Radiant heat transfer occurs when infrared light leaves the surface of an object and travels to the surface(s) of other cooler objects. Unlike conduction and convection, radiant heat transfer does not require a fluid or solid material between the two objects transferring heating. It only requires a space between the two objects. The radiant energy only becomes sensible heat when absorbed by a surface.
The radiant heat emitted by the relatively low temperature heat emitters used in hydronic heating is technically described as infrared electromagnetic radiation. It’s simply light that the human eye can’t see. However, other than the fact that it’s invisible, infrared light behaves just like visible light. It travels in straight lines at the speed of light (186,000 miles per second), and can be partially reflected by polished metallic surfaces. Unlike warm air, radiant energy travels equally well in any direction. Up, down or sideways, direction simply doesn’t matter. This characteristic allows a heated ceiling to deliver radiant heat to the room below. The radiant heat emitted by a warm floor, wall or ceiling is a completely natural phenomenon that’s literally as old as the universe itself. A surface warmed by sunlight gives off infrared radiation just like one warmed by embedded tubing. The latter simply uses a different heat source and transport system to deliver heat to the surface. Most low temperature radiant heat panels emit less than 1/10 the radiant flux of bright sunlight, and all of it is infrared as opposed to ultraviolet light. Even the human body gives off infrared radiation to cooler surrounding surfaces.