gʷerH-tl-
“means of warming, a furnace”furnace, furnish
Derivative from *gʷerH- "to heat", yielding Latin furnus/fornax "oven", English furnace, furnish.
Discussion
The Proto-Indo-European form *gʷerH-tl- is reconstructed as meaning "a means of warming" or "furnace, heating device" and represents a derivative of the root *gʷer- "to heat, to warm" with an instrumental suffix *-tl-. The formation suggests a tool or device used for the purpose of warming, consistent with the technology of enclosed fires and primitive ovens.
In Latin, the form developed into fornus or furnus "oven, furnace", which produced English furnace. The semantic shift from "a means of warming" to "an enclosed heating structure" is minimal and transparent. The same Latin word gave rise to fornax (a technical term for a furnace) and is preserved in the constellation name Fornax. French four "oven" is a direct descendant, as is Italian forno.
The initial Proto-Indo-European labio-velar *gʷ became Latin f- (in some phonological environments), a regular but sometimes debated sound change. The broader root *gʷer- "to be hot, to warm" also produced Latin formus "warm" and the related word thermal — though the latter comes through Greek thermos "hot", where the labio-velar developed differently (into th-).
Greek thermos "hot, warm" is the principal Greek reflex of the underlying root, giving English thermal, thermometer, thermostat, and thermos (the vacuum flask). The relationship between Latin f- words (furnace) and Greek th- words (thermal) from the same root illustrates the different phonological paths taken by the labio-velar in each branch.
Sanskrit gharma- "heat, warmth" is a further cognate, confirming the Indo-Iranian branch.
The form *gʷerH-tl- is valuable because instrumental nouns of this type provide evidence of Proto-Indo-European material culture. The existence of a specific term for a heating device suggests knowledge of enclosed or channelled fire — technology beyond the simple open hearth — pointing to a degree of domestic sophistication in the proto-community.