PRV, Thermal Expansion, and a Weeping T&P Valve
An Anne Arundel County homeowner calls after a new pressure-reducing valve was installed at the meter. Static pressure had been 110 psi; the PRV is set to 60 psi. The municipal meter has a backflow preventer. The 50-gallon gas-fired water heater is 8 years old, with a T&P relief valve rated 150 psi/210°F. One week after the PRV job, the T&P valve drips steadily when the burner runs, and the dishwasher now produces a single loud hammer when its solenoid closes. The T&P discharge is 3/4-inch CPVC, rises 12 inches above the heater outlet, then runs horizontally and terminates 18 inches above the garage floor. There is no thermal expansion tank. The homeowner wants the T&P valve replaced and the hammer blamed on the new PRV.
1. Diagnose the dripping T&P valve before replacing it. What measurements and system details would you check, and what is the likely fix?
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The T&P valve is acting as a final safety device, not as a pressure regulator. The meter backflow preventer and the new PRV both stop water from expanding back into the main, so the system is closed. When the burner heats 50 gallons from about 50°F to 140°F, the water expands roughly 0.8–1%, or about 0.4–0.5 gallon. In a closed system that volume raises pressure quickly, and the T&P lifts. I would measure static and dynamic pressure at a hose bibb with a calibrated gauge, confirm the PRV outlet is actually holding near 60 psi, check for an expansion tank and its precharge, verify the T&P rating, and check whether a check valve was added at the heater outlet. The correction is to install a properly sized thermal expansion tank on the cold-water side and set the PRV correctly. Replacing the T&P valve alone would hide the symptom and leave repeated relief discharges.
2. Evaluate the T&P discharge piping as described. Is it code-compliant? List the corrections.
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It is not compliant. The relief discharge must be full size, made of material rated for the relief valve’s temperature and pressure, and must terminate not more than 6 inches above the floor or outside the building. A 3/4-inch CPVC line is questionable for a 210°F relief discharge unless the specific product is rated for that condition; many CPVC ratings fall below it. The 18-inch termination above the garage floor also violates the 6-inch rule. The discharge must not be trapped, must not have a valve or reduced fittings, and must not be directly connected to drainage without an air gap or receptor. Corrections: replace the pipe with rated metallic pipe, secure it, terminate within 6 inches of the floor or to the exterior, and keep the termination visible and safe. Verify the T&P tag and rating before returning the heater to service.
3. What is the likely cause of the dishwasher water hammer, and what fix would you recommend that does not simply raise the PRV setting?
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The hammer is probably caused by the dishwasher solenoid closing quickly against a pipe run that is not adequately secured or lacks water-hammer arrest. The new PRV lowered static pressure, but it did not create the shock; it may have changed the system dynamics enough to make an existing loose run or missing arrestor noticeable. I would verify pressure at the dishwasher supply, secure the pipe, check for a failed solenoid or loose washer, and install a listed water-hammer arrestor close to the solenoid. Raising the PRV setting would increase pressure and make the problem worse, and it could also push the system back toward the T&P relief threshold. The expansion issue and the hammer should be corrected separately.