Re #210606
Thank you Bruce, that does help. 
In my pre PRT I have not detected such a flow. There are three routes in as far as I can see Two from jiggle valves and one from the water rail local to the stat or more probably the stat itself. Until I get better access I can't confirm that.
I have seen bubbles come up into the expansion tank once before not long after having to top it up (part of the reason why I'm investigating all this is to overcome the potential drips from hose joints).
As the 4-way "T" arrangement does not seem to include a "jiggle valve" whereas the "up and over hose" does I expect to see that flow Bruce describes unless there is another valve "somewhere" that I can't locate/define.
So if anyone else can confirm what Bruce has reported that would be good.

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