I will say they generally work well enough, but they must be influenced by the flow of hot water in the pipe, which is very close to them and hot. Not so much when the pipe has or is cooled, because they will be open and open or closed makes no difference to no flow.
I was surprised, here with an oil boiler will admit not perfect. However late mothers house, once set, they were spot on, within a °C.
The computer shows target and current, I set the lock shield valve by turning off, then slowly ¼ turn at a time back on until I could feel a little warmth in the feed pipe.
Once all set that way, then looked at the current and target, if current exceeded target then close a little, and if current always under target open a little. It did take a few months to slowly tweak the lock shield setting, but the whole system did work, far better than expected. Only one room with a problem, main living room, and that was due to the sun through the bay window causing room to over heat.
Before fitting electronic TRV heads I had walked into the room to find at 32°C when aim was for 20°C, after once set up, 24°C was the maximum it reached. The main thing was the radiator only got hot enough, so before sun came up likely sitting at 30°C to maintain the room at 20°C, so when sun hit the room, only needed to cool 10°C. Before when using a wall mounted thermostat, the radiator would heat to 70°C then switch completely off, so there was a massive hysteresis, the TRV heads got the boiler modulating as designed to do.
When the house was sold, new owners did not want the electronic heads, so all replaced with cheap wax type, but at this point all the lock shield valves now set, and we found hardly any difference between temperature control with mechanical compared with electronic except of course set at just one temperature.
The problem is a mechanical TRV head will start closing at 18°C and fully closed at 22°C and instead of saying 20°C it says *123456 which is useless, as one does not know which needs tweaking the TRV or lock shield, if the heating and ventilation engineer has set the lock shields to say 15°C drop across each radiator the home owner has a chance, but when I looked at late mothers house all lock shields had been left wide open.
Also there was no TRV in the hall where the wall thermostat was fitted, so impossible to set up, close the lock shield enough to allow rest of house to heat up, and then when front door opened hall would cool (specially since mother in a wheel chair so likely open for an extended time) and it would not recover, open the lock shield so hall would recover and rest of house cool.
Adding a TRV to hall radiator transformed the system, now TRV would open when front door opened, allowing fast recovery, but start to close before the wall thermostat would turn off boiler, so rest of house could also warm up. I was apprehensive as every book I had read said no TRV in room with wall thermostat, but it also said wall thermostat should be in a ground floor room normally kept cool with no outside doors and no alternative heating, really what home has a room like that?
I did fit two thermostats, one in kitchen and one in hall, in parallel so either could turn on boiler, however once the TRV was fitted in the hall, found kitchen thermostat not required.
