I see what you mean, I thought I'd mentioned unvented in the posts.. sorry. I think 'unvented' is a given nowadays though
Then why are you asking about the secondary return, if as you say it`s the norm in your world why are you asking how to install it??
Does anyone smell that...
Think it`s Bulls**it..
You did mention unvented. I apologise, you are not sure on the secondary return and how it works is my take on it. Sorry to burst in....