The T&P valve — temperature and pressure relief valve — sits near the top of the tank, usually on the side, with a small metal lever and a discharge pipe running down toward the floor. Its job is simple and serious: if temperature or pressure inside the tank climbs past safe limits, the valve opens and lets water out before the tank itself becomes the problem. A drip from that pipe is never just a drip. It's the valve reporting something.
That doesn't mean panic. Most drips trace back to an ordinary cause you can sort out in an afternoon. What matters is reading the pattern first, because the fix for high pressure, the fix for thermal expansion, and the fix for a tired valve are three different fixes — and swapping the valve alone won't cure the first two.
Put the bucket under the discharge pipe and watch the pattern over a day. The timing tells you more than the volume does.
| Drip pattern | Likely cause | First move |
|---|---|---|
| Drips after showers, laundry, or a long heating cycle, then stops | Thermal expansion in a closed system | Check the expansion tank, or plan on adding one |
| Drips steadily, day and night | House pressure too high, or the valve seat is worn | Test pressure with a gauge |
| Started right after you tested the lever | Debris caught in the valve seat | Flush the valve once more; replace it if the drip continues |
| Heavy discharge, steam, or a hot tank that won't settle | Overheating — thermostat failure | Shut the heater down and call a plumber |
Water expands as it heats — a 50-gallon tank can produce well over half a gallon of extra volume in one heating cycle. In an open system, that extra water just pushes back toward the city supply. Most modern homes aren't open systems, though. A pressure-reducing valve, a backflow preventer, or a check valve at the meter traps the water inside your plumbing, and the pressure has to go somewhere. The T&P valve is where it goes.
The tell is timing: drips that show up during or right after the heater runs, then stop. The standard fix is an expansion tank — a small tank, usually 2 to 4.5 gallons, installed on the cold-water line above the heater. It gives heated water a cushion to push against. If your house already has one, check it before assuming the valve is bad. An expansion tank that has lost its air charge or has a failed bladder acts like it isn't there. A plumber can test the charge in minutes; a waterlogged tank gets replaced, typically for $250–$500 installed.
Most relief valves are set to open at 150 psi. Normal house pressure should sit between 40 and 80 psi, with 50–60 psi a comfortable target. Municipal pressure, though, can run far higher — and a failing pressure-reducing valve can let it creep up over time, often spiking at night when neighborhood demand drops.
Screw the gauge onto an outside hose bibb or the washing machine hookup. Use a gauge with a lazy-hand needle if you can — it records the highest pressure reached while it sits there.
Open the tap fully and read the number. Under 80 psi is normal. Between 80 and 100 psi is high enough to explain a dripping valve and to shorten the life of other fixtures.
Leave the gauge on overnight. Nighttime spikes are the ones a quick daytime reading misses. If the lazy hand has climbed past 100 psi by morning, pressure — not the valve — is your problem.
If pressure is the culprit, the fix is at the pressure-reducing valve where water enters the house: adjust it down if it still responds, or have it replaced, usually $300–$650 installed. Replacing the relief valve without touching high pressure just gives the new valve the same job of dripping. High pressure rarely limits itself to one symptom, either — if faucets feel harsh and fixtures wear out early, work through why your water pressure is off in both directions, too high and too low, before you settle on a cause.
Valves age. Mineral scale builds on the seat, the spring weakens, and a valve that has sat untouched for a decade may weep at pressures it once held easily. One common trigger: somebody lifts the test lever — a legitimate check — and a fleck of scale lodges in the seat, so the valve never quite seals again.
You can test the lever briefly, and it's worth doing once a year: bucket under the discharge pipe, lift the lever for two or three seconds, let it snap shut. Water should flow while the lever is up and stop when it drops. Two warnings come with that test. The water is hot, so keep hands and feet clear of the pipe's outlet. And on an old valve that's never been tested, there's a real chance it won't reseat — test it when a same-day replacement or a plumber visit is an option, not the night before guests arrive.
Swapping a T&P valve is a common plumber job, and some experienced homeowners do it themselves. Know what it involves before deciding. The tank has to be drained down below the valve's level — the same drain-down covered in the guide to draining a water heater — the old valve unthreads, and the new one goes in with pipe sealant on the threads.
The non-negotiable part is the match. The replacement must carry the same or higher pressure rating (usually 150 psi), the same temperature rating (usually 210°F), and a BTU rating at least equal to the heater's input, all printed on the old valve's tag and the tank's label. A valve with the wrong rating isn't a bargain — it's a safety device that won't do its job. Expect $150–$350 for a plumber to replace one, part included; the valve itself runs $15–$40. If any part of that matching process feels uncertain, that's the line where the job belongs to a pro.
Call rather than tinker if the valve discharges more than a drip, if the tank is over 10 years old and showing other symptoms, if the discharge pipe was never run to within about six inches of the floor, or if pressure readings are high and there's no pressure-reducing valve to adjust. Also call if the valve body or the tank threads are heavily corroded — a valve that won't unthread cleanly can turn a small job into a tank replacement, and that risk is better carried by someone with the tools to handle it.
A slow drip is a warning, not an emergency — but it should never be ignored or plugged. The valve is the tank's last line of defense against excess pressure and temperature. Heavy discharge, steam, or a tank that keeps heating is an emergency: shut the heater down and call a plumber.
The valve itself costs about $15–$40. A plumber typically charges $150–$350 total, part and labor. If the real cause is high pressure or a failed expansion tank, fixing that cause adds cost — but it also stops the new valve from dripping the same way.
Nighttime is when municipal water pressure peaks, because neighborhood demand is low. A valve that's borderline — or house pressure that's borderline — often shows up only in those overnight hours. A gauge with a lazy hand left on overnight catches the spike.
Once a year is the common recommendation: lift the lever briefly, confirm water flows, and let it snap shut. On an older valve that's never been tested, be ready for the chance it won't reseat fully — that itself is useful information, and it means replacement is due.
No — never. Capping the pipe disables the safety function and leaves excess pressure with nowhere to go. That converts a $200 repair into a genuine hazard. Fix the pressure, the expansion, or the valve instead.