On Sat, 25 Nov 2006 15:58:00 +0000, Graham wrote:

I often wonder how normal TRV's can work effectively, as their sensor is so
close to the source of heat.

I have seen old ones like this
http://myweb.tiscali.co.uk/infoweb/TRV.JPG
but usually not in domestic situations.

Does a design with an external vial perform sufficiently better
to make them worth considering, in spite of their aesthetic problems?

I think the point is that _any_ TRV is a huge improvement over a
non-thermostatic valve. The type in the example you show is usually
specified where the TRV itself is likely to be covered by curtains or
suchlike so as to make it almost completely ineffective.

For a remote sensor to give better control over room temperature it
probably ought to be on the far side of the room. The Honeywell
system with even more aesthetically-challenged motorised TRVs is a
step in this direction.