Don Y wrote in :

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.

And that has absolutely nothing to do with expansion tanks. Water heaters explode due to a
combination of runaway heating *and* a failed temperature-pressure relief valve. If *that*
happens, no expansion tank is going to contain the enormous increase in pressure that
precedes a catastrophic steam explosion.