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Thread: Black pipe: unions & drip legs

  1. #1

    Default Black pipe: unions & drip legs

    Is it acceptable to use pipe dope on the male threads where the union comes together (I bet theres a nice name for that)? I've read not to put dope on the center faces of the union, but haven't been able to figure out about the threads. I see the unions in place in my basement have all three connections doped, but I thought I remembered reading somewhere not to dope the center threads.
    I don't fully trust that what was done before in my basement is correct, so can someone confirm which is the correct way?

    Also, currently there are no drip legs on the lines for my dryer or stove. Are they advised for these appliances? The furnace and water heater have them.

    Thank you.

  2. #2


    the mating faces of the union do all of the sealing, some plumbers dope the male threads just to lubricate them. Its usually not necessary to dope the mating faces but I have known plumbers including myself to use Blue Block on the mating faces from time to time just for piece of mind but usually I don't dope the mating faces.
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  3. #3
    Moderator & Master Plumber hj's Avatar
    Join Date
    Aug 2004
    Cave Creek, Arizona

    Default unions

    I usually dope both the union nut threads and the mating surfaces to give an extra degee of sealing.

  4. #4


    Thanks for the advice on the unions. That was my main concern.
    Any thoughts on using drip legs for the stove and dryer?

  5. #5
    Licensed Grump GrumpyPlumber's Avatar
    Join Date
    Jun 2007
    Licensed Grump


    I don't dope unions.
    I ALWAYS pressure test gas, the slowest leak can slowly add up to a mass of gas and as soon as it reaches a spark it can explode or trigger a fire.
    My thoughts on doping a union seal are that you might get a great seal, but as soon as someone opens the union to replace the fixture..the dopes breaks up in chunks.
    An unsuspecting homeowner might not know to clean those surfaces before tightening.
    "The biggest regrets we have in life are the chances we never took."

  6. #6


    Quote Originally Posted by BuffaloM View Post
    Thanks for the advice on the unions. That was my main concern.
    Any thoughts on using drip legs for the stove and dryer?
    drip tees are basically sediment traps to keep debree from clogging/damaging the regulator. some water heater manufacturers require this to maintain warranty. I did break a drip tee open one time in an apartment complex because the water heater was not getting gas, when I broke it open a large amount of slimy/watery goop came oozing out, after that it was fine. there was one good reason for the sediment trap.
    Borders First / No Amnesty / Protect the Trades


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