Expansion tanks used for potable water are similar to a diaphragm tank, but a
plastic liner on the liquid side keeps the water from contacting the steel wall of the
Again, an expansion tank is needed only if the cold water supply line to the storage
tank includes a backflow preventer or check valve. In this case, the expansion tank
acts to absorb the expansion of the storage water as it is heated. Without some
expansion capacity, the water pressure will slowly build up until the temperature and
pressure relief valve on the storage tank discharges.
Pressure Reducing Valves
Glycol-filled loops must never be equipped with automatic water make-up. If glycol
leaks out and is replaced by water, the system will not be protected against freezing
Oil-filled loops must never have water introduced to them.
Draindown systems do not normally require a pressure reducing valve, and it is
difficult to design an effective automatic water make-up system for drainback
systems. Therefore, automatic water make-up is rarely used on collector loops.
In some areas, code officials may require an approved backflow preventer in the
cold water supply to the storage tank. A check valve is not an appropriate
The discharge port of a backflow preventer must be piped downward to avoid
damaging electrical and other components, and to prevent injury to anyone in the
area during blowoff.
Whenever a backflow preventer (or a check valve, despite its unacceptability) is
installed, an expansion tank must be installed between the tank and the backflow
preventer. If the expansion of the storage water is not accepted somewhere, the
temperature and pressure relief valve on the tank will discharge heated water on an
almost daily basis. (Figure 2-36)
Some provision for draining the liquids from all the system loops should be made.
In water loops, a simple boiler drain at all low points is used. The storage tanks,
particularly those in DHW systems, must be drainable for service and maintenance.
2.7 COMPONENT OPERATION