T&P Relief Weeping After the Utility Added a PRV and Dual Check
Service call at a 1968 two-story house. The 50-gallon gas water heater's T&P relief valve discharges about a cup of water every evening, shortly after the family finishes showers. A gauge on the front hose bibb reads 58 psi at midday and 95 psi after the burner has run a while. Three years ago the utility replaced the meter and installed a new pressure-reducing valve with an integral dual-check backflow assembly at the meter yoke. The house has no expansion tank and no thermal expansion loop. The T&P discharge pipe runs about 8 feet horizontally with two elbows and terminates 3 feet above the garage floor; there is no drain pan. The heater sits in a closet inside the garage. The homeowner also reports that the downstairs lavatory faucet bangs hard when the handle is shut off quickly, and that it never did this before the meter work.
1. What is the most likely cause of the daily T&P discharge, how would you prove it on site, and what is the correct repair?
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Rule in, rule out. A T&P valve that relieves about a cup a day, always after the burner has recovered the tank, is the classic signature of thermal expansion, not a bad valve. The valve opens near 150 psi (or 210 F). The 95 psi hose-bibb reading is a spot reading taken at the wrong moment; it almost certainly misses the peak that occurs hours later.
Why the system is now closed. The utility added a PRV with an integral dual check. The check holds water in and the PRV closes the downstream side, so the plumbing is effectively a closed system. A 50-gallon tank heated from roughly 50 F to 140 F expands on the order of 0.6 to 1 gallon of volume. Water is nearly incompressible, so that volume pressurizes the piping until the T&P relieves.
Proof on site. Thread a gauge with a lazy, peak-holding needle onto the heater drain valve, draw hot water to fire the burner, and watch. A climb toward 150 psi followed by a drop when the valve weeps confirms expansion. Then check the PRV separately: if outlet static pressure is above about 60 psi or creeps up overnight, the PRV is passing and needs repair, which is a second and independent cause.
Correct repair. Install a listed thermal expansion tank on the cold side between the PRV/check and the heater, sized to system volume, with precharge matched to the PRV outlet setting; for a 40-50 gallon residential heater and 3/4-inch piping, a 2-gallon tank is typical, but confirm with the manufacturer's sizing table. Set or repair the PRV to about 50-60 psi. Do not simply replace the T&P valve; it is doing its job and a new one will leak the same way.
2. Evaluate the T&P discharge piping and the heater location against code requirements, and list what must be corrected.
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T&P discharge. The relief valve discharge must be full size, meaning the same nominal pipe size as the valve outlet (commonly 3/4 inch), must run by gravity with no traps, no valves, no plugged or threaded end, and must be of a material rated for near-boiling water. The termination is the problem here: it must end not more than about 6 inches above the floor, or outside the building, and must discharge where it cannot injure anyone or damage structure. Ending 3 feet up in a garage leaves a scalding stream at head height for a child or pet, and the two elbows plus 8 feet of run must be verified for continuous fall so water does not sit in the pipe and corrode it or freeze.
Drain pan. A heater in a closet where leakage would damage the structure needs a pan, with a drain routed to a safe place of disposal by gravity or a listed pan-drain pump. A garage closet floor may be able to drain to grade, but confirm the pan discharge path.
Location constraints. In a garage, the heater must be protected from vehicle impact, and gas-fired units must comply with the listing's ignition-source elevation and FVIR requirements. Also confirm the appliance compartment has combustion air and that the vent connector meets clearances, because a closet install that was acceptable years ago often is not once the door and framing are considered.
Order of work. Fix the pressure problem first (expansion tank), then correct the discharge termination and pan, then test the T&P using its lever with the burner off and the drain line open to confirm it reseats.
3. Is the lavatory faucet bang related to the expansion problem? Explain the mechanism and the fix.
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Separate problem, same job site. Water hammer is a momentum problem, not a pressure-rise problem. When the lavatory faucet closes quickly, the moving column of water in the pipe must stop, and its kinetic energy produces a pressure spike that travels back through the piping and rattles whatever is loose.
Why it appeared after the meter work. The new PRV/dual-check assembly changed the system in two ways. First, it removed the utility-side cushion that used to absorb some of the surge. Second, if the PRV has a worn or chattering seat, or if the work introduced air into the lines, the banging can be transmitted through the piping. Old air chambers in the walls are often waterlogged and no longer function. A loose faucet washer or a loose pipe strap will also convert a normal surge into a loud report; part of the diagnosis is listening at the fixture versus at the wall.
Diagnosis and repair. Purge air from the system at the highest fixture. Secure any loose piping and check the faucet itself for a loose seat or washer. Install listed water hammer arrestors at the quick-closing fixtures, typically at the lavatory here and at the dishwasher and clothes washer if present. Verify the PRV is not chattering under low flow. Do not assume an arrestor alone fixes a failed PRV; the two failures should be corrected independently.
Note for the invoice. Explain on the invoice that expansion and hammer are separate defects with separate causes, so the customer does not later argue that the expansion tank 'should have fixed the noise.'