to high-temperature vessels used to process materials rather than to heat spaces. Examples include blast furnaces for ironmaking, electric arc furnaces for steelmaking, and reverberatory furnaces for glass and ceramics. These furnaces may use solid, liquid, or gaseous fuels and are designed for continuous operation and high energy efficiency.

Historically, furnaces evolved from simple hearths to complex, tightly controlled systems, with significant gains in efficiency, emissions control, and safety in the modern era.