On 16/12/2019 08:23, Chris J Dixon wrote:
David wrote:

On Fri, 13 Dec 2019 10:43:38 +0000, Chris J Dixon wrote:

My Bulldog TRVs are about 15 years old. One of them has been fitted with
an electronic control, but has stopped turning off.

Having put back the original thermostatic head, it still seems to be
stuck open, though the spindle appears to move freely.

Before the TRVs were replaced, I had regular issues with them sticking
closed, but this is a new one on me.

I wondered if a some contamination was blocking the valve movement, and
left it fully open for a while in the hope that it might flush through,
but no luck so far.

Any thoughts? I know that in a pressurised system I ought to be able to
quickly dive in and have a look once I have let the pressure out, but I
really don't want a sudden gush of black liquid in my lounge.

My understanding is that TRV is Open when the pin is fully up/out and
closed when pin is fully down/in.
So if the pin will move down freely ( and fully) but the valve remains
Open something is broken inside.

I find a WET/DRY vacuum cleaner an essential tool when fiddling with
radiators and plumbing , amazing how good they are in preventing water
hitting the floor or worse still the carpet.