Closed Water System Causes T&P Relief Drip

The backflow preventer and pressure-reducing valve create a closed system, so water heated in the tank cannot expand back into the public main. As the water temperature rises from about 60°F to 140°F, the water volume increases and the system pressure rises. The T&P valve is acting as the only relief point, which explains the nighttime drip after recovery cycles. The water hammer after the dishwasher runs may be aggravated by the same pressure spike, although a quick-closing dishwasher solenoid can also cause hammer.\n\nThe immediate correction is to install a properly sized thermal expansion tank on the cold-water side between the backflow preventer/PRV and the heater, set its precharge to the supply pressure, and verify the PRV is functioning. Do not cap, plug, or defeat the T&P valve. Remove the threaded cap from the discharge pipe immediately because the relief valve must be free to discharge. If hammer persists after the expansion tank is installed and the piping is secured, add approved water-hammer arrestors at the quick-closing fixtures.

The expansion volume is approximately 1.6% of the system volume for a 60°F to 140°F rise. For 80 gal, that is about 1.3 gal of expansion. Use the tank relationship Vt = Vexp / (1 - Ppre/Pmax), where Vt is total tank volume, Ppre is the precharge pressure, and Pmax is the maximum allowable pressure.\n\nWith a precharge set near the 75-psi supply pressure and a maximum pressure of 150 psi at the T&P relief setting, the acceptance factor is 1 - 75/150 = 0.50. The required total tank volume is about 1.3/0.50 = 2.6 gal. A nominal 2-gal tank is therefore below the calculated requirement and should not be accepted as adequate. Use the next standard size, typically about 4.4 gal, and set the precharge to the actual supply pressure.\n\nIf someone instead used a 60-psi precharge with a 75-psi maximum, the acceptance factor would be only 0.20 and the required tank would be much larger, about 6.5 gal. That is why the precharge must match the PRV setting and the maximum pressure must be realistic. Verify the final selection with the manufacturer's sizing table.

The T&P discharge pipe must be full size, materially rated for the temperature, and laid to drain by gravity. It must not have a threaded cap, plug, valve, or trap, and it must terminate in a safe, visible location. A typical acceptable termination is not less than 6 in. and not more than 24 in. above the floor or exterior grade, with the end pointing down and not threaded. The pipe must be supported and protected from freezing, and in a garage it must be protected from vehicle damage. The water heater itself needs physical protection such as bollards if it is exposed to vehicles.\n\nThe permit was pulled for a heater replacement but did not show the expansion tank. Submit a revised permit drawing or amendment showing the expansion tank, T&P correction, and garage protection. The inspection should verify the T&P relief rating and discharge, the expansion tank precharge and location, the PRV and backflow arrangement, combustion air and venting, and the final plumbing connections. Do not leave the capped T&P discharge in service.