On Tue, 16 Aug 2016 18:03:49 -0700, "Tony944" wrote:

"bd" wrote in message
groups.com...

replying to trader4, bd wrote:
And the valve could eventually fail
I looked at expansion tanks today, and in the instructions it plainly states
that the tank WILL eventually fail. Given that both of my hot water heaters
are inside my home and in places where a flood of water would be destructive
and potentially VERY expensive, I think that I could dump a little water
every
now and then.
Actually the expansion tank ios also used in some homes with an
anti-backflow valve. If the water heater is filled with cold water and
then heated, the water expands and has to go somewhere. Without a
check-valve it goes back into the water main (small amount - but
enough to build incredible pressure in a closed system) With check
valve it can't get out so an expansion tank is used. A thermal
pressure relief valve on the water heater should NOT be depended on to
bleed off the pressure due to expansion - what happens if and when the
water heater is shut down and the system cools??? Also, the valves
tend to leak after a few operating cycles. If the relief valve has
been activated it SHOULD be replaced.