It’s a heating method in which heat is transmitted by radiance from a heated element to a surface directly connected to it. The heat is transferred to nearby objects and people in the room. This compares to convection heating or forced air, where air is being heated in a furnace and then transferred to you and the objects in the room. This is rather inefficient because air doesn’t carry heat all that well and can escape through windows and doors. Ultimately, it provides an uncomfortable draft in the house, resulting in a dry air environment in which dust and microbes float and travel.
Holding a hot mug of coffee and feeling the heat move through the mug to your hands is an example of radiant heat. Standing in the sun on a cold day is another good example: everything between you and the sun is cold, but the sun radiates its heat straight onto you, the sidewalk and the objects around you.
There are two main types of radiant floor heating systems: hydronic systems with water and glycol in pipes and electric heating cables which are embedded in floors, ceilings or walls.