safety valve: hotwater tank, test if bad ?

Discussion in 'Plumbing Forum, Professional & DIY Advice, Tips & ' started by w1ljm, Dec 12, 2006.

  1. w1ljm

    w1ljm New Member

    Messages:
    19
    Location:
    boston
    I have been having problems with my gas hot water heater. The 40gal tank keeps setting off the safety valve (gals. of water on the floor). I have an expansion tank on the line and I tested it to make sure that there is not water coming out the air valve (none). I also tested the pressure in the tank when the water line has not pressure from the street or house and there is 15psi. The water heater is not set very high in fact my wife asked if it could go higher because there are times that she can run straight hot water and it is not very hot at all. Is there something else that I need to test? Something that I am forgetting ! Is there a simple test to verify whether or not the valve is good or bad? It seems to operate fine. I do not know if the spring weakens after a time. FYI the tank is around 6yrs old.
  2. Gary Swart

    Gary Swart In the Trades

    Messages:
    7,308
    Location:
    Yakima WA
    I have questions about the pressure test. May I assume you have a pressure regulator valve on the water supply line, and if so, it is either malfunctioning, improperly set, or your gauge is broken. The pressure after the PRV should be somewhere between 40 and 60 PSI, give or take a bit. The air pressure in the expansion tank should be set to match the PRV. It is this balance of pressures that make the expansion tank work. First thing to do is to make sure your gauge is working, then adjust the PRV to a workable pressure...say 50 psi. Then adjust the air pressure in the expansion tank to 50 psi. It all components are functioning properly, your home free. If the relief valve continues to release, put the gauge on a hot water faucet and test the pressure before and during the time the heater is heating water. If the pressure skyrockets during the heating cycle, I'd replace the expansion tank. Go to a neighbor and check the gauge first so you know that it is recording pressure properly. 15 psi won't operate a home water system.
  3. w1ljm

    w1ljm New Member

    Messages:
    19
    Location:
    boston
    What is the max pressure that you can put on in the expansion tank? I was afraid of putting in too much and blowing the Diaphragm. Is the correct step for filling the air pressure to first take the water pressure off the tank?
    Last edited: Dec 12, 2006
  4. therm exp tank

    it should say the max working pressure on the therm tank

    usually it says 150psi on a bigger one....
    I have never pumped one up that high...
    I dont think they expect anyone to....



    if you have 90 lbs in comming water pressure I suggest you

    pump it up to about 95-98lbs and it will work within a proper range....

    then if the pressure rises above 95 it will go up into the therm tank...

    its basically nothing more than a heavy duty innertube----

    if you have a prv valve......
    whatever you have the pressure set at, go about
    5- 8 lbs higher....




    of course its probably wise just to change that t+p valve....

    most of the time they usually dont re-set themselves....
    Last edited: Dec 12, 2006
  5. Gary Swart

    Gary Swart In the Trades

    Messages:
    7,308
    Location:
    Yakima WA
    Not get into a nitpicky argument about acceptable psi, 90 is too high for a home. That's why we use PRVs. The procedure for adjusting the pressure is to first set the PRV to the desired pressure. Then pump the expansion tank to the same pressure. As far a replacing the TP valve, that's not a bad idea, they only cost about $10.
  6. w1ljm

    w1ljm New Member

    Messages:
    19
    Location:
    boston
    Do you have to take the pressure of the water off the expansion tank first before you add air to the tank. Sorry I have never done this before, but the concept makes since. (The pressure on the water side of the expansion tank should be equal to the pressure on the air side of the tank.) I am not sure if I understand this correctly. If you know that your pressure from the RPV is ie. 55psi then your air pressure should be about the same. But in order to do that, I would think that you would have to bring the pressure of the water side of the tank down to zero by shutting the water off from the main shut off and also opening a faucet. Then pressurize the air side of the expansion tank to 55psi. Then turn the faucet back on then the main shut off.
    I would be afraid that I would blow the rubber diaphragm???
  7. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,304
    Location:
    Cave Creek, Arizona
    tank

    As long as the air pressure is less than the system pressure, it will adjust itself to the system pressure as soon as the water is turned on. If the pressure is low, more water will enter the tank until the air reaches the stabile point. If it is almost at system pressure it will take less water for that to happen. If the air pressure is higher than the system pressure the tank will have no function until something increases the pressure to, or above, that point. Once the water is turned on, the air pressure will always be at the system pressure, unless there is absolutely no air in the tank, or the static pressure is too high to start with.
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