On 11/21/2015 10:55 AM, Doug Miller wrote:
Micky wrote in news:n3605btdvs9avi246kpt1j8vvfrfptkeas@
4ax.com:

How did we get along without these tanks for so many 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.

You can find lots of videos "exploding water heaters". There's a lot
of pent-up pressure in those systems and the heater is often the weakest
link.

Regardless, you don't want to subject the various appliances in your home
(water closets, dishwasher, washing machine, dryer, etc.) to elevated
pressures -- possibly ABOVE their design limits!

Prior to installing the PRV, showers here were delightfully "strong";
as if the water was scraping the dirt from your skin.

OTOH, we had the "fill hose" fail on one WC late one night, "spontaneously".
Similarly, the BRAND NEW (braided steel) supply hose to the washing machine
failed and the resulting water jet cut a hole in the drywall in a matter
of seconds!

Note that as municipalities grow, they RARELY go back and resize the
water mains to accommodate the new "loads". Instead, they just up
the pressure to force more water through the existing pipes. While
the systems *may* have been delivering water at "normal" pressures
originally, chances are these pressures have increased dramatically
over the years.

It's a cheap test: buy a pressure gauge that screws onto a garden
faucet (hose bibb) -- hard to create a pressure-tight seal
on an indoor faucet!

Or, corner the local water department guy when/if you see him in your
neighborhood and ask to borrow theirs (for 30 seconds). Ditto for
any plumber.