The old mechanical TRV in my draw has 2 to 3°C between fully open and fully closed, the electronic type you hear slowly altering the flow rate either due to room warming up or cooling down, or the programmed time for a change in room temperature. But when they exercise at 12 noon on Saturday they take around 8 minutes to cycle between fully open and fully closed and back to required setting, but each time you hear them run otherwise they only run for a few seconds, this is the whole idea of analogue control, everything slowly opens or closes and increases or decreases until the boiler is at minimum output and only then does the boiler cycle off/on, and not only does the mark/space ratio change, but also the total length of the mark/space cycle.
Today the word seems to be algorithms, we are told boilers have algorithms built in, and TRV heads have algorithms built in, and thermostats have algorithms built in, all designed to reduce hysteresis and running costs, but we rarely know how these algorithms work.
So one boiler may measure the temperature drop and adjust the output so the return water gets cooler as the output drops but not all, some only way to do that is for some thermostat connected direct to the ebus. And as the user it is hard to see how much if any the use of modulating thermostats help.
The idea of OpenTherm is good, but as to if it works better than using just return water temperature not so sure, one problem is so often the careful design is compromised by some other cleaver device where they don't work in harmony with each other, for example this
thermostat which is on/off and starts to cycle off/on before it reaches target temperature to reduce hysteresis, very good idea with an on/off boiler, but messes up a modulating boiler.
There are other ideas which when started were good, but not really suited for a modulating boiler, as the user we think the installation is designed for the home, but so often errors are made, and it is not helped because it would cost too much for a perfect system so we work with a compromise.
I was told as far back as I could remember you don't put a TRV in the same room as the wall thermostat, I was also told the on/off thermostat should be fitted in a room normally kept cool, on ground floor, with no doors to outside and no alternative heating. But that room rarely exists, so we need to think about what we are doing, not blindly follow rules which often no longer apply.
So I thought about it with mothers house, what I wanted is for the hall to recover quickly after front door has been opened, but to then slow down before is switches off the wall thermostat, and the TRV will do just that, it transformed the heating system adding a TRV to hall radiator.
We were also told one radiators to allow water to always circulate, often bathroom towel rail, but that was before we used by-pass valves and modulating boilers.
Theory says lock shield and TRV should be fitted to all radiators, but kick space fan assisted radiators also should not have flow restricted, but one odd one fitted in kitchen works well even if not following the general rules.
