T&P Discharge and Water Hammer After PRV Installation
A two-story house is on municipal water with a static pressure of 105 psi at the meter. A pressure-reducing valve at the meter is set to 55 psi. The 50-gallon gas water heater has a new T&P relief valve, but it drips whenever the irrigation system runs and then shuts off. The irrigation is supplied by a 1-inch line downstream of the meter and is protected by a pressure vacuum breaker. There is no expansion tank. The homeowner also hears a single loud bang in the basement when the irrigation valve closes, and a pressure gauge on a hose bibb spikes to about 90 psi for a few seconds after the valve closes. The water heater is in a closed system because the PRV prevents backflow to the main. The homeowner wants to know whether to replace the T&P valve, add a water-hammer arrestor, or change the PRV setting.
1. Diagnose the T&P discharge and describe the test and correction.
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The T&P valve is probably doing its job. The PRV at the meter creates a closed domestic system; when the water heater heats a full tank, thermal expansion has nowhere to go, and pressure rises until the T&P relief opens. A new T&P valve will not fix that. Put a calibrated gauge on a hose bibb and record pressure after a long draw and during recovery. If pressure climbs toward the relief setting, install a listed thermal-expansion tank on the cold-water side of the water heater, sized for the 50-gallon tank and the system pressure, with the precharge matched to the PRV setting. Check that the PRV is not passing high pressure and that the T&P valve has the correct rating and is piped to a safe drain. Do not cap or plug the T&P.
2. Diagnose the bang and the brief 90-psi spike. Will an arrestor fix it, and where should it go?
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The bang and brief pressure spike are water hammer, not a relief-valve problem. The irrigation valve closes quickly, and the moving column of water stops abruptly; the pressure wave travels back through the piping. Install a listed water-hammer arrestor close to the irrigation valve, and consider arrestors at quick-closing appliance valves. Secure loose piping and check that the PRV is maintaining its set pressure; a failed PRV can raise the baseline pressure and make hammer worse. A slower-closing valve or a soft-close operator can also help. The arrestor must be sized for the line and installed in an accessible location recommended by the manufacturer. After the repair, recheck the pressure spike; it should be materially reduced.
3. The irrigation pressure vacuum breaker is 6 years old and has not been tested. Is it the right assembly, and what installation and maintenance issues apply?
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A pressure vacuum breaker is a reasonable choice for an irrigation system with no chemical injection, but it is not adequate if fertilizer is injected downstream. The PVB must be installed above the highest downstream outlet-commonly 6 to 12 in, depending on the local rule-with no shutoff valve downstream that could be left closed with the PVB under pressure. It must be tested and winterized. If a fertilizer injector is present or added, replace the PVB with a reduced-pressure-principle assembly at the irrigation connection. The PVB is not the cause of the heater T&P discharge, because the domestic PRV already closes the system; the expansion tank is still needed.