Steam is an effective heating medium. It is adaptable to almost any type of building. One of the simplest steam-heating systems is the one-pipe gravity type. Installations of this type are usually limited to moderatesize buildings where the radiators can be positioned at least 24 inches above the water level in the boiler. This type of installation is simple to operate and the initial cost is low.
Some inherent disadvantages of the one-pipe gravity steam-heating systems are as follows:
1. Large piping and radiator valves are required to allow the condensate to return against the resistance offered by the steam flow.
2. Steam and condensate flow in opposite directions with a possibility of water hammer.
3. Air valves are required. Failure of these valves to always open to allow the escape of air may result in slow heat buildup and excessive fuel consumption.
4. Comfortable room temperatures are difficult to maintain unless the radiator valves are regulated by opening and closing. Automatic control of steam from the boiler may result in fluctuating room temperatures.
The two-pipe gravity steam-heating system was developed to overcome the difficulty encountered when steam and condensate flow against one another in the same pipe. This system has all the disadvantages (to some degree) listed for the one-pipe system.
Additional disadvantages are as follows:
1. The midsections of the radiators may become air-bound if they are not water-sealed. This happens during the warm-up period when steam fills the radiator nearest the boiler and flows through and enters the return piping.
2. Installation of a valve at each end of all radiators is required so that the steam may be shut off. If this is not done, steam may be present in both the return and supply lines.
3. The returns from each radiator must be separately connected into a wet return header or must be water-sealed in some manner.
