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Water Heaters 101> Temperature/Pressure Relief Valves

What you'll find on this page: Every tank heater has one, but few people do what they should with them to keep themselves safe.

On the top or side of every tank heater, residential or commercial, sits a safety valve. Its purpose is to open and vent if the temperature or pressure inside the tank passes safe limits. Commercial ones are fairly robust, but about a quarter of residential ones fail every year.They should be tested once a year bT&P drain line plumbed uphill

y pulling up on the handle. It should let water flow freely, and it should stop when you let go of the handle. Possibilities:

T&P valve

  • It won't open
  • It dribbles
  • It drips

Sometimes, if they drip or dribble, banging on the top of the handle with a hammer or pipe wrench will stop it.

The valves have a thermostatic probe that needs to be in the heater, so the valve itself should be screwed directly into the tank and not have any other plumbing placed between it and the tank. If the pressure becomes too great, the worst that will happen is that the tank or piping springs a leak somewhere. But if the temperature becomes too great, the heater will explode like a bomb and quite possibly demolish everything nearby. Both Mythbusters and the Watts Regulator Co. (which makes T&P valves) have conducted tests, and where temperature goes past the limit, the tank flies hundreds of feet into the air and lands hundreds of feet away and destroys the little shed it was parked in.

So residential ones should be tested once a year. Another issue is that many T&Ps have drain lines that sometimes run through walls and outside. Those should always run generally down and out. If they go uphill anywhere, then water can pool at the valve if it opens and it may fail to work when needed. The example above is from our Closet of Horrors. What usually results in this is when an old tank had a top-mounted valve, but a new one was either shorter -- likely the case here -- or had a side-mounted valve, and rather than do it right, somebody just connected the old to the new.

 

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