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

I fitted all mine at the bottom, and mostly on the return pipework
(which is cooler). Positioning at the bottom means they are in the
draft towards the bottom of the radiator, and so pretty accurately
reflecting the room temperature. The thermal sensor is also well
insulated from the pipework, so there's no heat conducted to it.
I was also sceptical, but the scheme does seem to work surprisingly
well.

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

I used the Drayton/Invensys TRV4. The instructions cover the remote
sensor version too (which I didn't use). It says the sensor is
ideally fitted at a height of 1.5m on the wall away from the radiator,
or on the wall under the radiator.

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

Also the fragility of the capilliary pipe (probably unsuitable if
small children around). The first 6" of capilliary pipe must not
be clipped as it moves where it enters the valve head.

It probably does work better, but I'm not sure to what extent the
extra accuracy matters. I used to have a proportional control gas
wall heater (Drugasar). That used a vial which suffered from self-
heating. I pulled it out to hang below the heater, and it then
maintained the room accurate to 0.1C (which was the limit I could
measure). Interesting though this accuracy was, it's not something
you can "feel".

--
Andrew Gabriel