During the night, the TP valve on our electric water heater went off. The noise, which sounded like a pressure cooker releasing pressure, woke us. It released a lot of water, but I have don't know why it happened. Can you tell me why and how to keep it from happening again?

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Originally Posted by ljhoward

During the night, the TP valve on our electric water heater went off. The noise, which sounded like a pressure cooker releasing pressure, woke us. It released a lot of water, but I have don't know why it happened. Can you tell me why and how to keep it from happening again?

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Hi howard and Welcome to The Plumbing Page. At AskMeHelpDesk.com. T&P valves can open up for several reasons. All of which MUST be checked outr.
1- High house pressure. Do you have a pressure reducing valve (PRV) installed. If so what's the house pressure? The average is 45PSI.
2- High temperature. That's when the water turns to steam . That's what you describe in your post.
3- A faulty T&P valve. I don't think so.
It sounds to me like one of your thermostats went bad and powered up a element that produced steam. Shut the power off to the heater and either replace the thermostats or call in outside help. Good luck, Tom

We don't have a prv, but when my husband bought the t&p, he got one. However, the plumber @the store said it was OK to put the prv on the water heater, but I read online that you should never do that. So, before we do it, which opinion should I take?

For openers you just have to know what kind of PSI your house has, Have it checked ASOP. The house PSI should run between 40 and 80 PSI with the average at 45 PSI. A pressure reducing valve(see image) does not install on the hot water supply. It installs where the city water enters your home and services the entire house. If your house PSI is high enough to trip the T&P valve then it's way too high. In time it will damage faucets and put a strain on your water pipes and fittings. Have it checked, install the PRV in the proper place and set it at 50 PSI. Good luck, Tom

Just to add to Tom's post.
Purchase a pressure gauge that attaches to a outside faucet to find out what your water pressure is.
Just screw on to faucet and open faucet. About 10 or $15 at Home Depot or Lowe's.

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pressure cooker releasing pressure

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However, that makes it almost a certianty that it is a defective thermostat.