<P> Despite its name, a radiator does not primarily heat a room by radiation . If positioned correctly a radiator will create an air convection current in the room, which will provide the main heat transfer mechanism . It is generally agreed that for the best results a steam radiator should be no more than one to two inches from a wall . </P> <P> Single - pipe systems are limited in both their ability to deliver high volumes of steam (that is, heat) and the ability to control the flow of steam to individual radiators (because closing off the steam supply traps condensate in the radiators). Because of these limitations, single - pipe systems are no longer preferred . </P> <P> These systems depend on the proper operation of thermostatic air - venting valves located on radiators throughout the heated area . When the system is not in use, these valves are open to the atmosphere, and radiators and pipes contain air . When a heating cycle begins, the boiler produces steam, which expands and displaces the air in the system . The air exits the system through the air - venting valves on the radiators and on the steam pipes themselves . The thermostatic valves close when they become hot; in the most common kind, the vapor pressure of a small amount of alcohol in the valve exerts the force to actuate the valve and prevent steam from leaving the radiator . When the valve cools, air enters the system to replace the condensing steam . </P> <P> Some more modern valves can be adjusted to allow for more rapid or slower venting . In general, valves nearest to the boiler should vent the slowest, and valves furthest from the boiler should vent the fastest . Ideally, steam should reach each valve and close each and every valve at the same time, so that the system can work at maximal efficiency; this condition is known as a "balanced" system . </P>

Where is air vented from a hydronic heating system