In article .net,
DaveC wrote:

"If the water pressure drops to zero while filling the tub, dirty water
could get drawn back into the city water main."

It still doesn't make sense. If main pressure drops to zero, the air break
has no water pressure -- it opens into the tub. There's no pressure in there
to "back flow" to the water main, even if the mix valve would allow it.

These devices are often called "vacuum breakers".

If the city water supply is interrupted, and somebody downhill from
your location turns on a tap, gravity will attempt to allow water to
flow back down your pipe, into the mains, and then into their pipes
and out through their faucet. This will generate a negative pressure
(i.e. a vacuum or suction) in the pipes in your house.

In the absense of a vaccum breaker, air pressure will force dirty
water into your house pipes through any open valve to a reservoir of
dirty water.

Outdoor irrigation systems are a primary culprit... if the faucet or
solenoid was open when the power went out, there can be
soil-contaminated water in the garden/lawn pipes leading to the
sprinklers, and this water can be sucked back into the water mains if
the mains pressure goes down through zero and a vacuum develops.

The cure for this is a vacuum breaker - a valve with access to outside
air, which will open if the water pressure drops below a certain
point.

I believe that in most areas in the U.S., a vacuum breaker is required
on any water outlet which can possibly have its output opening
immersed in standing water under any conditions.


--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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