Thermal Expansion and a Relief Valve That Will Not Seat
A tester is called to a 4-inch reduced-pressure assembly serving a large apartment building. The building has a new hot water recirculation system, and the assembly's relief valve has been discharging intermittently for two weeks. The tester finds that the relief valve discharges when the building's hot water system heats up, but not when the system is cool. Static pressure upstream is 85 psi. The first check holds 6.5 psi and the second check holds 1.4 psi. The tester suspects thermal expansion but must confirm the cause before recommending a repair.
1. How can the tester distinguish thermal expansion from a failed relief valve seat, and what tests or observations would confirm the cause?
Read the worked answer
The tester should first observe the timing of the discharge. If the relief valve discharges only when the hot water system is heating and stops when the system is cool, thermal expansion is likely. The tester can install a pressure gauge on the downstream side of the assembly and monitor the pressure during a heating cycle. If the downstream pressure rises above the relief valve's opening point and the relief valve discharges to relieve that pressure, thermal expansion is confirmed. A failed relief valve seat, by contrast, would discharge continuously regardless of temperature and would not correlate with the heating cycle. The tester should also check for a thermal expansion tank or other means of controlling expansion downstream of the assembly. If no expansion control exists, the closed system is pressurizing as the water heats, and the relief valve is doing its job by relieving that pressure.
2. What is the appropriate recommendation, and why is replacing the relief valve the wrong fix?
Read the worked answer
The appropriate recommendation is to install or repair a thermal expansion control device, such as a properly sized expansion tank, on the downstream side of the assembly. The relief valve is functioning correctly by relieving excess pressure caused by thermal expansion in a closed system. Replacing the relief valve would not solve the problem because the new valve would also discharge when the downstream pressure rises. The tester should document the downstream pressure readings during the heating cycle, the absence of expansion control, and the recommendation to install an expansion tank. The assembly itself should be retested after the expansion control is installed to confirm that the relief valve no longer discharges during normal operation. The report should distinguish between the assembly's condition and the system condition that is causing the discharge.