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The pressure release valve on my water heater will trip predictably after a significant amount of hot water has been used (e.g. dishwasher, laundry, long shower) releasing 16 oz or less of water. It only happens if a sizable quantity of hot water is used and the water heater as to heat up cold water, if the tank is sitting idle full of hot water it won't happen.

I've noticed that after using a lot of hot water, if you wait about 10-15 minutes then turn on the hot water there's almost an explosive blast of pressure as the water heater is heating cold water. Letting a faucet drip hot water usually prevents the pressure release valve from going off.

It's an electric water heater over 6 years old with a cold water expansion vessel. The temperature is set to about 110 and I've been slowly ratcheting it down to see if it affects this pressure build up, it doesn't.

What's going on?

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Your expansion tank has likely failed or isn't large enough for your hot water tank. Expansion tanks have a bladder in the middle that ensures it is half full of air and needs to be periodically checked to be sure it's properly pressurized. These bladders eventually fail, allowing water to enter the air side, which eliminates the ability to absorb the expanding water.

Replacing the expansion tank may be a simple process if it's just screwed on. With the water shutoff, you unscrew the existing tank, pressurize a new tank, apply some plumbing dope to the threads, and screw the new tank on.

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  • Sounds simple enough, what is entailed in pressurizing the new tank?
    – MyItchyChin
    Commented Mar 28, 2012 at 15:29
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    @MyItchyChin They have a valve on the air side of the tank, same as for a car or bicycle tire. Match the home water pressure if possible (you can get a pressure gauge to measure it at a garden faucet) or just pump it up to 60psi or so.
    – BMitch
    Commented Mar 28, 2012 at 17:23
  • Get a bigger expansion tank if possible. The one my builder installed failed in the third year, also at my neighbors'.
    – SteveR
    Commented Mar 28, 2012 at 17:36
  • @BMitch Thanks for the info! I put a tire pump on the existing tank and it registered 0 psi. I couldn't get it to pump up with a tire pump. Do you think I'd have better luck with a compressor or should I just replace the tank?
    – MyItchyChin
    Commented Apr 26, 2012 at 20:56
  • @MyItchyChin, when the bladder goes, you should still be able to measure some psi from the water supply before shutting off the water. Pressurizing with a failed bladder won't work since you're trying to pump up all the pipes in your home rather than half of the tank. My guess is that the valve itself has failed, maybe rusted from water passing the bladder. Best option is likely to replace the entire tank.
    – BMitch
    Commented Apr 29, 2012 at 12:44

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