On Jul 7, 3:56*pm, 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.

??
--
DaveC

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"There's no pressure in there to "back flow" to the water main,
even if the mix valve would allow it"

That's what I thought, but I didn't mention it because I didn't know
where the inlet was - top or bottom.

In theory, a washer that fills from bottom could backflow into the
main if there were no pressure/water in the main (think: break) and
the force of gravity was enough to force the water into the fill tube,
through the cold water pipe, and out into the street.

Now, considering your answer that this tub fills from the top, I don't
see how there could be a dirty water backflow issue.

Well, maybe...depending on exactly where the inlet was, I guess a
broken water level sensor coupled with a water main break that occured
*after* the tub filled to level of the inlet could cause a small
amount of dirty water back into the main.

I *must* be missing something and eagerly await the explanation.