This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol3", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
In affording warmth to buildings by means of hot water all the methods and systems adopted rely, for efficacy, on the action of Convection, or "obtaining a circulation," as it is termed. Without devoting some study, or at least consideration, to this detail of the subject the engineer must often fail for want of knowing the principle that his work is based on. A full explanation of the action of Convection has already been given, and it is recommended that the student master this information ; for although the explanation is associated with hot-water works for domestic supply, the principle involved is just the same, while the suggested models and the experiments that may be conducted are equally useful and instructive. Taking, therefore, this information as an introduction to the subject now being dealt with, the erection of practical works may be described.
There are three distinct systems of low-pressure hot-water heating works, and two of high-pressure. Low Pressure is a term applied to an apparatus which has an open expansion pipe - a free blow-off as the London Building Act describes it; while the High-Pressure apparatus consists of one or more circuits of pipe, which are sealed and have no open outlet whatever. The three low-pressure systems are the Two-pipe, One-pipe, and the Overhead. All may have radiators as the heat distributing surface (in good interiors), or large cast pipes may be used (in warehouses, etc.). Many works are erected which combine two or all three systems, more or less. It will be seen directly that one or the other may be best to adopt according to conditions. The two-pipe system is the oldest, but there are numbers of two-pipe jobs being done at this moment by quite up-to-date engineers. The one-pipe system is favoured most, perhaps, but this is largely due to its meeting general requirements more often than the others. In the writer's own house the apparatus is partly one-pipe and partly overhead work, and the arrangement was not a matter of choice but of absolute necessity. A two-pipe installation was practically impossible.