My main question: should I do more debugging or is it safe to assume I just need to replace the T&P valve? My issue seems more extreme than most videos/articles about fixing a leaking valve and I want to know if I should investigate more serious concerns with the tank or plumbing?


NOTE: My heater has a pressure expansion tank that was installed.

I know very little about home improvement (first time home owner). I had my hot water heater (located in my garage in the corner on a small pedestal) replaced in March of 2021 by a professional.

Last night, a neighbor came to my door to let me know my garage was leaking significant water. I went down and my entire garage was flooded (~3 inches tall). The water was flowing full force from the copper pipe attached to the T&P Valve.

After shutting off water to diagnose, I noticed the T&P valve was fully opened (by itself). Which, after much googling, appears to be unusual. It seems like when it opens and doesn't close, it doesn't fully open (from what I gather) and should only let out a moderate amount of water (not

I closed it and restored water to the house, but it still has a steady drip (once every 15-30 seconds). The drop does not increase when hot water is being used, so it seems steady.

Is it safe to assume that the valve failed? Or are there more serious concerns that I should investigate further?

  • The expansion tank is to handle the normal small amount of expansion that occurs when water is heated up to 160 F or so. It is not designed to handle super-heated water. Nov 8, 2023 at 19:02

1 Answer 1


Yes, it is safe to assume that the T&P valve has failed and needs to be replaced. Even without the flood, a constant dripping is a sign of a problem.

However, that leaves open the question of whether there is actually a water heater problem. It is possible that the T&P valve failed independent of the temperature and pressure of the water heater - i.e., just a T&P valve problem.

It is also possible that the water heater had problems - e.g., a failing thermostat - which resulted in an actual overtemperature/pressure situation. If that is the case then the T&P valve would open, as it should to prevent a tank explosion, but that it then failed to close after relieving the overtemperature/pressure problem. If that's what happened then yes, you still need a new T&P valve, but you may actually need something else replaced - anything from a thermostat on up to the entire water heater.

If the water heater is more than 10 years old then it is worth looking at a replacement sooner rather than later.

  • To be fair, the Mythbusters showed in several episodes that it's incredibly hard to get a water heater to explode, even when explicitly and intentionally disabling all the safeties. That said, the safeties should, not must be fixed if the heater is going to be kept (not replaced). (+1)
    – FreeMan
    Nov 8, 2023 at 18:25
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    Another potential failure is the expansion tank.
    – pdd
    Nov 8, 2023 at 18:31
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    Having a T&P valve slightly leak after being opened is not terribly unusual if it's old and likely some debris/deposits preventing the valve from sealing completely. Is it possible that someone might have purposely opened the valve (whether a child's mischief, vandals, etc.)? It would seem unusual if it were found wide open as a failed valve that it would then go back to operating normally (except for the slight leak).
    – Milwrdfan
    Nov 8, 2023 at 18:56
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    If it "opened itself" I'm betting on actual reason for it to actuate. Failure tends to look like dripping that won't stop. The only thing that will kick one all the way open is a darn good reason from inside the tank, AFAIK.
    – Ecnerwal
    Nov 8, 2023 at 19:46
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    @MrHappyAsthma If you're a belt-and-suspenders type, pick up a water pressure test gauge when you get a replacement T&P valve, and attach it to the water heater's drain valve and open the petcock before you turn the water heater back on. It should show your water main pressure. Monitor it for a couple of hours after startup. If pressure climbs more than a few PSI, you should turn off the water heater and investigate. Example: homedepot.com/p/Water-Source-Water-Test-Pressure-Gauge-WSPGH100/…
    – MTA
    Nov 8, 2023 at 20:17

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