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SteveBell SteveBell is offline
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Default Installing new vacuum breaker on a hose bib

dpb wrote:

David Nebenzahl wrote:
On 10/3/2008 10:24 AM dpb spake thus:

David Nebenzahl wrote:

By the way, after testing it I could see why these are good
things to install: after turning on the hose bib and letting
the hose reel fill up, I closed the bib, whereupon a large gush
of water sprayed out of the anti-siphon valve. Had it not been
there, all that water would have gone back inside the house
plumbing, along with whatever crap was in the hose (or in a
pool the hose was thrown into, in some cases).

How would that be if the the bib is close?

Right; duh. I should have written that some of that water might
have gone back inside the house. (A closed hose is basically a
small reservoir of water under pressure.)

But there's no pump there to build more pressure than the supply
pressure that filled it, hence no reverse flow.

If it is left out in the sun expansion will, of course build
pressure, but that is a very small volume even if the bib is
subsequently opened and the feed pipe is already full anyway. In
very small volume any that does manage to flow inward will be
flushed immediately the hose is opened and an open valve in the house
would have to be in pretty special location to cause the water in a
stub supply line to be pulled inwards.

It's just not a common event imo...again, possible, granted but
hardly a major concern.

The code-hawks are worried about the case where the city shuts off your
water, be it because of maintenance, a broken line, or terrorist
activity. If you have your hose running in the pool when that happens,
and you're at the top of a hill (or just higher than the water main in
some cases) water could be sucked out of your pool and into the main.

You're right; it isn't a common event, but it has a non-zero

Steve Bell
New Life Home Improvement
Arlington, TX