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.
Fig. 51 illustrates the Overhead System, with details of its special features, as will be explained. In theory, and to a great extent in practice, it is considered correct, to favour the circulation, to let the contents of the flow-pipe be as hot as possible, while the return pipe only should carry cooled water ; and the flow should have the least volume of water in it. Fortunately, the circulation never requires to be favoured to this extent, but the fact remains that the idea is a correct one, and on this account the overhead system is thought to have a more perfect action than any other. It probably does, but this is largely due to there being so much vertical pipe, as a rule ; and very little experience shows that, with vertical work, the circulation frequently wants severely checking to favour any horizontal section of the apparatus there may be. Correct as the principle of the overhead system is, it may be said to be unnecessarily so.
It will be seen that the flow main proceeds by the nearest possible route to the highest point, and carries no radiator branches on it. As it exists solely to serve the returns and their radiators, it should be well covered to prevent loss of heat. It carries the hottest water, and can lose heat the fastest. At its highest point the flow is branched in one or more directions, and from here all pipes are carried as returns. The returns can be branched (like flows) as they descend, but, as with other systems, they should all join the boiler separately if possible. The expansion pipe can be at the head of the flow-pipe, as shown, or may be at any other high point, provided a radiator receives the air at the head of the flow-pipe. This has been already fully explained.
Attention is called to the return circuit on the left side of the illustration. It will be seen that it ends running at (or it may be just beneath) the floor level that the boiler stands upon. It is an advantage claimed for this system that no boiler pit is needed ; that the boiler may be on the same level as some of the rooms to be heated. In riverside premises this is a good feature, as basements or boiler pits are commonly impossible. In city business premises, too, the basements are often made valuable use of as suites of offices, or stock-rooms, level with the boiler-room, and require to be heated. This is easy of attainment, as the circulation is such that the fact of the returns dipping, say, two feet, or even more, below the boiler has no prejudicial effect on general efficacy.
The radiator branches can be connected in any of the customary methods, but where the returns are long straight runs the branches should be run so as to "swing" a little to the push and pull of the main return as it expands and contracts. If the branch is a straight horizontal pipe, then the radiator should not be nearer than about 4 feet to the main ; when nearer, some method of cranking the branch must be adopted to give to the strain. On the right of the illustration one radiator is shown with top and bottom connections at one end. If appearance will admit, this is a favourite connection to single vertical mains, as it is the best, theoretically, and no air cock is needed to the radiator. What air may collect in the radiator escapes through the upper branch.
If the installation consisted of one circuit - one flow and one return - the pipe would have to be of one size throughout. It would, in fact, be an apparatus on the one-pipe system, but with vertical mains instead of horizontal. When the returns are branched or divided, as shown, then the pipes may be graduated in size, as described with the two-pipe system. The flow-pipe would be of the full size needed to carry the whole of the work, then each return would be of a size to suit the radiation upon it.
A useful purpose to which the overhead system can be put is in heating suites of rooms on one floor. These may be offices or residential flats, and as the floor beneath is probably occupied by someone else and not available for the boiler, the whole must be confined between one floor and ceiling level.
Fig. 52 illustrates how this is done, but it will be quite recognised that there has to be a ceiling pipe and a floor pipe, necessitating two casings (or being left visible). The ceiling pipe, too, cannot be kept close up, as it must be below the water-line in the supply cistern. This can only be overcome by either finding a higher place for the cistern, or by arranging with the tenants of the floor above that the cistern be in one of their rooms, in a kitchen apartment or in a cupboard.