New boiler, last rad heats up then goes cold

FataVerde

ThisIsWeird said:

I'll try and post some photos and dims later.
FV, if you hold your removed head upside-down and turn the control, can you see the 'piston' inside move up and down too? It should, by a good 6mm or more.
And, on a rad with a head still fitted, could you also turn the control from min to max and back again a few times, and report what you 'feel'? It should feel more loose and easy to turn from 5 down to around 2-3, and become quickly obvious it's firming up, and slightly increasingly so as turned down further. Does it?

The piston in my broken TRV head does not move up and down (that's what made me realise it's broken) although when connected to the rad (the only way I can turn the rad off) it still moves easier between 2 and 5. I have to struggle to get it to 0. The functional TRV head moves with ease to 5 and it does require a bit more pressure but hardly noticeable to get it to 0.

ThisIsWeird

FataVerde said:

They were fitted last April! Ridiculous. The others in the house have 'bulldog' as brand and they look like they've been there for 10years at least lol

Could you also, please, tightly wrap a strip of paper around the outside of that chromed thread, and mark the overlap point with a pencil. Measure and divide by 3.14159265358979323846264338

30mm seems to be the most common - almost considered 'standard' - but many are only 28mm.

If yours is 30mm, then there are seemingly numerous 'universal' heads that should fit.

I'm surprised at Kudox - a well-regarded make in the radiator market. And your photos show a decent-looking valve with no obvious shortcomings.

Could you try out the tests I asked about in the post above - when you are home and have time, of course :-)

(I have two makes of TRV. The 'Optima' looks similar to yours, is 30mm dia., and the pin sticks up around 6-7mm, but I wouldn't be too concerned about that last point as much will be 'play'. My other is a 'Westherm Four', old - over 20 years - and 28mm dia. It has a very skinny pin, but they still work!)

FataVerde

ThisIsWeird said:

Could you also, please, tightly wrap a strip of paper around the outside of that chromed thread, and mark the overlap point with a pencil. Measure and divide by 3.14159265358979323846264338

30mm seems to be the most common - almost considered 'standard' - but many are only 28mm.

If yours is 30mm, then there are seemingly numerous 'universal' heads that should fit.

I'm surprised at Kudox - a well-regarded make in the radiator market. And your photos show a decent-looking valve with no obvious shortcomings.

Could you try out the tests I asked about in the post above - when you are home and have time, of course :-)

(I have two makes of TRV. The 'Optima' looks similar to yours, is 30mm dia., and the pin sticks up around 6-7mm, but I wouldn't be too concerned about that last point as much will be 'play'. My other is a 'Westherm Four', old - over 20 years - and 28mm dia. It has a very skinny pin, but they still work!)

Hi, I measured around the chrome thread with my measuring tape and it seemed exactly 10cm/100mm. So the result of the division is 31.83 which probably means it's a 30mm. Measuring across the diameter with my rigid tape also seemed to give a 30mm result. This, plus the fact that it is only 1 year old would suggest 30mm. I plan to order this universal TRV on amazon and just return if it does not fit. https://www.amazon.co.uk/Thermostatic-Radiator-Thermostat-Replacement-Temperature/dp/B09MHYBDMK/ref=cm_cr_arp_d_product_top

ThisIsWeird

Almost certainly, yes, it's 30mm.
And most likely that new head will do the trick :-)