WRONG - WRONG - WRONG !!!

I just got done fighting this same battle and contacted 10 local plumbers
and not a single one had the correct answer ... but they all had an
incorrect EXPENSIVE answer.

The odds are that you have (or now have) either a check valve or PRV
(Pressure Reducing Valve) at your water meter. That makes your house
plumbing a CLOSED system. A water softener can also "close" a plumbing
system as they are reluctant to "backflow". A closed system does not allow
for the expansion of water heated in your hot water heater. Since "thermal
expansion" is a certainty when water is heated some accommodation for the
increased volume of water is necessary (and may be required by plumbing
code in your area).

That can be accomplished by a variety of solutions. One, do nothing and
allow your T&P valve on the water heater to "burp" the excess. That's bad
for the valve and cuts it's life. Second, install a "thermal expansion
tank" at the cold side of the hot water heater. This is costly, usually
requires a plumber, and can be a kludge as there is usually no where to
locate the tank as an after thought. Third, and a really cute answer ... go
to

https://www.wattsind.com/watts/showp...jectGroup_ID=2
47

This is a replacement ballcock for your toilet that incorporates a pressure
relief valve set to dribble excess pressure at 80 psi. It works flawlessly.
Just be careful to get the correct size for your WC tank. Their docs are
vague and you must measure the height of your overflow tube from the tank
bottom and get the valve which has a "CL" or "critical level" equal to or
higher than the height of the overflow tube. The valve comes in 10", 11.5",
& 12.5 " overall length and is usually sold only through Plumbing Supply
Houses ... no Home Depot or Lowes. You can buy direct through the Watts
Online Store or I have found them on the net but always at the same price.
The price is a bargain when considering you don't need a plumber or an
expensive expansion tank and installation.