On Wed, 25 Sep 2019 21:57:21 +0100, alan_m
wrote:

On 25/09/2019 11:22, AnthonyL wrote:
A question about the central pin in the thermostatic head of for
instance

https://www.screwfix.com/p/drayton-r...ing-head/3279r

One of my radiators is not coming on. I've taken the head off and if
I rotate the top part (ie hot to cold) the central assembly doesn't
move though I can push it in with a screwdriver and it springs back
but seems to require more pressure than I'd expect the little pin in
the radiator valve could exert.

What is the ambient temperature of the room or within the TRV haed when
in your hand whilst testing it? If its close to 29C then the wax in the
TRV head will have already expanded close to its maximum to shut off the
valve. First mark the position of the mechanism and then put the TRV
head in the fridge for 5 minutes before checking the position again.

There will be a lot of force pushing back when you try and compress the
wax in the TRV head with your screwdriver if the wax has already
expanded due to a temperature approaching 29C

When the valve head is reinstated it may only fully open the valve if
the temperature is below 12C. Any temperature above this and the valve
will already be closing down and at, say, 20C it may be 50% to 75%
closed. If in this sate the radiator is not getting hot then the system
may not be properly balanced. In my experience balancing the system in
hot weather that have TRVs is difficult and if I do any work on my
system during the summer I wait until a really cold spell to fine tune
balancing.

It may be possible to fit the head scewed. On a similar head I've found
that I have to push it down quite hard to properly seat the valve base
before doing up the silver ring/screw.



I think the penny is dropping slowly. There are at least two types of
valve head. One which is purely mechanical so the user has to turn
the knob to allow more or less heat through the radiator. The other
is thermostatic so it attempts to stop the room from getting warmer
than it is set to. Is that right?


--
AnthonyL

Why do scientists need to BELIEVE in anything?