Wouldn't it be the valves themselves closing down a bit? The temperature in the room might feel nippy to you, but the stat' might not think it is that cold. These valves never close off completely when the settings are selected, they just close down a bit to restrict the flow of hot water to the rad. This is also borne by the fact that loosening the collar of the stat' head opens the valve so the water flows un-restricted.
It might also be the stat on the boiler is set to low for heat you want at the radiators.
Funny you asking that, but I spoke to a plumber during last winter, and he said it's better to install a second smaller rad' in a room, rather than replacing small for large. His point was to put a second radiator in a position in the room that is nearly opposite the existing one. This gives the circulating air two heat cycles back into the centre of the room. So, rather than just one rad' trying to heat all the room, the smaller radiator acts as a sort of small booster, I suppose. The second radiator doesn't have to be as large as the main one, in fact, he says half the size is always about right, and it also doesn't need a thermostat valve because the main rad' fitted with one will keep the temperature regulated anyway.
I don't know if this actually works in practice, but he's a great plumber and has never had complaints from anyone, not that I know of anyway. His systems last for years without major over-haul, and he's sought after by many companies for sub-contract work and his problem fixing skills. So he might have a good point in this technique of fitting a second smaller radiator, rather than replacing.
Perhaps nippy was a little vague, it was actually uncomfortably cold. If the air temperature feels uncomfortable how are you supposed to warm a room up.
If I loosen the collar I assume I am losing the control over the temperatute so defeats the object for fitting the TCVs in the first place.
I've tried various boiler settings as it is a new unit (getting used to it) but I'm thinking that if opening the collar on the valves heats the rads nicely then it is down to the valves shutting or restricting the water flow.
I've tried swapping valves but no change is apparent; I might try a better quality valve as I'm sure all the ones fitted are cheap devices.
It sounds very sensible (a lot more sensible on the face of it than floor mounted TRVs - I see the OP says TCV's, I guess it's the same?), but the pipework would be daunting for me; concrete floors, all top/down pipes, the extra piping would have to be exposed etc.
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