Water is drawn off from the flow pipe. This pipe projects some distance
through the bottom of A, so that the hottest portion of the contents may
be drawn off first. A tank situated in the roof, and fed from the main
by a ball-cock valve, communicates with A through the siphon pipe S. The
bend in this pipe prevents the ascent of hot water, which cannot sink
through water colder than itself. From the top of A an _expansion pipe_
is led up and turned over the cold-water tank to discharge any steam
which may be generated in the boiler.

A hot-water radiator for warming the house may be connected to the flow
and return pipes as shown. Since it opens a "short circuit" for the
circulation, the water in the tank above will not be so well heated
while it is in action. If cocks are fitted to the radiator pipes, the
amount of heat thus deflected can be governed.

[Illustration: FIG. 192.--The "tank" system of hot-water supply.]

A disadvantage of the tank system is that the tank, if placed high
enough to supply all flows, is sometimes so far from the boiler that the
water loses much of its heat in the course of circulation. Also, if for
any reason the cold water fails, tank A may be entirely emptied,
circulation cease, and the water in the boiler and pipes boil away
rapidly.

THE CYLINDER SYSTEM

(Fig. 193) is open to neither of these objections. Instead of a
rectangular tank up aloft, we now have a large copper cylinder situated
in the kitchen near the range. The flow and return pipes are continuous,
and the cold supply enters the bottom of the cylinder through a pipe
with a siphon bend in it. As before, water is drawn off from the flow
pipe, and a radiator may be put in the circuit. Since there is no
draw-off point below the top of the cylinder, even if the cold supply
fails the cylinder will remain full, and the failure will be discovered
long before there is any danger of the water in it boiling away.

[Illustration: FIG. 193.--The "cylinder" system of hot-water supply.]

Boiler explosions are due to obstructions in the pipes. If the
expansion pipe and the cold-water supply pipe freeze, there is danger of
a slight accumulation of steam; and if one of the circulation pipes is
also blocked, steam must generate until "something has to go,"[38] which
is naturally the boiler. Assuming that the pipes are quite full to the
points of obstruction, the fracture would result from the expansion of
the water. Steam cannot generat