Because, generally, they work perfectly well.
Do they? I have looked at many homes where mechanical TRV's are fitted, and the owner has just turned them up to max, as no idea how they work.
After removing the mechanical type, I played with them, I could not believe the span, from first starting to close to being fully closed.
I started looking at central heating in general when my dad's was not working very well, mine (open-plan house) worked reasonable well, so never really looked at it, why should I, if it works don't fiddle. Only problem was when we went from tungsten bulbs to CFL we found the living room too cool in the evening, so moved to a programmable thermostat.
Dad had a new fangled condensing boiler, which could modulate, some rooms freezing and others stinking hot, the main problem was a bay window which caught the morning sun, and I was getting readings of 32°C in some parts of the room, with the radiator still hot. But like I am sure many others I looked at the lock shield valve, and TRV and thought which one needs altering.
I tried to set by turning off the lock shield valve, then slowly ¼ turn at a time, back on, until one pipe got warm, then moved to the next, it worked better, but not A1, so I bought a pair of Energenie electronic TRV heads, as my son had told me they work with Nest, which was likely the next step. The computer showed me current and target,
now I had something I could work with, the setting was easy, if the current exceeded target unless sun through the bay window, then the lock shield valve needed closing a bit, took a bit of time, but once set it remained spot on. The boiler did not switch on/off all the time, and once room warm the radiators remained warm, not hot, or cold, just maintained the room temperature remaining warm.
The wall thermostat had been turned full on while setting, and it was a completely inappropriate thermostat for the job,
it was designed to instigate a mark/space ratio, slowly decreasing the Mark time, so it did not over shoot, and cause a hysteresis, great idea for an on/off boiler, but for a modulating boiler, it stopped the boilers own controls working. Darn expensive thermostat, very clever, but not for a modulating boiler, each time it turned off, and back on again, the boiler had to start working out what output was required from scratch.
Also needed to add a TRV to the hall radiator, which had been missed, they said because the wall thermostat was in the hall, but when the front door was opened, and my mother who was in a wheelchair so not a fast thing, came in or out of the house, the hall was stone-cold. Set the lock-shield for fast recovery, and then the wall thermostat would turn off heating to the rest of the house prematurely. So with a TRV it allowed fast recovery outside temp to near temp required, then would close, so the last bit was slow, allowing the rest of the house to also get hot, only turning off the boiler on a warm day. This TRV was better being a mechanical type, as the large droop helped.
Until one wanted to reduce the house temperature overnight, since the wall thermostat was not programmable, the heating was simply turned off, 11 pm to 7 am, the re-heating at 7 am was a problem, the anti-hysteresis built into the TRV was OTT, so to get around this, at 7 am set to 22°C then at 8 am set to 20°C which worked well, but resulted in being unable to set up geo-fencing.
Armed with the knowledge gained from mother's house, I tried setting up this one. It did not work, boiler does not modulate, and the info given that Energenie works with Nest was false. And I found hall cooled slower than living room, so needed a thermostat in the living room as well, still needs one in hall, as open fire in living room, so two thermostats in parallel. Also, wife's bedroom turned out to be the coldest room in the house, so a linked TRV head used in that room, so have Nest Gen 3 and Wiser single channel working together.
It now works A1, but it did take some time to get it all working as we want it, well there is one slight problem, wife's radiator on the outside wall, so until heating has got a circulation of air running, the TRV can run the boiler a little prematurely, wasting a little energy, but not enough to really worry about.
Problem is we are the elderly, at 74 I feel the cold, also damaged hand does not help, I don't want rooms either under or overheated, and I want to be able to control the temperature with ease, so saying "hey google set living room to 22°C" it will set both TRV's and the wall thermostat with the one command. Without my leaving my seat.
I can see how young wippa snappers may find going around the room altering the setting on two TRV's and a wall thermostat no problem, but at my age I want the easy life, and turn heating up/down, lights on/off, or dimmed, or even change colour, I keep it simple, I tell Google Nest what I want, and it does it, how could one make it any simpler for the old?
Well, even my son does the same, although he uses Alexa. And he is really young, under 50.
