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sealing radiator connection

Bigtim Member Posts: 9
I removed several steam radiators (one pipe) to paint the rooms.  When I put them back, one of  them leaked at the big nut that attaches radiator to the valve.  What can I use to seal this connection? I was considering pipe dope or plumber's putty. Anyone know what works best. ?


  • Jamie Hall
    Jamie Hall Member Posts: 21,541
    The union?

    I presume you mean the leak is visible around the union nut.

    First thing, take it apart again and check the sealing faces of the union -- they should be smooth and clean.  No nicks, dings, burrs, whatever.  If it is grubby, clean both faces thoroughly.  If you have nicks or whatever, you may have to use a sealant.

    Second, carefully align the radiator with the pipe, and move it into position so that the faces of the union meet and are right together.  Unions can take up some misalignment, but very little -- and they are not good at pulling a radiator together with the pipe.

    Then put it back together and see if it still leaks.  It may be helpful to rock the radiator -- just a little -- while you are tightening the nut.  But it shouldn't need to be reefed on that hard.

    If you still have a leak, I suppose pipe dope or, if you really have a problem (a big nick or something like that) try form-a-gasket or red silicone...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Big-Al_2
    Big-Al_2 Member Posts: 263
    edited October 2009
    RTV Silicone

    I have one radiator valve that will drip at the union, no matter how well aligned or tight it is.  I put a thin coat of automotive-type RTV silicone sealer on the mating faces of the union (not the threads) tightened it up and let it harden overnight before turning the boiler back on  It seals just fine, is good up to several hundred degrees, and comes apart and cleans up really easy if you ever need to take it apart again.  I use silicone RTV for a lot of bonding situations where I want it to stick . . . but come apart without damage at some later time. 

    A plumber friend of mine uses pipe dope on every union, threads and all . . . just in case.  Of course it makes subsequent work on it a bit of a challenge.
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,575
    leaking union

    i have had good luck with a steel-wool cleaning of the mating faces of the union. make sure no traces of steel wool particles are left. sometimes the riser can sink down, when disconnected, and cause a crooked joint. levering up the valve, while rejoining is usually  successful. maybe your pressure is too high?--nbc
  • Bigtim
    Bigtim Member Posts: 9
    It worked

    Guys,  Thanks for the help.  I removed the radiator, cleaned off the mating surfaces with Scotchbrite, put on some pipe dope, make sure I had the alignment good, and put it back together.  No leak !!  I really like the idea of the red RTV alot, I've used it on cars for years, just never thought of it. But, I had the pipe dope so I tried it first and it worked. 
  • Big-Al_2
    Big-Al_2 Member Posts: 263
    edited October 2009
    Watch your sight glass

    My only reservation with using pipe dope on a union is that some of it has bound to wind up inside the pipe and on its way to the boiler.  I'm not sure if there would be enough oil in it to disrupt the boiling and make you skim . . . but I'd look for a stable water line in the sight glass after a few days and make sure all is well.
  • Bigtim
    Bigtim Member Posts: 9
    using pipe dope

    Thanks for the info Big Al.  In your earlier post you said you plumber friend used lots of pipe dope. I guess this led me to believe it would not cause any problems. I only used a very small amount, just a thin coat on the mating surfaces.  So, I think I am OK, but in the future I will use the RTV. 
  • Big-Al_2
    Big-Al_2 Member Posts: 263
    edited October 2009
    Probably not a Problem

    My friend does mostly industrial water and air piping . . . not steam. Putting pipe dope on the threads won't help the union seal.  It will only serve to make it harder to take apart and re-use . . . but he's the same guy who uses pipe dope and 10 wraps of Teflon tape on every threaded connection . . . seriously.  (He claims that the imported pipe and fittings that he's stuck using aren't very round.)  His jobs look a little funny, but he never has leaks.

    If I didn't have any RTV on hand, I'd probably try using pipe dope too.  I doubt there would be enough released to make a difference . . . but I'd check for a steady water line anyway.
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