When Does a Water Heater Relief Valve Need Replacing?
Replace a water heater relief valve when it drips all the time, is crusted or corroded, fails a lift test, has never been tested on an older heater, or has opened after the tank overheated. The new valve must match the pressure and temperature rating and the Btu rating on your heater's label, with a correct discharge pipe.
The temperature and pressure relief valve, or T&P valve, is the brass valve with a small lever near the top of the tank. It opens if the tank gets too hot or the pressure gets too high, and it sends that water down a pipe toward the floor. It's a cheap part that protects an expensive tank, and it's one of the repairs we do most around Apache Junction.
Not sure the valve is really the problem? Our relief valve leaking guide covers diagnosis: a steady drip usually means a worn valve, while water that comes out now and then usually points to thermal expansion. This page covers the replacement itself.
Safety: Never cap, plug or put a shut-off valve on a relief valve or its discharge pipe. A blocked T&P valve can't protect the tank if the thermostat or gas control fails. If the valve drips, fix the cause.
Signs It's Time for a New T&P Valve
Some valves give clear warnings. Others look fine until someone lifts the lever. These are the situations where we recommend a new valve instead of waiting.
- A constant drip from the end of the relief pipe, day and night, even when no one has used hot water.
- White or green crust on the valve body or around the lever. With water at about 13–16 grains per gallon here, minerals build up on the seat until it can't close.
- A failed lift test. You lift the lever and nothing comes out, or water comes out and won't stop when you let go.
- An old valve that's never been tested. On a heater that's 8 or more years old, a valve that's sat untouched may be stuck. Replacing it is often smarter than testing it and hoping.
- After an overheating event. If the valve opened because a thermostat or gas control failed, the high heat can damage the seat. We fix the control and usually replace the valve too.
- Water at the valve threads, where the valve screws into the tank. This one needs a closer look, since it can also mean corrosion in the tank.
Can a dripping relief valve be repaired instead of replaced?
Not really. T&P valves are sealed safety devices and aren't meant to be taken apart. Sometimes a single lift of the lever flushes out a piece of grit and the drip stops. If it doesn't, a new valve is the fix. The part costs little, so the labor is most of the bill.
Matching the New Valve to Your Heater
A relief valve isn't one-size-fits-all. The wrong one may fit the threads and still be unsafe. We check three things before installing a new valve.
Pressure and temperature
Most home heaters use a valve set at 150 psi and 210°F. The heater's rating label lists the maximum working pressure. The valve's setting can't be higher than that.
Btu rating
Every valve has a metal tag showing how much heat it can relieve, in Btu per hour. That number must be at least the heater's input rating, which is on the heater's label.
Probe length
The valve's sensing probe has to reach into the hot water at the top of the tank. Tanks with thick insulation often need an extended-shank valve.
Where do I find the rating on my water heater?
Look at the rating label on the side of the tank. It lists the model, the input in Btu per hour for gas or watts for electric, and the working pressure. The old valve's tag shows its own pressure, temperature and Btu rating, though on older valves the tag may be faded or missing. If you can read the heater label, we can match the valve.
Discharge Pipe Rules We Check on Every Job
A new valve won't help much if the pipe it drains into is wrong. In older Apache Junction homes we often find relief pipes that are too small, capped, threaded at the end or run uphill. We fix those at the same visit.
- Full size. The pipe has to be the same size as the valve outlet, usually 3/4 inch, all the way to the end.
- No cap, valve or plug anywhere along its length.
- No threads on the open end, so no one can screw a cap onto it later.
- Runs downhill so it drains fully and doesn't trap water.
- Ends where you can see it, usually within about 6 inches of the floor or the drain pan, or at another spot local code allows.
- The right material. Copper or another pipe rated for hot relief water. Plain PVC drain pipe isn't rated for it.
In many mobile homes the relief pipe runs through the floor or out the side of the outside closet. That's fine when it's full size and open. We check that it isn't crushed, kinked or buried in dirt where it ends. Our mobile home water heater page has more on those closets.
How We Replace a Relief Valve
- Shut off the heat and water. Breaker off for electric, gas control to "Off" or "Pilot" for gas, then close the cold-water valve. Our water heater shut-off guide shows each one.
