I don't understand your fixation with pluming.
To me its just a visible admission that the boiler is not fully condensing all the water vapour.
I think you're right Tony he's fixated by plume.
Fitted HRUs on Baxi combis, got the photos to prove it. I was pleasantly surprised by the amount it heats the cold water. Oh & there was a massive amount of f.cking plume John!!! Especially on a freezing cold Scottish winter morning!!
As for feeding the 15mm heated supply pipe to a vented hot water cylinder, are you having a laff??!! C'mon get real!!