Best Of

It's not the Fosta specifically and I don't know that I'd call it fragile, but my experience is that Viega tubing of any variety has a tendency to leak if you stress the joint after it's been in service awhile. Maybe the collars open up over time or something, because re-crimping solves it 99% of the time. I did have one recently on a 3/4" Viega domestic line that was dripping (I rotated an elbow to meet the heat exchanger I was installing) and it took about 20 crimps in different directions with my PureFlow jaws to finally make it seal, but I ended up replacing it anyway because I didn't feel comfortable leaving it like that. Heck, maybe it's the Viega fittings? I really don't know.

@Alan (California Radiant) Forbes and @GroundUp, here is a better shot of what is under the insulator.

don't know why there are dielectric unions there. they are unnecessary and they leak.

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.