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PeterD PeterD is offline
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Default Identify clothes washer part?

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.

??


Yes it does, when mains pressure drops to zero two things are possible
without the air brake:

1. Mains pressure actually turns negative (very common, just takes one
opening downstream to do this...)

2. Even if pressure is truely zero, contamination will backflow into
the pipe, and at a surprising rate. Technically it is possible even
with full pressure (this is caused by boundary layer effect where the
water at the side of the pipe or conduit is not moving.) A pipe run
underwater, with a pin hole, will draw contamination into the water
supply. If that 'underwater' water source is heavily contaminated, the
amount of contamination can be significant.