The answer to 'what does furnace do' is a furnace is a central heating system that generates and distributes warm air throughout a home. Furnaces use a fuel source, such as natural gas, propane, or electricity, to heat air that is then circulated through ductwork and vents in the home. This warm air helps maintain a comfortable indoor temperature, especially during colder months.
Furnaces also include a blower fan that pushes the heated air through the ductwork and into the living spaces. Many modern furnaces also have features like air filters, humidifiers, and programmable thermostats to improve efficiency and indoor air quality.