On Saturday, November 21, 2015 at 2:08:07 PM UTC-5, Micky wrote:
On Sat, 21 Nov 2015 17:55:45 -0000 (UTC), Doug Miller
wrote:

Micky wrote in news:n3605btdvs9avi246kpt1j8vvfrfptkeas@
4ax.com:

How did we get along without these tanks for so many years?

For the record, I'm not talking about the pressure valve. I can see
that that's a good thing, and the house was built with one, which
hasn't required a bit of maintenance in 36 years.

Because for many years, municipalities allowed the sort of behavior described by Don Y -- if
the pressure in the residential system rises above the supply pressure, water will be forced
out of the residence and into the supply.

This is now prohibited in many locations, and homes are required to have backflow
preventers to insure that this cannot happen.

Heated water has to expand somewhere, and if it can't expand into the municipal supply,
you'd better have an expansion tank.

But what's wrong with my water going into the city supply. It's the
same water they gave me in the first place. If it was good enough
for me, why isn't it good enough for them again? Do they think I
squeezed into the pipes and poisoned it?

And even if it does back up, is it going to back up all the way
through my basement, under the front yard to the water main? Seems
to me it will back up no more than 20 feet, and when I use the water
again, the backed up water will come back into my own house.

the reason for anti backflow prevention.....

lets say you have a garden hose filling your swimming pool. the main fails and the input pressure is now zero.

your icky pool water now siphons into the main all the way to your neighbors, they drink it and can get very ill