This provision consists in running a pipe from the highest point in the hot-water piping up and over the tank. Through this pipe any expansion of the hot water may vent itself into the tank. If several separate lines are fed from the same tank, a separate expansion pipe should be provided for each line. In some cases, a lead safe is placed under the attic tank, and a drip carried down, usually to the cellar or basement, to carry off any leakage from the tank. In general, however, this is not considered necessary.

Fig. 255.   Tank System of Supply for Residence

Fig. 255. - Tank System of Supply for Residence.

In Fig. 255 is shown a tank system of supply for a residence, as ordinarily put in, and in Fig. 256 a pressure system of supply for a two-flat house.

In the pressure system, separate lines of supply are carried up to each kitchen boiler, with branches to the kitchen fixtures, and also separate lines to each bath room, each line being provided with its own stop and wastecock.

In shutting off a supply system and allowing the water standing in the pipes to drain out through the waste of the stop- and wastecock, it will be found that by simply opening the cock, the desired result cannot be obtained. It will be necessary to open a faucet at the highest fixture in order to drain out the whole line. If a faucet on a lower floor is opened, the pipe will simply drain out up to that level. In other words, in order to allow the water to flow out of a pipe, it is necessary to have atmospheric pressure act upon the top of the column of water to be drained out. In order to completely drain the piping, it is often necessary to open faucets at certain of the fixtures. For instance, in Fig. 240, to drain the water in the pipes running to the kitchen fixtures, it will be necessary to open a faucet on these lines.

In the case of the pressure system, it is necessary to provide against the siphonage of the contents of the range boiler.

Fig. 256.   Pressure System for Two Flat House.

Fig. 256. - Pressure System for Two-Flat House.

The boiler tube, shown by dotted lines in Fig. 256, dips down into the boiler to a point near the bottom, the supply to the boiler being much longer than the tube, and terminating at a point much lower than the level of the boiler. Now the siphon has a short arm and a long arm, the latter ending at a point much below the short arm. Therefore it will be seen that the boiler tube and boiler supply in connection with the house main and service pipe reaching to the street main forms a perfect siphon apparatus, and a break in the house main or in the street main might allow the water in the pipe leading to the boiler to waste back and siphon out the entire contents of the boiler. If a break should occur in the night, for instance, this result might easily occur. The siphonage of the contents of the boiler will leave a more or less perfect vacuum in the boiler, which might result in its collapse, this being especially true of certain makes of copper boilers.

Fig. 257.   Use of the Vacuum Valve.

Fig. 257. - Use of the Vacuum Valve.

In the case of the boiler supplied from the attic tank, this danger is entirely lacking, owing to the location of the source of supply above the boiler rather than below it. There are two methods of preventing the siphonage of range boilers. It may be done by drilling a hole in the boiler tube at the top of the boiler. This, however, is open to the objection that rust and sediment will in time fill the hole and render it useless for the purpose for which it is designed. .

A better and surer method is shown in Fig. 257, a vacuum valve being used for the purpose of preventing siphonage. This valve should be placed on the supply to the boiler at the point where it turns from the vertical line to the boiler. It is necessary that this valve should be located at the highest point of the siphon, just as it is necessary to vent the trap from its crown, for the reason that in order to prevent the operation of the siphon, air must be admitted at its crown. The operation of the vacuum valve is the following: In the event of a break on the supply pipe, the contents of the boiler start to siphon out, owing to the creation at the top of the boiler of a partial vacuum. The instant a vacuum is created on the inner side of the vacuum valve, atmospheric pressure opens it, and air enters, thus effectually breaking the siphonic action. At other times, the pressure of the water against the valve keeps it closed.

Fig. 258.   Wrong Use of Check Valve to Prevent Siphonage.

Fig. 258. - Wrong Use of Check Valve to Prevent Siphonage.

The vacuum valve should be set to point downward, as shown, in order that any passage of water out of it may fall upon the floor rather than upon the walls or ceiling. It is a good plan to have the valve, if possible, located over the sink, so that any leakage from it may enter the sink instead of falling upon the floor. The only disadvantage in the use of the vacuum valve is that if it has not operated for a considerable length of time it may stick and refuse to work

A dangerous method of preventing siphonage by the use of a check valve on the boiler supply is shown in Fig. 258. Ignorant workmen sometimes place it on this line, with the idea that as soon as the water in the boiler attempts to flow in the opposite direction to its natural flow, the check valve will close and prevent siphonage. It will certainly perform this duty, but in its use it acts in another way, which is very unsafe. Under normal conditions, it closes the supply pipe against expansion, and as the expansion of the boiler is naturally through the supply pipe, the use of the check valve at any point upon it prevents the natural expansion of the water as it heats. This means that the boiler is in danger of bursting.