Pipe dope on steam heat fittings?

[RonABC]

I recently bought a house that has a one-pipe steam heat system. It was a vacant bank-owned property that I bought as-is, and I will be renting the property out. I never had steam heat before. The heating system works well but two or three of the radiators have a very slight drip where the radiator valve connects to the radiators. I just noticed it today when I moved the radiator covers to paint them.

I briefly tried tightening the fittings with a pipe wrench, but that didn't seem to work. I'm thinking the next thing I'll do is turn off the heat, loosen the connections that are dripping, clean them to make sure there is no grit or dirt on the threads and contact surfaces, and then tighten them again.

But, I am wondering..., would it also help to try adding a little bit of pipe dope to the fittings to try to help assure that they will seal completely? If so, is there any special type of pipe dope to use that is better for steam heat fittings?

My hunch is that no pipe dope is required or recommended, but I thought I'd ask.

P.S. I am going to also try posting this question at

is a website that someone here suggested to me in the past. I also bought a book there called "We Got Steam Heat!", but I am only up to page 31 so far so I don't know if this question is answered later in the book.

[Ed Pawlowski]

We always use pipe dope or tape on steam joints.

[Jeff Wisnia]

You HAVE to use some sort of sealant on nearly all kinds of pipe threads save for a few known as "dryseal threads" which are generally made of brass, used for gasoline and other solvents, and seal by deformation of the thread metal itself.

If you look up a drawing of the thread form for NPT pipe threads you'll see that there's clearance between the apex of the male threads and the "crotch" of the female threads. That clearance will create what's called a "spiral leak", unless it is filled with some sealant, or you overtighten the joint to the point of metal deformation and get real lucky.

Jeff (Who is old enough to remember when string was commonly used as a pipe thread sealant.)

[RonABC]

[ransley]

Dope and teflon tape on top, no leak. That was from my repair guy that only does boilers in Chgo.

[Don & Lucille]

Dope and teflon tape on top, no leak. That was from my repair guy that only does boilers in Chgo.

[Phisherman]

The Teflon tape or dope makes sense. Both is like having a belt with suspenders.

[Wayne Whitney]

For a union, the mating surfaces should be kept free of the pipe dope. Pipe dope on the threads that bring the mating surfaces together is OK, in this case it is acting only as a lubricant, not a seal. If the union won't seal with clean mating surfaces, and the alignment is correct, it is time for a new union.

Cheers, Wayne

[RonABC]

[Stormin Mormon]

Rectorseal #5 works nicely. Paint it into the external male threads, and then crank the pipes back together.

String as a sealant? That's before my time. I'd like to hear about that.