Rinnai Leaking Help

Your cold inlet is the blue line entering the combi unit above and to the left of the joint in question.

The leaker is on the space heating return. It is called a dielectric union and it uses a flat rubber gasket, this is an extremely common problem with dielectric unions. Not sure what sort of hydraulic separator is in that insulation block, but eliminating the dielectrics would be a good move if possible. If not, replace the gaskets for $10 and wait for another leak to appear.

i'm not so sure that you wouldn't find the rest of the union if you take the insulation off. no need for a dielectric union in to plastic pipe even if you think you need it on a closed system.

is there a name on the tube? those look like viega fittings of some sort.

i think the 2 halves of that styrofoam insulation pop apart to expose the manifold. i'd look at what sort of fittings go in to the manifold. it looks like it might just be npt or that might be a npt to bsp adapter. i'd make a short piece of copper to screw in to the manifold with an npt adapter to screw in to the manifold and adapt the ell to the pex but if you don't know how to sweat pipe black iron nipples and an ell or pex fittings for all of it might be easier.

That is Viega Fosta Pex and is no longer made. It appears to be a 1.25" nominal NPT dielectric union and a 1.25" X 1" black reducing bushing and then a 1" MPT X PEX adapter. It's not exactly clear what is leaking, but if it's the top side of the elbow below the union, you can zip the collar off of the tubing with your Dremel and carefully remove the vertical tubing from it. If it were mine, I'd replace the union and bushing with brass ones then screw a new adapter into the bushing (or buy a 1.25" MPT X 1" PEX adapter and skip the bushing) then run a new piece of oxygen barrier PEX down to the existing elbow. Any crimp system will work here, whether copper rings, stainless collars, or cinch clamps. Be very careful though, that Viega tubing has a tendency to leak at the joints after it's been disturbed and may open up a can of worms for you. You can usually re-crimp them to seal, but that requires the same collar type crimper that was used originally. You cannot use a copper ring crimper to crimp the stainless collars. 9 times out of 10 when I work on these Viega systems, a new leak appears that wasn't there before and I have to re-crimp a few joints.

@lindylex the copper ring has a slightly larger outside diameter than the stainless collars, so using the copper crimper on the collars doesn't squeeze them all the way. If you have the copper ring tool, you can just replace the collar with a ring. The FostaPex has an aluminum layer on the outside which requires a stripping tool or a steady hand with a knife to strip it down to the black tubing, which is just a standard PEX sizes and will allow any crimp/cinch type of fitting.