A vacuum breaker remains closed only as long as the system it is piped into has
pressure. When the system pressure falls below atmospheric pressure, it opens.
Vacuum breakers are used at the top of draindown and drainback systems to allow
air to enter the loop to facilitate rapid drainage. They are sometimes installed above
the cold water inlet of storage tanks and water heaters to eliminate vacuum
conditions that could collapse the tanks. (Figure 2-43)
A Vacuum Breaker
As is the case with automatic air vents, no vacuum breaker manufacturer endorses
the use of their product outside.
Three-Way Diverting Valves
Some space heating applications, particularly those involving hydronic back-up, use
motorized three way valves for automatic control of the distribution of solar heat.
The valve must be rated for the water pressure and temperatures expected in the
system, but these are not normally a problem.
Manual three-way valves (Figure 2-44) are sometimes used to isolate or bypass the
storage or back-up heaters in DHW systems. Because slight amounts of leakage
are typical, they cannot be used as shutoff valves for service purposes. However,
as an isolation or bypass valve, they are more convenient than a pair of two-way
2.7 COMPONENT OPERATION