- Relieve pressure and lower the water level. We open a hot tap, then drain enough water that the level drops below the valve opening. On side-mounted valves that's a few gallons. On top-mounted valves it's less.
- Remove the old valve and pipe. Old valves can be tight from scale. We brace the tank so the fitting doesn't twist.
- Check the tank opening. Rust or soft metal in the threads tells us the tank may be near the end. If we see that, we tell you before going further.
- Install the matched valve with thread sealant rated for potable water, then fit a correct discharge pipe.
- Refill, test and restore heat. We refill with a hot tap open to push out air, check for drips, turn the heat back on and watch the valve through a heating cycle.
Should I replace the relief valve myself?
Handy owners do it. The common mistakes are buying a valve with the wrong Btu rating, cracking a fitting by forcing an old valve out, and reusing a bad discharge pipe. Hot water and gas controls add risk. If you're unsure about any step, it's worth having it done.
Why a New Valve Can Still Drip on Hot Afternoons
If a new T&P valve starts dripping again within days, and mostly in the afternoon or after the heater runs, the valve is likely fine. The pressure is the problem.
When water heats, it expands. If your home has a pressure-reducing valve on the main or a check valve at the meter, that extra water can't push back into the street. Pressure climbs until the relief valve opens. Summer makes it worse here. Cold lines come in warm, and garage heaters sit in hot air when July highs average about 105°F.
The fix is an expansion tank on the cold line, or a new one if the old tank has lost its air charge. See our expansion tank installation page for how that works. We check house pressure on every relief valve job so we can tell you which problem you have.
Tip: A cheap pressure gauge with a red "max" needle screws onto any outdoor hose bib. Leave it on for a day. If the max needle shows a spike well above your normal pressure, thermal expansion is the likely cause.
After the Job: Testing and Care
How often should a T&P valve be tested?
We recommend lifting the lever once a year, which is also what many heater manufacturers suggest. Put a bucket under the pipe, lift the lever for a few seconds, and let it snap back. Water should flow freely and stop when you let go. The water is scalding, so keep hands and feet clear of the pipe end.
What else should I check after a new valve goes in?
- The end of the relief pipe stays dry for the next few days.
- No moisture where the valve meets the tank.
- Hot water temperature feels normal. The U.S. Department of Energy suggests about 120°F for most homes.
- No new water on top of the tank. If the fittings up there drip, see our water heater leaking from top guide.
A relief valve is one of many repairs on our water heater repair service page. If water is on the floor right now, our water heater leak repair in Apache Junction page walks you through the first steps. Call (480) 555-0147 any time, day or night.
Relief Valve Replacement FAQs
How long does a T&P relief valve last?
There's no fixed number. In hard water, mineral buildup on the seat is what usually ends a valve's life, and that can happen years before the tank wears out. A valve that's tested once a year tends to stay free. One that's never been touched for a decade is more likely to stick or drip.
Can I use any 150 psi relief valve on my water heater?
No. Pressure is only one of the ratings. The valve's Btu rating must be at least the heater's input, and the probe has to reach the hot water at the top of the tank. A valve that fits the threads can still be the wrong valve. Match it to the heater's rating label.
Does the relief valve pipe have to go outside?
Not always. In many homes it ends a few inches above the floor or the drain pan, where you can see it. Some homes, mobile homes especially, run it through the floor or wall to the outside. What matters is that it's full size, runs downhill, has no cap or threads on the end, and meets local code.
Should I replace the relief valve when I replace the water heater?
Yes. New heaters come with a new T&P valve already matched to the tank, and it should be used. Reusing an old valve from the old heater is a bad idea. The discharge pipe should also be checked and replaced if it's undersized or damaged.
Why is there a little water after I test the relief valve?
A few drops right after a test are normal while the seat settles. If it keeps dripping, grit may be stuck in the seat. Lift the lever once more to flush it. If it still drips after that, the valve is worn and should be replaced.
Does a tankless water heater have a relief valve too?
Yes. Most tankless units have a pressure relief valve on the hot outlet or at the service valves. It has the same rules: never cap it, and pipe it to a safe spot. If it drips, the causes are similar, including high house pressure.