Hydronics - Single-pipe Steam

Single-pipe Steam

In the oldest modern hydronic heating technology, a single-pipe steam system delivers steam to the radiators where the steam gives up its heat and is condensed back to water. The radiators and steam supply pipes are pitched so that gravity eventually takes this condensate back down through the steam supply piping to the boiler where it can once again be turned into steam and returned to the radiators.

Despite its name, a steam 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.

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 installed.

In order to work correctly, these systems depend on the proper operation of thermally closed air-venting valves located on radiators throughout the heated area. When not in use, these valves are open to the atmosphere, and radiators and pipes contain regular air. When a heating cycle begins, the boiler produces steam, which expands, rises, and displaces the regular air in the system. The regular air exits the system via the air-venting valves on the radiators, as well as air-venting valves placed on the steam pipes themselves. The valves close when steam reaches them, due to a small amount of alcohol in them turning into vapor and exerting mechanical force to close the valve. When the heating cycle ends, the steam in the radiators cools, the air-venting valve reopens, and regular air again enters the system.

To increase heat delivered to an area served by a radiator, a larger air-venting valve can be installed. Some more modern valves can also be adjusted so as 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.

The most common problems with air-venting valves occur when they are painted over, crushed, or clogged with rust, often leading to homeowner frustration. Improperly trained service technicians often respond to complaints by increasing boiler steam pressure rather than replacing air-venting valves. This actually makes matters worse, by causing high pressure steam to leak or otherwise, as well as wasting heating oil and energy. Investing in new air-venting valves for an old or troublesome single-pipe steam system, as well as taking the time to correctly size and adjust them, will reduce or eliminate many headaches once completed and lower heating fuel use and bills.