The wiring before boiler change will be to motorised valve, boiler, and tank thermostat, and after it will be to the boiler only, and before it will be a low voltage control (230 volts) and after an extra low voltage control (0 - 48 volts) so may be same unit, but basic re-wiring from scratch.
Also the location may need to change.
With a hot air central heating or with an open plan home, a single central thermostat can control the whole house. However if we have doors without vents in the doors to allow air to circulate room to room, then each room is individually controlled, this is done with the thermostatic radiator valve, (TRV) however the basic valve can't turn the boiler on/off it can only turn it up/down due to the return water temperature.
So traditionally we fit a on/off thermostat in a room keep cool, so in warm weather it will not run the boiler, on the ground floor as heat raises, and in a room with no outside door, or alternative heating this includes sun through windows. Most homes no such room, so it is considered the hall is near enough, as front door only opened for visitors.
But if we are using opentherm, then the hub or what ever name the manufacturer has given to the information collection device is, controls the boiler output, so we need sensors to tell the hub what leave of heat output is required. In the main we build these sensors into the TRV, Nest however is an odd one out, it uses sensors
and for some reason known only to Nest, these have for a long time only worked with the USA version of Nest, and with a pack of three costing £140 it is a rather expensive way to do it. As one still needs a programmable TRV head unless all rooms are turned on/off together.
I have Nest Gen 3 and it says "The Nest Temperature Sensor requires a 3rd generation Google Nest Learning Thermostat or a Google Nest Thermostat E. Identify your thermostat at nest/which thermostat." But the USA version is also called Nest Gen 3 and is a completely different thermostat. So not sure if the sensors work with British Nest Gen 3, I have already bought Energenie MiHome TRV heads which were claimed to work with Nest, they both will work from same app, but that is where the linking ends, and even if the linking did work, it is the wrong way around, we want the TRV to tell the wall thermostat what to do, the Nest system the wall thermostat tells the TRV what to do.
So I have given up, and have installed Drayton Wiser, in my case using a wall thermostat as two radiators in the same room, but there is no need for wall thermostats, it can control the hub just from the TRV heads, this also has opentherm, the same with Honeywell Evohome, and the EU version of Tado (Opentherm was withdrawn from UK version, but understand now can be got again).
Nest Mini speakers control most of my automation throughout the house, including the new Wiser, but the Nest Gen 3 has been a flop, it was got because the unit can be powered and control both central heating and domestic hot water by just two wires, which was all I had from boiler to main house, still used for DHW if required, but central heating now controlled by Wiser, I don't use OpenTherm as using an oil boiler, I did not like the idea of relying on batteries for the thermostat, done that before, and was left in the cold due to failed batteries, but still have Nest as a back-up should the batteries become discharged, and every TRV head also has a pair of AA cells, so start of each season I fit 20 AA cells, 9 TRV's and one wall thermostat.
I know Nest Gen 3 will work from a USB power supply, but that would mean getting an UPS (uninterruptible power supply) to the USB unit as well as to the central heating, so seems a bit pointless, the internal battery with Nest Gen 3 does not last long, less than 8 hours, and if let to discharge, it needs to charge again before Nest will run again, so when I turned off boiler for heating engineer to work on it the next day, on turning it back on he could not test boiler until it had recharged.
Also had problems with geofencing, central heating turned off when the EE mast was damaged in high winds.
