Tom, two reasons you may see downstream pressure increase. One you may have debris from installation in the seat of the valve body allowing pressure creep. Even a small copper piece in the seat can cause it to over pressurize the system. Second you may have thermal expansion. When water is heated it can expand and increase pressure in the system. A system expansion tank may be needed to absorb the thermal expansion.
Tom, two reasons you may see downstream pressure increase. One you may have debris from installation in the seat of the valve body allowing pressure creep. Even a small copper piece in the seat can cause it to over pressurize the system. Second you may have thermal expansion. When water is heated it can expand and increase pressure in the system. A system expansion tank may be needed to absorb the thermal expansion.