Another efficient method of heating for country houses, is by direct steam radiation. This consists in circulating steam through radiators or coils placed directly in the rooms and halls to be heated. This may be done under high or low pressure, but the low pressure gravity return system is generally used for house heating, a pressure of from two to ten pounds being rated as "low pressure".

Fig. 58. Two-Pipe System.

Piping

Three systems of piping are used in this form of heating called the "two-pipe system", the "one-pipe relief system", and the "one-pipe circulating system". In the two-pipe system the main pipe is taken from the top of the boiler and carried by branches along the cellar ceiling, pitching gradually down until all of the "risers" have been taken off, when it drops down and returns with a slight pitch to the boiler, below the water line. (Fig. 58.) This is called a "wet" or "sealed" return. Sometimes the return pipes are run overhead and this is called a "dry" return. This dry return is more productive of noise in the pipes than the wet return and is not so often used. In either case one set of pipes is run from the main supply pipe in the cellar to the different radiators and another pipe carries the condensation from the radiators to the return pipe. This requires two sets of pipes, valves, and connections and is more expensive than the one-pipe systems which are generally used.