Oh what a can of worms, to my mind Nest Gen 3 is really for hot air central heating used in USA not our British water system. For a single thermostat to work needs to be an open plan house, my last house Nest would work well, but this house had doors.
So many modulating boilers can be modulated using either of two methods, one is a modulating thermostat (OpenTherm) the other is the return water temperature, using TRV's to control the room temperature the boiler is modulated by the return water temperature, but it has one problem.
Once you turn off a boiler, the water stops circulating, so it does not know when to turn on again, so you can't turn it fully off, it would without some interface cycle on/off all summer.
So by fitting an on/off thermostat in a room normally kept cool, (so will not run in morning in summer) with no outside doors, and with no alternative heating in the room (including sun through windows) and on the lower floor (heat raises) this can turn off the boiler when not required, but also turn it back on when required.
However I have not got any room which fits the bill, and most homes are the same, so we look for some compromise, using the hall for example, because there is an outside door, we need to have a TRV set to match the wall thermostat so fast recovery once door is opened, yet will not switch the wall thermostat off before rest of house has warmed up.
The easy way done by most smart thermostats other than Nest is to link at least some TRV heads to the wall thermostat. Hive is another odd one out with no OpenTherm option, but it does have linked TRV heads, so as long as the wall thermostat is below 22°C a demand for heat from a TRV will switch boiler back on.
Drayton Wiser and Honeywell Evohome have add on modules for OpenTherm, but it seems the British building regulations have been miss read by many. The LABC for many local authorities have said they see use of TRV's as zoning, but installers have not seen it that way, and at the moment as far as I am aware only EPH do a thermostat which can be set to master/slave and run zone valves with OpenTherm, and then clearly no need to also have TRV heads, really it should be either/or but installers have not installed that way.
So the big question, if set up correctly, what advantage is there using OpenTherm to return water temperature? Well it seems OpenTherm will run the boiler cooler, and will turn off boiler earlier, but it seems to be some debate as to if the energy saved is really that much.
But the whole of the central heating system raises the same question, if this house had gas, then would having 9 electronic TRV heads not linked to the main thermostat be much worst to having 9 linked TRV heads and a OpenTherm wall thermostat, so take one make, seems about the best, as the TRV heads it is claimed have algorithms to work out how long it takes to reheat a room, so work faster than most, so Drayton Wiser so £43 each for TRV heads, and £130 for wall thermostat, so around £520 for the system. Or £15 for the electronic TRV heads and £35 for a simple on/off programmable hard wired thermostat £170 so around £350 difference.
If we consider it may save 10% of fuel costs, not not really sure that is true, and oil costs me around £400 a year, so possibly saving £40 a year minus and interest on money now not invested but plus price increase which will likely cancel each other out, looking at around 9 years to pay for it's self, so big question is in 9 years time will we still be using gas for heating?
And I am not convinced it would save 10%, so at 5% will unlikely pay back in my life time.
So more interested in comfort, returning home to a nice warm house. So now geofencing verse time control? Well not impressed with geofencing, I did have it enabled with my Nest, until a storm took out the EE mast and my heating went daft. It took some time to realise what had happened, as Nest also has a built in PIR so every time I walked past the thermostat heating would come on, but I play on this PC in my bedroom, so not walking past the thermostat, and getting cold, so walked down stairs to see if heating on, which it was, only when I used the PC to look, did I realise heating had auto gone to eco mode due to no EE signal from my phone.
