T&P Overflow Discharging? Do Not Cap It
The Short Answer: Water from the T&P overflow means pressure or temperature beat the valve — usually 150 psi / 210°F. Do not cap it. Check house pressure, the expansion tank, and a stuck thermostat. A hard stream is an emergency: shut the cold inlet, kill energy, call a licensed plumber.
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Search Intent & The Real Problem
You found a wet discharge pipe, a pan full of water, or a stained ceiling under the heater. Hot water still works, so it feels like a nuisance leak. It is the last mechanical fuse on the tank. On a closed plumbing system the T&P becomes the expansion tank. On a failed thermostat it is the only thing between 120°F and a scalding dump — or a ruptured vessel.
Tankless T&P drips are a different heat exchanger and a different valve location — Rinnai T&P leaking. Water at the base that is not coming from the T&P pipe is leaking from the bottom. This page is the overflow/discharge on a tank.
What other guides get wrong
They start with “replace the T&P.” A new 150 psi valve on a 90 psi house with a waterlogged expansion tank weeps again in two days. They also confuse a dripping drain valve, a wet jacket, and T&P discharge — three different repairs. And they still say cap it overnight. That is how tanks become incident reports.
Step-by-Step Troubleshooting
Step 1: Name the wet pipe
| Wet location | What it is | Next |
|---|---|---|
| Brass valve with a test lever, pipe to a drain or pan | T&P discharge | Pressure, expansion, overheat, then the valve |
| Plastic hose-thread valve at the bottom | Drain valve | Not overflow — Rheem drain valve |
| Rusty wet foam at the base / jacket | Tank | Replacement, not a T&P |
| Cool drip from a high-efficiency drain | Condensate | Trap / pump, not T&P |
| Pan full, T&P pipe dry | Inlet fitting, drain, or tank | Paper-towel test |
- Required tools: Flashlight, dry paper towel, phone photo of the T&P stamp (psi / °F).
Wipe the lever, the threads, and the discharge opening separately. A weep at the threads is often a bad gasket. Water from the discharge opening means the valve opened.
Step 2: Measure static water pressure
- What to look for: Hose-bib gauge over 80 psi. IPC/UPC want a PRV when street pressure can exceed 80 psi. City pressure plus a failed or missing PRV will lift a healthy 150 psi T&P once the tank heats — and sometimes even cold, if the street is that high.
- Required tools: $12 hose-bib pressure gauge.
Screw it onto an outside faucet, open fully, read static. Over 80 psi: the house PRV is the repair, not the heater. Under 80 psi and it still dumps while recovering: expansion or overheat.
Step 3: Test the expansion tank (closed systems)
- What to look for: A tank-style expansion tank on the cold line. Schrader valve that spits water = waterlogged. Precharge at 0 psi = failed. No tank at all plus a check valve or PRV on the city line = the T&P is doing the expansion tank’s job.
- Required tools: Tire gauge, bicycle pump. Isolate and drain the expansion tank before you set precharge to match static pressure, typically 50–60 psi.
A 40–50 gallon tank heating from 50°F to 120°F has nowhere to put that extra volume on a closed system. The discharge you are mopping is that volume.
Step 4: Overheat — stuck thermostat or failed ECO
- What to look for: Discharge that is steaming or too hot to touch. Faucets hotter than your 120°F setpoint. Electric: upper thermostat welded closed, ECO that never opened. Gas: control that will not drop the burner. This is a temperature lift at 210°F, not a pressure drip.
- Required tools: Contact thermometer at a nearby faucet. Energy off before any cover comes off.
If it is dumping near-boiling water, you are done troubleshooting in the closet. Shut it down and hire it. A thermostat that burned up is the same failure chain as thermostat burn-up.
Step 5: Only then condemn the T&P itself
- What to look for: Debris on the seat after someone “tested” the lever, a valve that dribbles after it lifted, or a 15-year valve that never reseated. Replacement without fixing pressure or expansion is a callback.
- Required tools: Two wrenches, matching 150 psi / 210°F T&P, potable-rated dope or a new gasket, bucket.
Discharge piping must be the full outlet size, no threads on the outlet, slope to drain, terminate where you can see it, no valve in the run. That is code, not a suggestion. Do not reduce it to 3/8” tube.
When DIY Ends: Calling a Licensed Pro
Stop if the valve dumps a hard stream, the water is scalding, you cannot find a PRV on a 80-plus-psi house, the expansion tank is on a live cold line you cannot isolate, or the tank jacket is also wet. You may gauge a hose bib and look at the Schrader valve. You may not cap the T&P, “snug it,” or epoxy the discharge. A licensed plumber will set or replace the PRV, charge or replace the expansion tank, verify the thermostat/ECO, and only then swap the valve — with a permit if your city requires one.
Frequently Asked Questions
Q: Why does my water heater overflow keep discharging water? A: The T&P opened because pressure or temperature exceeded its rating, or the valve itself is stuck open. Closed-system expansion, street pressure over 80 psi, a stuck thermostat, or a failed seat. Do not cap the pipe.
Q: Can I cap a dripping T&P valve overnight? A: No. A capped relief valve is a code violation and a rupture/scald risk. Close the cold inlet and shut the heater down instead.
Q: Does a discharging T&P mean the tank is bad? A: Not by itself. A wet jacket, rusty water, or a leak from the shell does. Fix pressure and expansion before you buy a 50-gallon.