Questions first is type of boiler? Non modulating boilers use on/off thermostats only and only consideration is main room or room normally kept cool. This depends on if using TRV heads or not, direction of wind how it affects temperature, sun light through windows how that affects temperature. You don't want the thermostat turning off when other rooms still cold, so fitting for example in living room with large bay window would mean when sun comes out heating turns off early. There is nothing to stop two wall thermostats in parallel, so if wind changes which side of home is coolest you can use two thermostats, if TRV's fitted the wall thermostat set minimum temperature only, the TRV sets maximum temperature.
As you move to modulating boiler the whole idea changes, either use a modulating thermostat, or the on/off wall thermostat is only there to turn off boiler on warm days, on cold days it does nothing, the TRV is king.
The condensing boiler modulates to ensure the return water is cool enough to gain the latent heat from flue gases, if it is turned on/off with an external control every time it turns off heat lost out of flue, and every time turned on it does so flat out, so all the cleaver stuff to gain this extra heat will not work as the boiler can't modulate like it should.
So unless using a modulating wall thermostat, the wall thermostat is only in theory there to turn off boiler on warm days. In practice it is also used to turn down home temperature at set times, when we go out, or to bed for example, but while we want heat, the wall thermostat stays on. We are told in a room normally kept cool, on lower floor, with no outside door, and no TRV in that room, however that room often does not exist, so fitted in the hall which does have outside door, but also fit a TRV so speedy re-heat but slows down before it reaches the wall thermostat temperature.
So the two main controls are the TRV and lock shield valve. The radiators must have a temperature drop across them, this ensures the hot water is shared between all radiators, but it also gives time for the TRV head to adjust, if the lock shield valve was wide open, then as boiler starts the radiator would heat to maximum temperature, even if room not far below set temperature, and the TRV even when it closed would do nothing as radiator already hot, so room will over shoot, even with electronic TRV heads it can take 10 minutes for the TRV to adjust, so want it to take around 15 minutes to ½ hour to heat radiator fully, other wise one gets a hysteresis as it over shoots then over cools with a see saw effect.
The most important thing with central heating is the lock shield valve setting.
Also you must realise the TRV sets room temperature, air circulates in the room, so the temperature of the return air is the best temperature to measure to reflect the whole room temperature
and the TRV is ideal placed to measure that. I use cheap electronic programmable TRV heads
cost me £15 each, only down side is 18 x AA batteries every year. There are more expensive versions, this one
connects to wifi, so I can see room temperature when away from home, and also use geofencing, I do check room temperature in the summer and decide if I want the AC on before I return home, and am told I can use IFTTT to get it to auto turn AC on, but heating wise I rely on the wall thermostat to turn whole boiler on/off so really being able to alter using wifi rather than blue tooth is very little advantage.
However the next stage is for the TRV to talk to a hub or wall thermostat to tell boiler when to run. There are many systems Hive, Evohome, Wiser etc. Some like Hive have limitations, Hive will only accept a demand for heat if under 22°C, so with Hive don't want wall thermostat in a room kept close to that or the system could fall over, needs to be in a room kept cool. Also clearly don't want that room to over heat, so does need a TRV in room with thermostat.
In the main we want to monitor two or three rooms carefully as sun or wind direction can change which rooms are coolest, so Hive wall thermostat and one Hive linked TRV head means two rooms monitored to decide if boiler runs, it is likely rest of rooms can have a cheap TRV head.
But re-heat time can be important, using iVector fan assisted radiators and a building management system we can heat and cool rooms quickly as very little water being circulated to be heated or cooled, and only when fan assisted can same radiator be used to heat and cool, however once you see the price you will realise why not used much, although may be more popular as heat pumps arrive. But what we are looking at is a lot cheaper, like the Wiser TRV head which is claimed to have algorithms so it can work out when to close to get the fastest reheat time without over shooting, so question is if worth it?
Looking at geofencing, to be any good, it needs to reheat rooms which are going to be used on arrival home in the time it takes you to travel home, so it will need to hold the room at a temperature which relates to your distance from home, if your travel time is one hour, then it needs to reach target in an hour, if it can raise temperature 5°C in an hour, and target is 22°C then it has to hold the room at 17°C while your out. If however only 2°C in an hour then 20°C while your out, this means often simple time is often better. So with the TRV head I have
the anti-hysteresis is OTT so set at 22°C for an hour then 20°C to get room to 20°C so don't need the clever algorithms.
I have spent more time switching functions off on my Nest wall thermostat than anything else, hind sight a mistake, a simpler thermostat would have been better. Note I use oil so not a modulating boiler.
So we use a compromise to suit our home, and to do that you need to understand the system, I was intending to fit Nest in mothers house, first was to be Energenie MiHome TRV heads then Nest which at that time worked with those TRV heads, but the heads worked so well, never bothered with Nest, when house was sold new owners did not want the electronic TRV head so they were brought here, and I needed a wall thermostat so got Nest, however in the mean time Google had taken over Nest and it no longer works with Energenie MiHome TRV heads support has been withdrawn, so the cheap £15 bluetooth
actually work better than the Energenie MiHome TRV heads. I keep repeating this I hope people learn from my error. Don't get me wrong Nest Gen 3 works well, and once the remote temperature sensors are released in UK it will work better.
For my brother-in-law who has family in Germany and lives up the road from me in Mid Wales geofencing will likely work well re-heating home as he lands in Manchester and travels to 100 miles to Mid Wales, but to be frank not that hard to get out phone and manually set, for the few times a year when geofencing works likely not worth it.
Using TRV heads I can set a sequence, so boiler fires, and kitchen TRV opens, 15 minutes latter dinning room one opens, 30 minutes latter living room one opens, and 2 hours latter bedroom ones open, so heat directed to rooms as required, needs over sized radiators to work well, but with a house on three levels so not small, I use a 18 kW oil boiler. It is the programmable TRV heads which allows such a small boiler to reheat a large house in the time scale I require.
