This section is from the book "A Treatise On Architecture And Building Construction Vol4: Plumbing And Gas-Fitting, Heating And Ventilation, Painting And Decorating, Estimating And Calculating Quantities", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
Radiator Connections. The ordinary mode of connecting a radiator to the riser in a one-pipe system is shown in Fig. 61. The pipe a serves as a spring piece to allow the riser to expand without lifting the radiator, and the drop b insures that the water shall drain away readily. If the radiator is located close to the riser r, the valve should connect to the other end of the radiator.
Returns. The downward grade given to return pipes should be uniform, as nearly as practicable. There should be no upward bends or loops, because air is likely to collect in them and impede the flow of the water.
When the returns are connected to a main which is located above the water level of the boiler, the arrangement is Called a dry return. If there is any perceptible difference in the pressures at the various radiators thus connected, the steam will flow backwards, through the return pipes towards the points of lowest pressure, and in most cases will spoil the drainage and cause water hammer.
158. When the return main is located below the water level, it is called a wet return, and the water which it contains acts as a barrier to prevent the passage of steam from one return to another. Thus the steam is compelled to pass through the system, in the direction it was intended to go, instead of making a short circuit or by-pass.
Water Level In The Returns. There is always more or less difference in the pressure of the steam in the boiler and at the end of a line where the return is connected; therefore, the water will rise in the return to a height above the water level in the boiler sufficient to balance the difference in pressure. As this difference varies in the several returns, the water is likely to stand at different heights in each. The hot water rises about 29 inches for each pound of difference in pressure. In a properly designed heating system this difference in pressure should not exceed 1 pound.