In by Jackie Dunn

A heating system that uses the principle of thermal convection. When air is heated, it rises, and as the air cools, it settles. Ducts are installed to carry the hot air from the top of the furnace to the rooms in a home. Other ducts, called cold-air returns, return the cooler air back to the furnace.

Merriam-Webster Online Dictionary
furnace (noun)
an enclosed structure in which heat is produced (as for heating a house or for reducing ore)
furnace (Wikipedia)
Industrial furnace from 1907.

A furnace is a device used for high-temperature heating. The name derives from Latin word fornax, which means oven. The heat energy to fuel a furnace may be supplied directly by fuel combustion, by electricity such as the electric arc furnace, or through induction heating in induction furnaces.

In American English and Canadian English usage, the term furnace refers to the household heating systems based upon a central furnace, otherwise known either as a boiler, or a heater in British English. Furnace may also be a synonym for kiln, a device used in the production of ceramics.

In British English, a furnace is an industrial furnace used for many things, such as the extraction of metal from ore (smelting) or in oil refineries and other chemical plants, for example as the heat source for fractional distillation columns. The term furnace can also refer to a direct fired heater, used in boiler applications in chemical industries or for providing heat to chemical reactions for processes like cracking, and is part of the standard English names for many metallurgical furnaces worldwide.

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