On Tue, 7 Jul 2009 12:56:13 -0700, 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.

It's a siphon break.

Remember the demo where you can get water to "flow uphill" inside a
plastic tube as long as the source end of the tube stays submerged and
the discharge end is below the level of the source end?

If the municipal supply loses pressure, there are likely to be several
"discharge ends" (other households) that are open and lower than the
wash tub. If the fill line was just a hose that terminated below the
water surface then it would suck the laundry water back into the supply.
The siphon break prevents that. You'll find similar gizmos on fill lines
for, e.g., swimming pools, hot tubs, etc.

--
Rich Webb Norfolk, VA