On 11/20/2015 6:52 PM, Tony Hwang wrote:

Pressure regulator valve is not enough? Our house has regulator and when
water is running it is 60psi. We don't need anything extra.

Run your shower (or otherwise use most of your hot water).
Then, shut off water (don't use any -- hot *or* cold) and
watch pressure slowly climb *upward* from that 60psi as
the COLD water in the heater expands -- into a fixed
geometry space (i.e., pipes remain the same size/volume).

E.g., out "inside" pressure climbs from 50 (the setting
on the PRV) to 100 after a long shower. It *stops* at
100 because our municipal supply happens to be 100 psi
AND our PRV has a builtin bypass (when inside pressure
exceeds supply pressure, PRV "opens" to vent pressure
into the LOWER municipal supply). If our municipal
supply had been 120 psi, then the clamping action
would limit the inside pressure to 120 psi instead of 100.