There are TRV units where the flow rate is controlled as well in the same unit.
The same here, the operative word is used, I have seen valves stuck where the head is set at maximum, which means they do not do any work, and are not being used. Set around mid-way, they will move, most of mine have two setting, day and night, so will motor to open and closed when the program calls for a change.
If I take my wife's bedroom as an example.
View attachment 399753 She wants it on to get room warm before going to bed, but does not like it hot overnight, but wants it a little warmer in the morning to get dressed, so the valve will naturally move every day in winter, summer there may be days when the room does not drop below 17°C, so valve does not move, but unlikely to go even 2 months without moving, I know as on the odd day we hear the boiler fire up.
With the mechanical TRV, the start and finish temperature, when the valve first cracks open, to when it is fully open is around 2°C, this is the problem with them, set at 2.5 with a small radiator in a large room and the room will likely stabilise at 18°C, but same setting with a large radiator in a small room, and looking at 21°C with same temperature outside.
This means as the outside temperature drops, so does the room temperature. If anything, I want the reverse, on a bright warm day having the room at 17°C is fine, on a dull winter's day I want more like 22°C, I have heard about weather compensation, but I have not seen how this works with the TRV head? Or even wall thermostat? I assume it controls the temperature of circulating water, but my boiler is not that clever.
Living room the TRV heads are not linked to boiler, so if set at 24°C it will only heat the room if boiler running due to wall thermostat. The two radiators are at 90° to each other, so there is no opposite wall for the wall thermostat to be mounted on. And I don't want the boiler to run when the TRV is closed. So they are set on the high side. So
View attachment 399761I am careful to set the TRV a bit on the high side, and I check to see the target (21°C) is above the current (17°C) when the wall thermostat is likely to call for heat. The eQ-3 TRV does not show current, but I do not have them in rooms where there is a wall thermostat.
Since Brexit it seems the cost of a TRV head has shot up, the eQ-3 Bluetooth version, I got 5 of them for £15 each in 2019, today the manual version
View attachment 399765£22.28 but there are other makes,
View attachment 399766for second one shown, so why anyone would fit a mechanical head I really don't know?