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Trying to flush/clear T&P Relief Valve

bob young
bob young Member Posts: 2,177


  • D107
    D107 Member Posts: 1,849
    Was told to flush when boiler temp is highest

    Well I did my annual relief valve flush the other day, a quick mass of crud flew out, and surprise surprise it didn't re-seat completely. (a drop every 15 seconds or so afterwards). Re-flushed today for two seconds, some crud again, now drips every 20 seconds. Any point in flushing more? I don't want to let this go on too long. It's an old WM gas-fired. Relief valve is Watts M1 3/4inch. Valve is only a few years old but I know this is very common.

    Wish they would have put a (full port?) ball shutoff valve between the boiler and the Watts M1 so I wouldn't have to drain the boiler for the valve change. Can that be done? Any distance requirements between boiler and ball valve and ball valve and Watts M1? (see attached photo.)

    2. Also fortuitous that the original 3/4" B&G SA flow control valve for basement zone also seem to be leaking and rusting out. Can a shutoff valve be installed on this line as well?

    3. This system has an air vent atop the Amtrol Air purger. (Visible rust is old) Given that, do I need that air vent atop the boiler as well or should that old one be removed and plugged? (most visible rust is also years old) This is piped on the return with no serious air problems.

    I hope to replace this boiler in 3-4 years as other house projects subside and I can fully tighten the house envelope.
    Circs have recently been replaced, boiler combustion tuned, and runs well.


  • Big Ed_4
    Big Ed_4 Member Posts: 2,703
    No Valve....

    ..... Before a relief valve. What ever could happen will happen. Some one will shut it and leave it off. Install a better valve like a 174 if you flush them.

    You can install as many valves as you want as long as they can't isolate a safty valve or control.
    I have enough experience to know , that I dont know it all
  • Darrell_4
    Darrell_4 Member Posts: 79

    If you read the very small print on the tag attached to a relief valve it says that the valve is to be removed, cleaned and tested periodically. The rate varies from manufacture to manufacture but is usually 2-4 years. I don't know how you'd bench test a relief valve, and I know parts are not available for rebuilding the seats or springs and it would cost a forutne if you had a pro do it. Just replace the thing on a regular basis. The draw down of the boiler also gives you the chance to check/replace the airvents, the fluid quality, (the gunk you referenced), the fill-valve train, the expansion tank, the low-water cutoff and various other componets any of which probably contributed to the original problem in the first place.
  • Did you know...

    that you DON'T have to completely drain the system to replace items like relief valves, drain cocks etc..

    Step 1: close ALL automatic air vents.

    Step 2: Isolate the expansion tank if possible.

    Step 3: Bleed the pressure down to static ZERO. This could take a while if you have air in the upper parts of the system pushing back against you. If it does, its an indication that you need to go through and bleed the radiators anyway.

    Step 4: Pre dope the inside or outside threads of the replacement item.

    Step 5: Removed the defective component. Be prepared for a small loss of water (bucket, towel etc) As soon as the old component comes out, toss it in the buckect and quickly screw the new component in to place.

    Step 5: Restore all closed items to their original position and make sure the pressure stops where it is supposed to.

    If all goes well, you'll be done in about 10 minutes, and on your way upstairs to talk to the owner about the benefits of replacing thier old beast with a newer mod con.

    If everything goes to hell in a hand basket, you will end up having to drain the system any way, so don't try this unless there is a functional floor drain real close....

    I have done this hundreds of times, and never had a hand basket situation :-)


    The above scenario assumes that there are NO isolation valves anywhere on the system except the make up and expansiopn tank (typical).
  • D107
    D107 Member Posts: 1,849
    thanks Ed, Darrell, Mark

    Great tutorial Mark; of course this system has no isolation on the expansion tank either, just a zone gate valve downstream of it. I'm wondering what the downside of your method would be without that tank isolation--if the worst happened and boiler drained, I've heard on the wall that you can re-pump the tank in place....

    I also wondered if those on-boiler air vents are redundant--or worse than redundant by actually letting in air--when you already have an air vent above the air purger. I'm pretty sure in a new system on-boiler vents are never used.

    Great post by the way a year ago by you on 'hydronic myths and rumors' on air purging...



    PS My correction: it's a pressure relief valve for boilers, not t&p, which is for hot water tanks. (I think.)
  • D107
    D107 Member Posts: 1,849
    Bob, thanks for the comic image but I want to be sure

    that I get it. Is it the torrent of water exiting an open 3/4" pipe or possibly an orbiting expansion tank?

    If the former and one was alone and if the open pipe was aiming down, couldn't two 5 gallon buckets be filled and dumped in relay if a slop sink was a few feet away. Or is it that flood scene from the Sorcerer's Apprentice?


This discussion has been closed.