As it got cold outside (not sure if this has anything to do with it), water heater pressure relief valve started dripping water after long hot showers. Trying to figure out if it's just the valve going bad or if there is anything more serious.

Water heater is ~9 yrs old. I see there is a drain discharge pipe installed going to outside, which I tested, working fine. House water pressure measured at an outside spigot around ~60-70PSI.

I can stop the valve from leaking for a few days by releasing the water from the drain pipe by opening the bottom ball valve that's located below the PRV. But it comes back eventually.

I suspect the valve is going bad and needs replacing. What's your thoughts ? enter image description here

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3 Answers 3


This Old House has a video explaining this. You need a potable water expansion tank.


The pressure relief valve is doing precisely what it is designed to do: Release water when the pressure is too high from heat expansion.

  • 2
    Did you see the photos I posted ? Isn't the blue tank on top of the heater exactly that ?
    – Lukas D
    Dec 1, 2023 at 15:57
  • 1
    It looks like one. But in the video, they put the expansion tank on the cold side...not the hot side. And he's a master plumber so I assume he did it the right way. Although I can't reason out why that would make a difference! You said the dripping stopped if you manually release some pressure...is there a way to test that the expansion tank is actually working? Dec 3, 2023 at 5:45
  • @SteveWellens The OP's expansion tank as seen in photo is also on the cold side, although it's a bit hard to see that.
    – Armand
    Dec 31, 2023 at 22:19

You may have a bad valve.

Or you may have a bad expansion tank and a good valve.

Or both may be bad.

Or the expansion tank may not be all bad yet, but might not have enough air in it. Since it SHOULD hold air pressure that's not a great sign, but a careless install might have left it with too little pressure - but if it's not been leaking for 9 years and started leaking now, that points to something changing now, which would mean a leak if the air pressure is low (but usually any leak eventually results in water on the air side.)

Your troubleshooting so far does not provide data to disambiguate which has failed.

A peak recording pressure gauge would show if the pressure is or is not going above the setting of your relief valve. If it is not, the valve is bad. If it is, the valve is just doing its job.

Shutting off the water heater, and the water supply to the water heater, and releasing all pressure on the hot water system would allow checking the pressure in the expansion tank. A quick blip of the Schrader valve with a tool (screwdriver or similar) before releasing pressure on the hot water would let you know if air or water came out (then you'd release pressure on the water side and check/set pressure on the air side, if not failed.) If water comes out the air valve, you need a new expansion tank. Don't ruin an air gauge by putting it on before checking for water. Per your measurement of "normal pressure", it should be about 70 PSI air with no pressure on the water side for best effect.

  • Thank you for the thorough explanation. I blipped the valve on the exp. tank and no air or water came out. It also sounds solid/full, not hollow when tapping on it. This might require a plumber call, as I'm not sure I want to mess with replacing/recharging the tank myself.
    – Lukas D
    Dec 1, 2023 at 18:02
  • It's not a terribly difficult job, but your call. Check the thread size, buy a new tank, and some pipe dope or tape if you don't have them. stop by an air pump on the way home if you lack a decent bicycle pump or compressor, shut off the heater and the water supply to it, open the faucets, drain a few gallons from the bottom, unscrew the bad one, apply dope or tape, screw in the new one. Water back on, bubbles out of hot water faucets, turn heater back on. Or you could try pumping air into it (as in last paragraph of answer) before buying a new one. It holds, or it doesn't.
    – Ecnerwal
    Dec 1, 2023 at 19:06
  • Ok did more research and you're right. This doesn't seem bad at all. I went ahead and repressurized the tank, we'll see if it holds after few days. as a follow up question, I checked the pressure just now, it seems to be going up as the water is heated, which I'm assuming is normal, what sort of fluctuation should I be expecting inside the expansion tank?
    – Lukas D
    Dec 1, 2023 at 19:59
  • With pressure on the water side, whatever that pressure is. The point of releasing pressure on the heater (not draining the whole thing) to set the pressure in the first place is to set it so the tank is essentially empty of water, full of air, at your service pressure, giving it maximum room to accept expansion above the service pressure, which should keep it below the release pressure if it's properly sized.
    – Ecnerwal
    Dec 1, 2023 at 20:07
  • Just wanted to say big thanks to you. The expansion tank never came to mind, and after some research, I feel confident I can replace both the tank and TP valve myself if the pressure doesn't hold up. Big win. Thanks again!
    – Lukas D
    Dec 1, 2023 at 22:29

You surmise correctly.

You need to replace the valve. When the water is heated and the pressure builds it allows water to "weep". It will only continue to get worse.

Age makes it suspect and all other conditions do not indicate that it is performing as needed.

  • Thanks for your confirmation. Now, I'm trying to devise the best way of replacing this valve and preserving the "contraption" of piping. Any insights of what the best way of doing this would be and maybe coming up with a solution so any future work on the valve is easier ? It seems to me that I might just need to rebuild the whole intersection since I won't be able to cleanly unscrew what's there now ?
    – Lukas D
    Dec 1, 2023 at 14:46

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