One-Pipe, Reverse-Flow System
The one-pipe, reverse-flow system is the simplest and cheapest of this type to install. This system is easily distinguished by the absence of any wet or dry returns to the boiler. Supply mains from the boiler are inclined upward and connect with the room heat- emitting units, there being no other piping. This system is called reverse flow because the condensate flows back through the mains in a reverse direction or opposite to that of the steam flow.
The operation of a typical one-pipe, reverse-flow system is shown in Figure 8-3. Steam from the boiler flows into the main or mains (inclined upward) through the risers to the heat-emitting units at the bottom. The steam pushes the air out of the mains, risers, and heat-emitting units and escapes through air valves placed at the other end of the heat-emitting units, as shown. The condensate forming in the units flows back to the boiler through the same piping, but in the opposite or reverse direction to the steam flow.
For satisfactory operation, every precaution should be used to install the correct pipe sizes, especially for the mains. If the piping is too small, it will be necessary to carry excess pressure in the boiler to ensure proper operation. Without the excess pressure, operation of the remote heat-emitting units will be unsatisfactory. Satisfactory operation of these remote units can be achieved without excess boiler pressure if the pipes used in the system are the correct size.
The smaller the main, the greater the speed of the steam flow and the greater the resistance to the flow of condensate. It is very difficult for the condensate to flow back to the boiler against the onrushing steam in a long main that is in an almost horizontal position. The main should be inclined as much as conditions in the basement permit.
Leaky radiator valves can be a problem. When the valve does not close tightly, steam will work its way into the radiator and stop the condensate from coming out. The result is that the radiator soon fills with water, and when turned on again, there is difficulty getting the condensate out. This produces gurgling, hissing, and the more violent effect known as water hammer.