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How does a Temperature and Pressure Relief Valve Work?

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  • How does a Temperature and Pressure Relief Valve Work?

    Maybe a little off topic, but today I replaced the T&P relief valve in the water heater because it had been relieving massive amounts of water for minutes at a time. Turning the thermostat so low I couldn't stand to shower in it did not solve the problem, so I replaced the valve.

    Along the lines of the recent science quiz topic I found lots of info online as regards when to replace them but nothing I could make sense of as to how the temperature sensing part works. Massive flow indicates temperature relief. The original valve was a Watts 100XL 210 degree 150 PSI. The new was some other brand, same ratings. So far no purging and I have the t'stat back up.

    But I don't understand exactly what they are doing with the dangling white rod that goes into the tank. It is floppy and has end play (new and old alike). And yes, it does seem to lose some play when I boil that sucker, but boiling is only 2 degrees warmer so... how does it work?

  • #2
    That floppy thing is the equivalent of the thermocouple sensor that sticks out into a pilot flame. I think a T&P valve has two relief mechanisms, both acting against the same spring and valve. One is a pressure relief valve, held seated by spring pressure. The second, the one that is activated by that floppy sensor, is more like an auto engine thermostat activator that reacts to temperature. Whether it uses a paraffin pellet or not, I don't know. Same valve, but controlled by two separate mechanisms. I could be wrong.
    A man studies until he's seventy and dies a fool.


    • #3
      IIRC The mechanism is similar to an automotive thermostat.That long spindly thing sticking out the bottom is the thermal unit.It is basically a hydraulic cylinder where the working fluid is a wax.As the wax heats up beyond the desired set point temperature it expands and pushes the piston in the cylinder,in this case just a metal rod,and forces it out of the cylinder.

      As it extends it pushes the relief valve seat open which is normally held closed by a heavy spring.There are two common modes of failure in these valves.One is the Silicon rubber valve seal gets torn or damaged and the valve just leaks water out the relief constantly,but more so when the water is hot(at control thermostat cutoff)

      The other is when the rubber seal between the thermal unit cylinder and rod fails and the wax leaks out.The first failure mode is an inconvenience,the second is an explosion waiting to happen if the control thermostat also fails and the pressure builds beyond the rupture point of the tank(boiler explosion)

      EDIT-Ponder Creek beat me to it
      Last edited by wierdscience; 04-25-2013, 10:08 PM.
      I just need one more tool,just one!


      • #4
        I have been a homeowner well over 50 yrs and have never had one of those valves fail. I Always use a new valve when I replace a tank. Bob.


        • #5
          Weirdscience -- I may have beat you to it, but you explained it much more clearly.
          A man studies until he's seventy and dies a fool.


          • #6
            Anyone who has a hot water heater in their home should see this video produced by Watts Regulator


            • #7
              Ironic this subject came up when it did as I had just looked it up myself and still had a good link to the very answer of the OP's question in my recycle bin.

              Here's a very good description as to what it is and how it works.

              Home, down in the valley behind the Red Angus
              Bad Decisions Make Good Stories​


              • #8
                Hot water heater???
                If you have hot water why would you bother to heat it. :>)
                The OP had it right...

                In my racket hot water heaters are really called booster heaters.

                A leaking relief valve is not as dangerous as a homeowner who thinks that thread on the outlet of the TP relief is a convenient place to install a plug... The Mythbusters/Allstate commercial scenario is possible. The bottom of the heater tank often has an accumulation of crud which insulates the tank bottom and can encourage corrosion... better idea to just replace the TP valve...

                Just a crotchety old retired plumber

                ARS W9PCS

                Esto Vigilans

                Remember, just because you can doesn't mean you should...
                but you may have to