The usual arrangement of this system of water heating apparatus is illustrated in Fig. 26. The flow pipe should proceed from the extreme top or highest point of the water-back, preferably from the top plate, and not project through to the inside of the waterback in the least degree. If it is impossible to connect the flow pipe in the top plate of the waterback it should be located in the side or back, but as close to the top as possible. From the waterback the flow pipe should proceed to the tank and terminate in it about three-fourths of the way up, that is one-quarter of the height of the tank from the top. It may pass through the bottom and reach up inside as a stand pipe as shown in Fig. 26, or it may enter the side at the required height.
The return pipe should leave the bottom of the tank, being connected directly in the bottom or in the side of the tank near the bottom. It should never be more than an inch from the bottom. From the tank the return pipe should proceed directly to the waterback, and if entering the boiler through the top, should extend downwards, three-fourths the height of the waterback.
The draw-off pipes are taken from the flow pipe as shown. It therefore follows that the flow pipe should be carried in a direction which will bring it as near to all the faucets as possible. Instead of this, the most common practice appears to be to carry the circulating pipes by the most direct route from the waterback to the tank, and to consider the running of the branch pipes afterwards. There is no objection to the return pipe taking the shortest route, but the flow should be diverted to pass the work as near as possible. Failing this, there would have to be long single-pipe branches, and the fault of these is that so much cold water has to be drawn before the hot issues. This is not so much a fault at a bath, at which some cold water will probably be needed. At a lavatory basin, however, the fault is very pronounced, the faucets being small and slow-running, and at no point is the quick arrival of warm water appreciated more than at this one.
Cylinder-Tank System. This is simply a combination of the two systems previously described.
The tank system and the cylinder system both have good features which are retained in the cylinder-tank system, and also certain bad features which are eliminated in the combination system which may be here described briefly, the tank system ensures a good flow of water from the high faucets, while the cylinder system commonly has a very unsatisfactory issue of water from any faucets that are near the top of the house. On the other hand, the cylinder system is safest where the cold water supply is at all uncertain, as the cylinder - the reservoir of the apparatus - cannot be emptied. The object of the cylinder-tank system is therefore to ensure a good outflow at all taps by having a store of hot water above them, and to have a store of water which cannot be exhausted unknowingly if the cold water supply fails.
Fig. 27 illustrates this system of appartus in outline, and the parts need no general description more than that given already. As to the sizes of the tank and cylinder, the best practice for general requirements is to make them of equal capacity, and the two together should be no larger than one would be if alone. Thus, if a 50-gallon boiler would be the suitable size for a job erected on the ordinary cylinder system, then with the combined apparatus the boiler should be 25 gallons and the tank 25. In the cylinder-tank system illustrated in Fig. 27, the cold water supply is delivered into the tank directly from the cistern, while in the system shown in Fig. 28, the cold water supply is carried down to the cylinder.