Just a point on possibilities for limiting the thermosyphon, I've been aware for some time of what appear to be limiting devices provided by Nuova Ricambi.

http://www.nuova-ricambi.de/shop/de/FAEMA/Blatt-13 and check out 591734, 591735 & 409607.

Anyone aware of or tried these ? It would appear possible that you could drill them out to suit although not quite sure how they would fit and be secured in the joint..

Derek, it is a good point. I haven't played with it though.