which the gas-heater connection is made is not the first fitting
and placed as close to the outlet as possible, the water will not
circulate freely into the boiler. This connection according to the
drawing should be studied and memorized.

[Illustration: FIG. 72.--Instantaneous gas heater. Showing
circulation heater or booster.]

INSTANTANEOUS GAS-HEATER CONNECTIONS.--An instantaneous gas heater
is placed in the basement. The copper coil in it is connected at
the bottom with the cold-water supply and the top outlet of the
coil is connected with the hot-water system of piping. There is no
need of a storage tank with this heater. When a faucet is opened in
any part of the hot-water piping system, the water passing through
the water valve at the heater causes the gas valve to open so that
the whole set of burners in the heater is supplied with gas, and
the burners are lighted from a pilot light. When the faucet is
closed, the gas supply is shut off and the burners are put out. The
pilot is lighted all the time. Space will not permit going over
these connections in detail. It is a large field and requires
considerable thought.

SAFETY AND CHECK VALVES.--When a meter is used on a water system,
the water company demands that a check valve be placed on the
hot-water system to prevent the hot water from being forced back
into the meter in case the pressure got strong enough in the
boiler. If a check valve is used for this purpose, or for any other
purpose, a safety valve must be placed on the boiler piping system
to relieve any excessive pressure that may be caused by having the
check valve in use. There is today, with meters of modern type, no
reason to use a check valve or a safety valve. If an excessive
pressure is obtained in the boiler, it is relieved in the water
main.

When water is heated, it expands. If the heat becomes more intense
and steam is formed, the expansion is much greater, and some means
must be provided to allow for it. This expansion can be allowed to
relieve itself in the water main as explained above. When a check
valve is placed on the piping, this means of escape is shut off and
a safety valve must be employed. Without these reliefs, the
pressure would be so great that an explosion would result. When
steel pipe and steel boilers are used for storage tanks and
connections, the pipe and tank will shortly start to rust and parts
of the piping are stopped up with rust scales. The water also
becomes