I've been going through severe problems with my Heatline Compact S30
combi-boiler and recently had an engineer out to it under warranty. He
noted that the TRV's the instaler had fitted were one-directional, said
this would cause knocking and stress on the boiler and advised that
they be changed to bi-directional. He also advised a room-stat which
the installer also neglected.

I have drained the system and bought new Mistral bi-directional TRV's
(with self seal tail). Here's where I am confused. The valve section
allows the tail to move freely and does not appear to seal at all. All
that is in the boxes are a two-way valve, two brass o-rings, two nuts
and the sliding tail. I bought from PlumbBase, but I found the same
item on ebay just now and it appears to have an extra brass part that
did not come with the units I purchased.
Am I missing parts from these or am I failing to realise how they work?

Also, the previous fitting had a female coupling on the bottom and the
copper pipes seem to be attatched to what I presumed was the old copper
o-rings! Unless I can get these off, the old fitting nut wont come off,
and I cannot fit the new parts anyway. Any help would be gratefully
appreciated. Thank you in advance.