In a vapor system, thermostatic traps are used at each radiator and at the ends of the steam mains. The radiator inlet valves used in this system are of the graduated or orifice type. The steam pressure necessary is very low, often less than one pound. This type of installation can be used in buildings where 24 inches or more can be provided between the boiler water level and the end of the return line.
Advantages of a vapor steam-heating system include:
1. Air cannot enter the system as it is closed. Thus, a moderate vacuum is created by the condensing steam, producing steam at lower temperatures.
2. An even, quiet circulation of steam is provided with no air-binding or noisy water hammer.
3. Room temperatures can be closely and automatically regulated by thermostatic controls.
4. Radiator air valves are not required.
Disadvantages of a vapor steam-heating system include:
1. Comparatively large pipe sizes are necessary.
2. Only low steam pressure is possible.
3. The condensate must return to the boiler by gravity.
The condensate must return to the boiler by gravity in this type of heating system and may back up in the vertical return pipe when there is excess steam pressure in the boiler. As a result, an air eliminator must be installed well above the level of the water in the boiler, yet low enough for it to close before the return water is of sufficiently high level to enter the return main. Close control of the boiler pressure is required in this type of system.
