A radiator is a heating surface that transfers a significant part of its heat by thermal radiation and the rest by convection.

Convection is the process by which heat is transferred when cold air flows over the warm surface of a solid material. Radiant heat is the thermal energy transferred from one object to another without those objects being in direct contact.

The heat transfer from a radiator is typically distributed equally between thermal radiation and convection, though this can vary depending on the design of the radiator. Radiators can be fitted with convection panels behind or between the thermal-radiating panels, allowing them to transfer a larger amount of heat via convection.

Thus, the total heating capacity is increased. The thermal radiation from a radiator does not immediately warm the air in a room, but is instead absorbed by the objects, walls and people in the room. As objects heat up, they heat the air around them by convection. A given radiator’s heating capacity is generally dependent on the temperature difference between the medium flowing through the radiator and the air temperature of the room.

A larger temperature difference will cause a larger heat output. A radiator’s nominal heating capacity is specified for a set of standard conditions in terms of supply, return and room temperatures. The actual heat output of a given radiator for given set of non-standard conditions can be calculated with the use of conversion tables or radiator exponents.