In the many instances where taps are scattered about a building, and not all placed near the cylinder as in Fig. 42, the single-tube expansion pipe has the objection that at many points it entails a considerable quantity of cold or cool water having to be drawn from the taps before the hot issues. This is a fruitful source of complaint, whether in the kitchen-offices, or upstairs, and it has to be obviated in all good jobs. The customary method of doing this is to provide what is termed a "secondary circulation," this name being used to distinguish it from the circulation between the boiler and cylinder. It is a desirable thing, even if the taps are all in a line (but extending some distance away), and when the taps are scattered it serves a particularly good purpose.

The right-hand drawing on Plate V. illustrates an apparatus with secondary circulation, and the important points concerning it are as follow:-The secondary flow-pipe starts out from the top of the cylinder, and takes a course (with the necessary rise, as previously described) such as will bring it past one set or line of taps, and on reaching its highest point an expansion pipe extends up to the customary distance above the house cistern level. It might be said that the whole is an ordinary expansion pipe so far, similar to that shown in Fig. 42. At or near the level of the highest branch this secondary circulation proceeds towards and past the other taps, on its way back to the cylinder ; but, from the point where the true expansion pipe extends up, the circulation ceases to rise, and is given a fall all the way down the cylinder. This will be seen in the illustration. The secondary flow therefore proceeds from the top of the cylinder up to the expansion pipe, while the secondary return extends from there back to the cylinder. As this circulation may have tap branches on it at any point it is important that both its ends should terminate where the store of hottest water is - which is at the top of the cylinder. It has been the custom to take the secondary return into a low part of the cylinder, or into the primary return, but there is only one correct place for it, and this is the upper part of the cylinder, about 4 to 6 inches from the top, according to the size of the apparatus. With the pipes ending thus all taps will be served with the hottest water, and whether there be little or much it can be drawn without having cold water mixed with it.

A reliable table of the proportions of parts in the cylinder system is given on opposite page.

Draw-off branches can be: lavatory basins 1/2 inch, sinks 3/4 inch, baths 1 inch.

In the previous pages it has been shown that, while the ordinary Tank System of apparatus is considered to be faulty, in the fact that the tank and pipes might be emptied should the cold water supply fail, the system is at an advantage in affording a good outflow of water at the taps, due to the tank being above the branches. The Cylinder System, on the other hand, quite overcomes the fault described above, but it introduces a new one in usually having a sluggish, sometimes very bad outflow at high taps, due to the absence of a tank above the branches. With the idea of avoiding the faults and gaining both advantages the Cylinder Tank combined system has been designed.

The left-hand drawing on Plate V. illustrates the details of this as usually erected, and it will be seen to resemble the cylinder system of apparatus with secondary circulation, except that a tank is inserted at a high point and the expansion pipe is connected to it. In an apparatus of this kind the cylinder would not be so large as that ordinarily used, as the customary proportion of parts must be maintained. If a 50-gallon cylinder would be the correct size if used alone, then with the combined system this must be divided between the two reservoirs. As a rule, it is found best to have a little more water in the cylinder than the tank, and for 50 gallons a 30-gallon cylinder and 20-gallon tank would be correct. The only purpose of the tank is to ensure a good outflow at open taps, so that its use is limited to the brief periods when taps are opened.

Expansion Pipe (or circulation above Cylinder, when used).

Shape of Boiler.

Width of Fire.

Capacity of Cylinder.

Circulating' Pipes from Boiler.

Number of Taps, including

Baths.

Inches.

Gallons.

Inch.

Inch.

Square, with

9

24

I

3/4

2

arched flue.

Flued boot .

9

30

1 1/4

1

3

Square, A.F.

10

28

1

1

3

F. boot . .

10

35

1 1/4

1

3 or 4

Square, A. F.

12

32

1 1/4

1

3 or 4

F. boot . .

12

40

1 1/4

1

4

Square, A. F.

14

42

1 1/4

1

4

F. boot . .

14

52

1 1/2

1 1/4

5 or 6

It is sometimes thought that the taps on the flow side of the tank must be served more quickly and always with hotter water than those on the return, but this is not so in practice. The hottest water is always gathering up in this tank, and what is not there must be in the upper part of the cylinder, therefore both secondary flow and return have about the same communication with the hot water that is available.

It is very desirable that the upper tank be well covered to prevent loss of heat. This will, as a rule, ensure a supply of warm water being available at early morning hours, as the hot water will all gather up at this high point at night.