No it's not union to union, it's union - the valve mess - union.

I just connected a Rinnai hot water heater and have the same problem as lay3r3. How did lay3r3 stop the leak?

I'm sorry for not circling back around to provide the outcome. I pulled the fitting apart and cleaned the threads completely and redid it with dope that sets up rather than non-setting dope. I was a little shy before to tighten the fitting on the unit because of fear of over tightening so this time I snugged it up a bit more. I did put dope on the threaded component of the union but was extra careful to make sure that the area for the rubber gasket was completely clean of dope or debris of any kind.

All good now!

In the end, it was one of three things or a combination of all three...
1) had residual crap on the threads from the first failed attempt
2) the non-setting dope
3) too lose on the threads

Honestly I am pretty sure it's mostly item 1. Glad to be fixed though and the unit ROCKS

Thanks for the follow-up. I will clean the threads and look for dope that sets up. Home Depot didn't know what I was talking about when I asked for dope that sets up. I assume it is different from the typical tephlon pipe dope.

I noticed that when I tightened the threaded joint to the Rinnai heater, there was a small brass shaving that came off. It was like a thin strand of brass. That may be part of my problem. I will try clean up the threads on both connections (male/female) including any residual tephlon tape, then us the pipe dope.

Thanks again for your help!

Yes I had the same thing with the fine shaving. And I did get the dope at home depot. It was right next to the other dope but it says something about soft set.

That shaving is a small piece of thread that didn't get removed when manufactured.

Are these connections flared?? If they are , do not use any type of dope or tape.