In the cylinder system the principal difference from the tank system lies in the fact that the cylinder or reservoir of hot water lies beneath the draw-off pipes and not above them, as with the tank system. This being the case it is impossible to empty the reservoir unknowingly or accidentally, should the cold water supply be shut off.
Referring to Fig. 24, the flow-pipe proceeds from the extreme top of the waterback, and does not project through inside the waterback in the least degree. If it cannot be taken from the top, it must be connected to the side or back of the waterback as close to the top as it can be got, but the top connection should always be used if in any way possible. From the waterback the flow-pipe proceeds to the boiler and terminates five-eighths of the way up from the bottom. The pipe can enter the side of the boiler at the correct point, or it can come through lower down and be extended up inside with a bend and short piece of pipe together without making two holes.
The return pipe leaves the side of the boiler as close to the bottom as possible, or it can come from the bottom if desired. It then proceeds to the waterback and enters either through the top or the side, terminating half-way down with a saddle boiler. Both of these pipes, the flow and the return, must have a rise from the waterback to the boiler of not less than 1 inch in 10 feet.
From the top of the boiler is carried the expansion pipe. This also should rise 1 inch in 10 feet from the boiler to its highest point. The highest point can be above the cold water cistern or through the roof.
The cold water supply to the system is a pipe direct from a cistern, as shown. This pipe must not be branched for any other purpose.
It is of the highest importance that the cold water supply pipe should be of full size, and not choked or reduced in bore anywhere. The outflow at the hot water faucet is exactly in ratio with the down-flow of water through this pipe, less friction, therefore everything possible must be done to give the water full and free passage and lessen the friction. This is done by having the pipe of good size, using bends and not elbows, or lead pipe, and seeing that the stop-cock, if there be one, has a straight full way through it. The stop-cock should be put near the boiler, so that the man who cleans the waterback, or effects repairs, does not have to traverse the house to shut the water off and afterwards to turn it on. A tee should be put on the cold water supply connection, inside the boiler to spread the inflowing cold water over the bottom of the boiler. If this is not done the inflowing cold water will bore its way up through the hot water above, unless the pressure be quite low.
An emptying cock should be put somewhere beneath the boiler, but this cock must be provided with a loose key, so that only an authorised person can withdraw the water from the boiler.
The draw-off pipes are all taken from the expansion pipe as shown. This pipe should therefore be carried up by the best route to touch at the points where the faucets are, otherwise long single branches must be run. The expansion pipe, being a single tube, has no active or useful circulation in it.
It must never be forgotten that, on opening a faucet, on a secondary circulation, water will proceed from both directions to reach that faucet. The circulatory movements all cease, and quite a new action takes place. Water will come up from the top of the boiler and this will be hot. There will also be water coming up the secondary return, and the temperature of this will depend on whence it comes. If connected as shown in Fig. 25 then whatever water comes to the faucets will be hot, all there is of it, and when the temperature of the issuing water falls it may be known that the hottest has all been withdrawn. There have been several points at which the secondary return has been connected with bad results, notably at the bottom of the boiler, into the primary return (between the boiler and waterback), into the boiler, and even into the cold supply pipe just beneath the boiler. These are wrong, and only one position is correct, as shown in Fig. 25. The point is from 3 inches to 6 inches from the top of the boiler according to its size. The latter would be for a 100-gallon boiler. A 50-gallon size would have the connection 4 inches from the top.