New T&P Valve Still Leaking? Why
The Short Answer: A brand-new T&P that weeps is rarely “a bad valve from the box.” Chips on the seat, house pressure still over 80 psi, a waterlogged expansion tank, or a 150 psi valve doing the expansion tank’s job will wet the same pipe. Do not cap it. Gauge the house before you swap the second one.
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Search Intent & The Real Problem
You already replaced it. The discharge is still wet. That is not the generic “overflow keeps discharging” diagnosis — that page is T&P overflow. This page is after the swap: why a new 150 psi / 210°F valve behaves like the old one. Tankless location and scale overshoot: Rinnai T&P. Lever you lifted yesterday: leaking after testing. Hard stream: stuck open.
What other guides get wrong
They say the new valve is defective and to warranty-swap it. I have thrown away good valves because nobody gauged the hose bib. They also put pipe dope on the discharge outlet, reduce the drain to 3/8” tube, or install a 75 psi boiler valve on a 150 psi tank. Wrong stamp, instant drip.
Step-by-Step Troubleshooting
Step 1: Threads vs discharge — name the wet part on the new valve
| Wet location | Meaning | Next |
|---|---|---|
| Weep at the NPT threads into the tank | Dope/tape, cross-thread, missing gasket | Isolate, remake the joint — do not cap |
| Weep from the discharge opening, lever at rest | Valve is opening or the seat is dirty | Pressure, expansion, debris |
| Discharge pipe pushing up on the lever | Pipe is holding the valve cracked | Re-hang the pipe; full outlet size, no threads on the outlet |
| Lever tied / zip-tied “so it would stop” | Someone capped it with a lever | Cut the tie. Hire it. |
- What to look for: Stamp on the new valve (psi / °F). Match the old one unless a licensed plumber changed the listing.
- Required tools: Paper towel, flashlight, phone photo of the stamp.
Step 2: Measure static pressure — the reason the old valve died
- What to look for: Hose-bib gauge over 80 psi. IPC/UPC want a PRV when street pressure can exceed 80 psi. A new 150 psi T&P on a 90–110 psi house will drip every heat cycle, same as the last one. Thermal expansion on a closed system (PRV or check on the city line) spikes higher when the tank recovers.
- Required tools: $12 hose-bib pressure gauge.
Over 80 psi: the house PRV is the repair, not T&P number two. Under 80 psi static and it still weeps while recovering: expansion or a dirty seat.
Step 3: Expansion tank — still missing or still waterlogged
- What to look for: No tank on the cold line plus a PRV = the new T&P is the expansion tank. Schrader that spits water = waterlogged. Precharge 0 psi = failed. Precharge should match static, typically 50–60 psi, with the tank isolated and empty.
- Required tools: Tire gauge, bicycle pump.
A 40–50 gallon tank heating 50°F to 120°F has nowhere to put that volume on a closed system. Replacing the relief valve does not create that volume.
Step 4: Debris on a new seat — the 20-minute failure
- What to look for: You cut the old valve out and chips of PTFE, dope, or tank scale went into the opening. First lift after install never reseated. Discharge pipe that was not blown out. A valve installed with the outlet pointing up so grit sits on the seat.
- Required tools: Bucket. If you must exercise the lever, do it with a discharge path, then let it reseat. Do not pump the lever ten times.
If it still dribbles after a clean install and pressure is in range, replace it once — after you have gauged the house. Two new valves in a week is pressure, not luck.
Step 5: Wrong valve, wrong rating, wrong heater problem
- What to look for: 125 psi or 75 psi valve on a tank listed for 150. Temperature-only or pressure-only part. Discharge reduced below the outlet size. Steaming water: stuck thermostat / ECO — temperature lift at 210°F, not a “bad new T&P.” That is thermostat burn-up territory. Wet jacket: the tank, not the valve — bottom leak.
- Required tools: Contact thermometer at a nearby faucet. Energy off before covers.
When DIY Ends: Calling a Licensed Pro
Stop if it streams, the water is scalding, you cannot find a PRV on an 80-plus-psi house, or you already installed two valves. You may gauge a hose bib and look at a Schrader. You may not cap the T&P, “snug it,” or epoxy the discharge. A licensed plumber sets or replaces the PRV, charges or replaces the expansion tank, verifies the thermostat, and only then leaves a matching 150 psi / 210°F valve with legal discharge piping.
Frequently Asked Questions
Q: Why is my water heater relief valve leaking after replacement? A: The new seat has debris, house pressure is still high, the expansion tank is missing or waterlogged, the discharge pipe is holding the lever open, or the wrong psi valve went in. The valve is the symptom until you gauge the house.
Q: Can a brand-new T&P valve be defective? A: Rarely. Treat it as defective only after static pressure is under 80 psi, expansion is working, the discharge pipe is not lifting the lever, and the seat was not packed with chips. Then swap it once.
Q: Should I cap the new valve overnight until the plumber comes? A: No. Close the cold inlet and shut the heater down. A capped T&P is a code violation and a rupture risk, new or old.