Why Is My Water Heater Relief Valve Leaking?
A water heater relief valve leaks because the valve is worn and no longer seals, or because it's doing its job and letting out pressure or heat that's too high. A constant drip usually means a worn valve (or very high house pressure). Water that comes out now and then points to thermal expansion, which an expansion tank usually fixes.
The temperature and pressure relief valve, or T&P valve, is the most important safety part on a tank water heater. It's the brass valve on the top or upper side of the tank with a lever and a pipe running down toward the floor. When it lets water out, that water usually lands at the bottom of the heater, so many people think the tank is leaking when it isn't.
Safety: Never cap, plug, or put a valve on the relief pipe to stop a drip. A blocked T&P valve takes away the one thing that keeps an overheated tank from bursting.
What Does a T&P Relief Valve Do?
The valve opens if either of two limits is reached: tank pressure gets too high (most residential valves are rated for 150 psi) or the water gets too hot (most open around 210°F). When it opens, hot water flows out through the discharge pipe instead of building up inside the tank. Once things are back to normal, the valve should close tightly again.
The pipe from the valve should be full size, run downhill, and end a few inches above the floor, the drain pan, or another approved spot where you can see it. It shouldn't be capped, threaded at the end, or reduced in size.
Steady Drip or On-and-Off? What Each One Means
Watch the end of the relief pipe for a day. The pattern tells you a lot.
| What you see | Most likely cause | Usual fix |
|---|---|---|
| Slow, steady drip all day | Worn valve seat or spring, often with mineral buildup | Replace the T&P valve |
| Water now and then, often after showers or laundry, or overnight while the heater reheats | Thermal expansion with no working expansion tank | Add or replace the expansion tank |
| Drips after you test the lever | Grit stuck in the seat after opening | Test again to flush it; replace if it keeps dripping |
| Hot water or steam pouring out | Tank overheating, possibly a failed thermostat or gas control | Turn off power or gas now and call |
| Drips much of the day, even when the heater isn't running | House pressure over about 80 psi, often from a failing pressure-reducing valve (PRV) | Check pressure; adjust or replace the PRV |
| Water around the valve threads, not from the pipe | Leak where the valve screws into the tank | Reseal or replace the valve; check the tank |
High house pressure strains every joint near the heater, not just this valve. Our page on high water pressure and a failing PRV covers what to look for.
Thermal Expansion: Why the Valve Opens Now and Then
If water comes out in bursts, mostly while the heater is reheating after showers or overnight, thermal expansion is the likely cause. Heated water expands, and a pressure-reducing valve or meter check valve can keep it from pushing back to the street. Pressure climbs until the T&P valve lets some out. A working expansion tank on the cold line absorbs that extra water.
Do I need an expansion tank?
If your plumbing is a closed system, usually yes, and an existing one can wear out and stop working. Our expansion tank installation page explains how they work and when to add or replace one. If the dripping started right after a new heater, meter or PRV went in, see our new water heater leaking guide too.
Why does the valve seem to drip more in summer?
Thermal expansion happens every time the heater runs, in any season. You notice it most when no taps are open, such as overnight or in a home closed up for the summer, because the extra pressure has nowhere to go. Heat matters in a different way: a valve in a garage or outdoor closet, where July highs average about 105°F, works hot for months. That wears its seat and spring, so an already-worn valve starts to weep.
When the Valve Itself Is Worn Out
T&P valves don't last forever. With water at about 13–16 grains per gallon of hardness, minerals build up on the valve seat. Once that happens, the valve can't close fully and you get a constant drip. Rust on the spring can also keep it from seating.
Why did my relief valve start dripping after I tested it?
Lifting the lever lets water rush through the seat, and a bit of scale can lodge there so the valve won't close fully. Lifting it once more can flush the grit out. If it still drips, the seat is worn and the valve needs replacing.
Can I replace a T&P valve myself?
It means draining some water and fitting a valve that matches your heater's rating label, so it's a careful job. Our relief valve replacement page covers how it's done and what to check after.
When a Leaking Relief Valve Is Serious
- Hot water or steam is pouring from the pipe
- Your hot water is much hotter than usual
- The tank makes loud rumbling or banging noises along with the discharge
- Water is coming from around the valve threads, which can point to tank corrosion
If any of these happen, turn off the gas or the breaker and the cold-water valve, and call us at +1 480-863-0807. Our guide to shutting off a water heater shows where everything is.
If the relief valve checks out and the leak is lower down, see our water heater leaking from bottom page or the full water heater leaking guide.
Have a tankless unit? It has a pressure relief valve too, and our tankless water heater repair page covers its leaks. For everything else, our Apache Junction leak repair service is open 24/7.
Relief Valve Leaking FAQs
Is it normal for a water heater relief valve to drip?
No. A relief valve should stay closed and dry unless pressure or temperature gets too high. Any regular dripping means either the valve is worn or something is pushing pressure up, most often thermal expansion without a working expansion tank.
Can I cap the relief valve pipe to stop the leak?
Never. Capping or plugging the relief valve or its pipe removes the tank's main safety protection. If pressure or temperature climbed with the valve blocked, the tank could burst. Fix the cause instead: a worn valve gets replaced, and pressure from thermal expansion gets an expansion tank. Until then, put a bucket under the pipe.
How much does it cost to fix a leaking relief valve?
Replacing a T&P valve is usually a service call plus the part, far less than a new water heater. Adding an expansion tank adds the tank and labor. For context, Angi's Apache Junction water heater repair average is about $646 (typical $422–$1,196) across all repair types. Our repair cost guide explains what moves the price.
Why does my relief valve leak after showers or overnight?
That timing usually points to thermal expansion. The heater fires to reheat fresh cold water, the water expands, and in a closed system with no working expansion tank the pressure spikes until the valve lets some out. It shows most when no taps are open. If it drips steadily even while the heater sits idle, check for high street pressure instead.
Can a worn-out expansion tank make my relief valve drip again?
Yes. An expansion tank only works while it holds its air charge. Once the bladder fails, it fills with water and can no longer absorb expansion, so the relief valve starts letting water out on reheats again. If drips return a few years after a tank was added, check it. Water instead of air from the small air valve on the tank means the bladder has failed.
Can a leaking relief valve mean the tank is about to fail?
Not usually on its own. But water around the valve's threads, rather than from its pipe, can point to corrosion in the tank wall. If the heater is older and rusty around that spot, replacement may make more sense than a new valve.