While the principle of the double boiler is simple and its connections straightforward, there are comparatively few who understand the manner in which it should be installed. The double boiler is used in city buildings of such height that the water under city pressure will not at all times reach the upper floors.

It consists of two boilers, one inside the other, the outer boiler being connected in the usual manner with the heater, and the inner boiler receiving its heat from the hot water in the boiler which surrounds it.

Plate LIII. Double Boilers

Double Plate 53.

Boilers

Double Boilers 128

This form of boiler is much used in large residences, and often in apartment buildings.

In most of the largest buildings, however, where very large amounts of hot water are required, the water is pumped into the house tank, and the entire hot-water supply for the building delivered under tank pressure.

The outer boiler is supplied by city pressure, while the inner boiler is under tank pressure. The lower floors, which can be reached by city pressure, are supplied from the outer boiler, and the upper floors, which cannot be reached by city pressure, are supplied from the inner boiler. The connections for the double boiler are to be seen in Fig. A, Plate 53.

The hot-water supply line from each boiler should be provided with an expansion pipe taken from the high point on the line and emptying over the house tank.

The supply to the latter is delivered by a pump or water lift. From the tank an overflow should be carried, generally into some open fixture which has a sufficiently large waste to insure the passage of all overflow water that may enter it. A tell-tale pipe should also be run from the tank to a fixture conveniently located, so that the pump operator may be warned when the tank has been sufficiently filled. Beneath the house tank a drip pan should be provided to collect any leakage that may come from the tank, and from this pan a drip pipe delivers such leakage into some open fixture.

In the event of a breakdown of the pump, or from other cause, there is always danger that the house tank may lose its supply. If this condition should continue for some time, it might result in danger to the inner boiler, to guard against which a connection is made from the pressure supply to the outer boiler, into the tank supply to the inner boiler, a check valve, C, being used on this connection. When the system is working normally the check valve remains closed, owing to the pressure of the tank supply, but when this is withdrawn, as would happen after a time if the pump were not in operation, the street pressure will open the check valve, and thus keep the inner boiler supplied with water. A check valve, B, prevents the siphon-age of the contents of the outer boiler in the case of a break in the service pipe. It is the use of this check valve that necessitates the use of an expansion pipe on the hot-water supply from the outer boiler, the check valve cutting off the natural means of expansion.

The valves A and D control the use of these two lines. If circulating pipes are used, as they should be on such work as this, the tank circulating pipe should connect into the return of the inner boiler, and the pressure circulating pipe should connect into the return to the heater.

Special attention should be given to properly draining the double boiler. If the inner boiler is drawn off first, there may be danger of collapsing it, due to the creation of a partial vacuum inside it and street pressure outside of it. This danger is eliminated by arranging the draw-off in such a way that the outer boiler must be drawn off first or both boilers drained at the same time. This is accomplished by the proper placing of valves, as shown in Plate 53, Fig. A.