The most commonly used backflow preventer is the
vacuum breaker shown in Figure 6–7 on the next
page. It is a combination check valve and air relief
valve in one integral unit. If the pressure in the
water supply lines drops below a predetermined
level, the check valve will close, shutting off the
water supply. With the check valve in the closed
position, the air relief part of the unit is open,
allowing air to enter the system. Thus, the vacuum
created by the negative pressure in the supply lines
is broken and water is prevented from backflowing
into the potable water supply. This type of valve
should be installed between the last control valve of
the supply system and the fixture being served. For
example, the vacuum breaker should be installed
just before the intake to a fertilizer injector.