The steam boiler is an old convection heating technology dating back over 200 years and is commonly found in older homes. Because steam boilers operate at a higher temperature than hot water boilers they are inherently less efficient than their hot water cousins. They are also a lot more finicky to operate. A steam boiler is similar to a hot water boiler in that, like a hot water boiler, the steam boiler also relies upon a heat radiating convection device in each room. However, a steam boiler will usually use a large cast iron radiator instead of a steel or baseboard convection device.
Steam boilers operate like a tea kettle. As water heats to a boil it builds pressure as it reaches the state of steam. Unlike hot water boilers which rely upon a pump to circulate the hot water, steam boilers use this pressure to move the steam through the piping system.
The piping system of a steam boiler is usually a one-pipe or two-pipe system. One-pipe (or single-pipe) systems were historically most common due to simplicity and economy in construction. A single-pipe system uses the same piping to supply steam to a radiator and to collect return water as it condensates from a steam state after going through the radiator.
However, the one-pipe system has its limitations. You cannot control or close off steam to an individual radiator since that would trap condensate in the radiator, and the one-pipe system has limited ability to provide higher volumes of steam (and therefore heat). As a result, even though the one-pipe system was inexpensive it is no longer commonly used.
Air Valves / Vents
One-pipe steam systems are vented (the vents or valves are found on the radiators and steam supply piping) and must push air ahead of the steam and out through the radiator vents during a heating cycle. This “breathing” provides the characteristic pipe banging and hissing sounds of a one-pipe steam boiler system.
The radiator air valves for single-pipe systems come in six different sizes to allow different amounts of air to escape from the radiator at different rates, necessary for the system to be balanced (see steam radiator air vents for more information). Balancing the system involves adjusting the rate of steam flow provided to the radiators both with the supply valve and venting air valve so that an individual radiator produces heat appropriate for the room it is heating. Larger vents are generally used at the end of long pipe runs (mains) and in colder rooms. Smaller air vents are used nearer to the steam boiler and in rooms which have a thermostat.
A two-pipe system, more commonly found on hot water boilers and newer steam boiler systems have a separate supply piping line to the radiators and a separate return piping line back from the radiators to the boiler. Vents are only installed on the piping to and from the steam boiler, not the radiators. Steam rises from the supply side of the radiator (radiator steam supply valve is on this side) and pushes air and condensate out through the return or condensate drain side of the radiator.
The steam boiler has basic controls to protect the safety and correct operation of the system including the following items:
- Pressure Gauge and Regulator: Monitors pressure and shuts off the fuel supply to the boiler if allowable boiler pressure is exceeded.
- Temperature and Pressure Relief Valve: Releases steam if the pressure gauge/regulator fails and pressure builds dangerously high.
- Water Level Sight Glass: Glass tube that shows water level in the boiler.
- Low Water Cutoff: Shuts off the system if boiler water level drops below required levels.