Weeping T&P Valve After the PRV Was Installed
A 50 gal, 40,000 Btu/h gas storage water heater installed in 2019 sits in a garage corner. Last spring the city replaced the main, static pressure in the house reached 105 psi, so a pressure-reducing valve was installed and set to 60 psi; the meter also has a check valve. There is no expansion tank and no other thermal expansion control. Since then the T&P relief valve drips about a quart a day. The discharge is 1/2 in. copper, capped at the end, and terminates 3 ft above the floor beside the heater. Tank temperature is 138 F with the thermostat at 140 F; the burner cycles normally and there is no sign of a failed thermostat.
1. Diagnose the drip. What test confirms it?
Read the worked answer
Closed system plus thermal expansion. The PRV and the meter check valve together create a closed system: on every recovery cycle, roughly 40 gal of water heated from about 55 F to 140 F expands about 1 percent - on the order of 0.5 gal - and it has nowhere to go. Pressure ratchets up until the T&P opens (typically 150 psi) or a fixture weeps.
The tell is the timeline: the drip started when the PRV went in, is worse after heavy hot-water use, and the thermostat and burner are fine.
Confirming test: put a gauge on the heater drain valve and log pressure cold, then through one full recovery with no fixtures open. A climb from 60 psi toward the relief setting with no flow proves expansion. If pressure stays flat and it still drips, suspect a failed T&P seat or a thermostat/control holding temperature - but here the sequence points to expansion, and repeated lifting has likely damaged the seat anyway, so plan to re-test or replace the valve after fixing the cause.
2. What is wrong with the T&P discharge as installed, and what does the code require?
Read the worked answer
Almost everything about it is wrong:
- 1/2 in. discharge on a valve with a 3/4 in. outlet is a reduction, which is prohibited - the discharge must be the full size of the valve outlet.
- The cap is the dangerous one. A capped or plugged relief discharge defeats the valve; if it cannot open, the tank can fail. It comes off.
- Termination must be gravity, with no traps, no valves, no threaded end, and the discharge point arranged so it will not cause injury or damage - within about 6 in. of the floor or to the exterior at a safe location.
- Material must be rated for the temperature (a listed T&P discharges near 210 F).
Also verify the valve itself: a listed temperature-and-pressure relief valve rated 150 psi / 210 F, with a rated relieving capacity at least equal to the heater's input, installed in the designated tapping and not obstructed. Never solve a weeping T&P by raising the relief setting, capping it, or turning the thermostat down.
3. Specify the expansion control: size, location, and settings.
Read the worked answer
Location: on the cold-water inlet, downstream of the check valve/PRV and upstream of the heater - the closed side of the system. Nothing between the tank and the heater but pipe.
Precharge: equal to the system pressure, 60 psi here, set with the tank isolated and the system side drained.
Sizing: use the tank's acceptance volume at the relief setting, not nominal gallonage. Expansion is about 1 percent of heated volume: 50 gal x ~1 percent ≈ 0.5 gal. A nominal 2 gal tank typically accepts about 0.5-0.6 gal between 60 and 150 psi, so it works - but read the acceptance chart and pick from it, and if fixtures are added or a larger heater goes in, recheck.
Settings: keep the PRV at or below 80 psi (60 psi is fine); leave the T&P at its listed setting. After installation, run a recovery cycle and confirm pressure holds near the precharge rather than climbing.
Also confirm the PRV is not bypassing and that no other check valve traps a second closed segment (irrigation, recirculation loop, or a second regulator).