This image is a square with three vertical lines inside of it. These lines are slightly wavy and parallel and appear to signify the rising of warm air through a vent or furnace.
This Heating icon is made in Material Design style
developed by Google. This style is based on extremely simplified shapes, thin two-pixel lines, and is optimized for 24x24 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 5,700 icons, all in the same style and quality.