Why would the pressure in the cold water lines increase whenever the hot water heater runs?
This is a problem on systems that have check valves or pressure reducing valves (PRV).
What is happening? The system fills up to line pressure or what the PRV is set to.
With no leaks and the water heater at its lowest temp the pressure is say 60 psi and now the water heater starts heating the water starts expanding but has no place to go so the pressure builds until the water heater turns off and cools a little and the cycle repeats.
How can this be prevented ? Addition of a small pressure tank literarily only about 3 gallons on the cold water inlet will completely eliminate the problem.
The pressure tank has a air space that the water expands into and the pressure only slightly increases a few PSI and things won’t be breaking.