Flue

Webster's Practical Dictionary- A Practical Dictionary of the Englih Language · 1884 · p. 160

flu, n. An air-pa age, esp. for conveying smoke and flame from a fire. (Steam Boilers.) A pa age surrounded by water, for the gaseous products combustion, — disting. fr. tubes, which hold water, and are surrounded by fire: small flues are called fliie-tubes.