A radiator is a heat exchange device that allows the warmth from heated water, oil or steam to pass through into a home to warm it. Radiators rely on a large surface area to maximize heat transfer into the surrounding air, as well as careful placement to create convection currents.
Radiators for home heat were originally powered by steam. A central boiler heated water to create this steam, and a series of pipes carried it throughout a building to radiators set against outside walls in order to create heat. As the steam cooled, it flowed back down through the same set of pipes to be re-heated. These systems were difficult to control and inefficient methods of heating. Modern hydronic systems use a closed plumbing loop to ensure a steady supply of heated water to radiators, allowing the cooled liquid to return to the boiler through a different set of pipes.
A home heating radiator consists of a coiled series of pipes, usually encased in a structure designed to maximize the surface area of the device. Large, flat fins, for instance, can help heat move out of the interior pipes and into the surrounding air. This warmth not only radiates into the room, but it can also warm the air nearest the radiator, causing the air to rise. This draws in air from near the floor, creating a circulating air flow to increase the spread of warmth.