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pressure relief valve

troyz Member Posts: 7
3 months ago the pressure relief valve on my boiler started leaking and would not close completely. I tested and replaced the expansion tank and PRV with the same Watts number valve and PSI.

Fast forward to today. The pressure relief valve is now opening when the heat is on often. This drops the pressure of course and then the system refills. The PSI is only around 20lbs. I've tested the expansion tank I just replaced a few months ago by dropping the system pressure to zero and it measures 12#.

The PRV doesn't shut off on its own either so I'm thinking I need to replace that again. Also system sounds like it has a lot of air in it. I'm guessing related to the constant opening of the PRV and drop in pressure. It is a high efficiency peerless boiler 8 years old with 2 zones. There is not a water heater connected. Also checked the zone circulator motor is functioning. I've turned off the boiler for now until I get this fixed.

My plan is to bleed all branches using spigots at boiler, replace PRV again, bleed radiators, Any other ideas of things to check? My thinking is that since the gauge doesn't read over 20 PSI and the PRV still opens it has to be faulty?


  • heatpro02920
    heatpro02920 Member Posts: 991
    edited April 2014
    we get this question often

    First we need a little more info maybe a picture..

    Your problem can be from a few different areas- But first make sure it is actually going to 30+psi, if it isn't getting that high than it is a bad PRV opening too soon{run the system through its paces, check temps run pumps}, make sure your PRV is installed vertically with an air gap, I see prv's facing down or sideways all the time, that is wrong...

    next if all of that is ok, and the boiler is indeed going over 30psi, then you can either have First-"new water" coming in, this can only come from 3 places, one is high pressure piped into the boiler, this is very rare, I have seen it though, someone puts a hose end on a boiler drain then connects that to faucet and it over pressurises the boiler...

    You can also have a faulty pressure reducing valve, this is the valve assembly that allows water to feed into your boiler, it turns your "street" pressure of say 60psi down to a boiler safe 12-18psi, if these are stuck open or "leaking by" they will over pressurize your boiler...

    Another problem could be a faulty indirect water heater , flate plate HX, or tankless water heater, these devices share a water bath with the boiler and have 15psi boiler water on one side and 60psi domestic pressure on the other, if there is a pin hole in there the 60psi will bleed into your boilers water supply and over pressurize the boiler...

    Another potential cause is a faulty expansion tank {or undersized/improperly installed}, If it has a broken bladder, is not properly charged {the pressure in the bladder should match your feed pressure}, ect.. Hot water takes up more space than cold so it needs to go somewhere...

    Next is a rare one, you could have a pump/circulator causing system havok, if it is installed in a bad spot a pump can create suction on its supply side and suck water in from the feeder, if it is installed wrong and large it can raise the boiler pressure and open the prv... This is rare, but it happens...

    Last make sure your boiler isnt getting too hot, you dont want a 250 degree high limit, lol...

    So if I were going to your house for a service call, I would first make sure the unit was indeed going over 30 causing the prv to open, I would plug your prv for a second {I am a pro, I don't recommend you do this, I can open the boiler drain to relieve pressure if need be, but some prvs are leaking and wont allow you to test the system without changing them and I dont want to abuse a new one, sometimes they open and dont close}...

    Once I seen it go over 30, I would set your water feed to 12psi {or what ever psi your home needs, more floors more psi}, set and check your expansion tank, then shut off your supply water the valve before the water feed}, if it over pressurized again with all of that done, I would move onto dhw, check tankless or indirect by shutting their supplies off, if that isnt the issue and your piping and cirs are up to par with nothing else out of the ordinary {boiler isnt running to 250 degrees}, I would call a ghost expert because that is all that can be wrong...

    Most of the time it is a faulty water feed or expansion tank, then you will sometimes find a faulty dhw system, everything else is pretty rare...
  • troyz
    troyz Member Posts: 7
    other factors

    Thanks for the response. I haven't seen it go over low 20's for pressure so I'll pickup a new one. It is installed vertically.

    Since this is the second PRV to fail within a year are there other factors to consider other then pressure? The first one just leaked and the expansion tank was bad.

    I haven't paid to close attention to the temperature so I'll look at that next.
  • RobG
    RobG Member Posts: 1,850

    With the system empty or the tank removed, the pressure in the tank should be set to the fill pressure of the system. If you only have twelve pounds in the tank and the fill pressure of the system is twenty, you are not leaving a whole lot of room in the tank for expansion. How many floors is the house and where is the boiler located? This will determine what the system pressure should be.

  • jonny88
    jonny88 Member Posts: 1,139
    relief vale

    by any chance do you have an indirect piped of your boiler
  • dooododoo
    dooododoo Member Posts: 25

    Another potential cause is a faulty expansion tank {or undersized/improperly installed}, If it has a broken bladder, is not properly charged {the pressure in the bladder should match your feed pressure}, ect.. Hot water takes up more space than cold so it needs to go somewhere...

    Does it do any harm to get a bigger expansion tank? (Besides paying extra for a larger tank.)
  • Zman
    Zman Member Posts: 7,569
    No harm in over sizing an expansion tank...
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein