Most home furnaces are forced air, which means cold air is heated by a fuel source, passed through a heat exchanger and then directed into the home via heating ducts. As warm air circulates in the structure, cooler air is pulled back into the furnace through cold-air return vents.
The hot air is forced into the duct work using a furnace blower. Each room has one or more duct work exits covered by a register, or air vent. Levers on the registers open and close the slatted grills to regulate air flow.
Furnaces can be fueled by heating oil, natural gas, propane or electricity. A few systems incorporate a boiler, but that is most often used for radiant and baseboard heaters. The actual furnace is usually placed in a basement, garage or large closet. The duct work fans out through the home from that installation point.
Most furnaces require a thermostat to keep temperatures inside the home constant. If the temperature falls below the prescribed setting, the furnace kicks on and stays on until the temperature is back up. This cycle may repeat several times through the day. The idea is to only have the furnace running when needed, saving on energy costs. Manual and programmable thermostats are available.