In article .com,
Don wrote:
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?

Right. If you refer to:-

http://i29.photobucket.com/albums/c2...PLODEDVIEW.jpg

the chrome tube with the thread on it screws into the rad. Use PTFE tape
on the threads - and you'll likely need a 'special' tool to tighten it.
There's usually a hex on the inside for this purpose and the hex key can
be bought from a PM, etc. Don't use grips on the outside as any damage
here may prevent a seal. Now the nut shown next to the tube, then the
brass olive, then the valve. When you tighten the nut to the TRV, the
brass olive crimps onto the tube to make a seal. It's called a compression
joint and is common in plumbing.

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!

If the old unit was identical you could have re-used that part - but as
you've found some are different fittings entirely. The type you have were
common some years ago and IIRC sold by HomeBase.

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.

What you need to do is saw a groove into the brass olive 'in line' with
the pipe, ie across the side of the olive - but not all the way through,
to avoid damage to the pipe. Then with a screwdriver twist the slot. The
olive should then snap off. Just keep your fingers crossed that the pipe
is long enough for the new style of TRV, as again this isn't standard
between makes.

Clean the pipe with wire wool - again from a PM etc - until it gleams. Put
the nut on first then the new olive. Make sure the pipe goes fully into
the valve - there is a shoulder to set the right depth in the valve.

If it's too long it will need cutting to suit - use a proper pipe cutter
again from a PM. If it's too short it will have to be extended. If the
latter is the case, post again for advice on how.

--
*A woman drove me to drink and I didn't have the decency to thank her

Dave Plowman London SW
To e-mail, change noise into sound.