/ ˈfɜːnɪs; NAmE ˈfɜːrnɪs/ noun
1.
a space surrounded on all sides by walls and a roof for heating metal or glass to very high temperatures :
It's like a furnace (= very hot) in here!
—see also blast furnace
2.
( especially NAmE ) = boiler
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WORD ORIGIN
Middle English : from Old French fornais(e) , from Latin fornax , fornac- , from fornus oven.