Fixing Small Leak in Black Pipe Gas Union

[Wraujr]

While inspecting my gas furnace I had my nose nearly touching the black pipe gas union between main run and furnace when I caught whiff of gas odor. I can't smell it in the room or even 6" away. My nose must be almost touching the union to detect the smell. Suspect this leak must be very tiny so I expect soapy water won't detect. I tried putting nose on other unions and joints and absolutely no whiff of odor, so I must assume there is a leak.... Not even sure how long it may have been like this..

What is best way to attack. Should I start with a simple quarter turn on the union nuts?? or does it need to be completely disassembled and redone. The union has been in place for 14 years and is located between the shutoff valve for furnace leg and the furnace.

Any experience appreciated.

 

[Dunbar Plumbing]

I would try tightening and if that doesn't help, replace it.

Do not use dope/sealant on that union facing; it's not designed for that.

Sometimes there are imperfections in the ground joint facing that cannot be sealed by tightening.

And soap testing will detect even the smallest leaks.

 

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[Wraujr]

Never worked with black pipe gas union. Typically is there some form of washer (copper, rubber, etc) between the two mating surfaces or is it truly black pipe-to-black pipe (albeit machined surfaces). I didn't think black pipe was as soft as say brass or copper... The reason I ask about washer is that if I disassemble union, I tend to assume I would need to reassemble with a new washer/gasket that has not be formed from previous assembly.

 

[Jadnashua]

Unions like that sometimes never seal properly if removed then reassembled. There is no washer, it is metal to metal, and if tightened, it can leave a (hard to detect) ridge, and you'll never align it exactly the same way, so you have a (very small) depression crossing your primary sealing point when reused. Try tightening it, if that doesn't work, I think replacement is the next step.

 

[Gary Swart]

The surfaces that seat together are copper faced and there are no washes, gaskets, or O rings. I suspect that if tightening will not stop the leak, you need to replace the union (both male and female sides) and take care that the two surfaces are aligned when you assemble the joint. If they are not aligned, they may or may not pull together perfectly straight when you tighten them and if they do not, the joint will leak. If you have the alignment right, you should be able to hand tighten the big nut easily quite a bit before using a wrench to finish up. I prefer old fashioned pipe dope on threaded fittings, but nothing should be put on the tightening nut or the inner part of the union.

 

[Wraujr]

Thanks for all the advice. I just had the HVAC tech out to the house for an A/C Freon check (which is why I was "nosing" around furnace/air handler in first place), so he smelled it and put soap solution on union and saw bubbles. He used two wrenches to tighten coupling (not sure if it was even 1/4 turn) and bubbles stopped. He was surprised that I even detected it and I told him it was coincidence that my nose was that close. Thanks again.