4 Domestic Side (Tank) Piping
Basic Domestic Piping
Figure 4-2 shows typical domestic water piping for a tank.
The function of the components shown are as follows:
a. Shut-off valves (recommended) - Used to isolate the
tank for servicing.
b. Backflow Preventer (required by some codes) -
Used to prevent water from backing out of the tank in
the event that inlet water pressure drops.
c. Expansion Tank (required when a backflow
preventer is used) - This expansion tank absorbs the
increased volume caused by heating water.
Use an expansion tank designed for use on domestic
water systems. Refer to the expansion tank
manufacturer's literature for the proper size expansion
tank to use.
If an expansion tank is used, do not put any valves
between the expansion tank and tank inlet.
d. Unions (optional) - Used to disconnect the tank in the
unlikely event that this is necessary.
e. Drain (required) - Used to drain the tank for inspection
Multiple Tank Domestic Water Piping
The two pipe reverse return piping uses more pipe than
the two pipe direct return piping, but the flow is more
balanced and even in the two pipe reverse return piping
layout (see FIG. 3-4).
Each tank must have its own T&P valve. It is recommended
that each tank be equipped with its own isolation valves,
unions, and drains so that one tank may be removed from
the system. If local codes require a backflow preventer,
check with the appropriate authority to find out whether
one backflow preventer may be used for tanks or each tank
must be equipped with its own backflow preventer. If each
tank must have its own backflow preventer, each tank must
also have its own expansion tank. If a common backflow
preventer is permitted, an expansion tank must be sized to
accommodate the expansion volume of all tanks.
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Domestic Water Piping for Distant Fixtures
In some cases the furthest fixture may be quite distant
from the tank. Such an installation would result in an
unacceptable delay before hot water reaches these distant
fixtures. Even if all the fixtures are relatively close to the
tank, the building owner may want hot water at all fixtures
as soon as they are opened.
A solution to this problem is that a pipe runs from the
furthest fixture on each branch back to the return of the
tank (reference FIG. 4-2). A small DHW recirculation pump
is mounted in this line and is wired so as to run
continuously. A check valve in this line permits flow
towards the tank inlet only.
When no fixtures are drawing water, the DHW recirculation
pump moves hot water from the tank to the end of the
branch just below the last fixture, then back to the inlet of
the tank via the return pipe. When a fixture is opened, hot
water is already out in the branch very close to the fixture
and hot water appears at it almost immediately. The check
valve prevents cold water in the tank's inlet pipe from
passing around the tank and heading directly to the fixture.
Because hot water is always circulating in the hot water
branch the entire branch should be insulated to prevent
excessive heat loss.