I spoke with Wiser tech support and they suggested that the back plate supplied with Wiser needs to be installed by removing the old thermostat. But they aren't able to give wiring advise over the phone.
The old thermostat has two cables connected to it: A red cable which goes into "N/O" A black cable which goes into "COM"
The new Wiser heat hub has got Following slots: N, L, 1, 2, 3, 4. It also has got OT1 and OT2
One is NOT a direct swap for the other. Your original stat as you point out only has two wires (Live and switched live) your new set up also requires a neutral. (Which your old stat did not require) It needs to be rewired, so I would suggest you get a competent person in to do the job for you.
Worcester Bosch does not support OpenTherm so it either needs a special which actually fits in the boiler front panel, or simple on/off.
As said your original does not use a neutral only wifi thermostat I know which does not need a neutral at the base is the Nest e, however both Nest e and Nest gen 3 do not connect to TRV heads, it was claimed the Energenie MiHome heads would work, but that is what I have and they are intermittent works OK from phone, but not schedule changes or changes on the dial. May as well use cheap eqiva eQ-3 heads they will also program but are not wifi (although can get bluetooth version).
The idea of a independent supply to the thermostat is frowned on, I think in the main as possible to end up with a borrowed neutral situation. The single channel Drayton Wiser has volt free contacts so in theory you could have an independent supply, the duel and triple channel have an internal link so can't be used with an independent supply.
I am not sure which you have? Since each TRV head is a zone, when it says multi zone not sure if referring to TRV forming the zone or a motorised valve forming the zone?
Not even sure if you are using the Drayton Wiser TRV heads? I have not fitted Drayton Wiser, but reports are good, the EvoHome, Hive, Tado and Wiser all work by letting the TRV control the room and the TRV tells the base when the boiler needs to run, but some can also work as stand alone, the Hive can for example, Nest was claimed to work in reverse, the wall thermostat told the TRV what to do, well it should have done, the rest the TRV tells the wall thermostat/hub what to do.
The idea is the radiator generates thermals which means the temperature of the air at bottom of radiator is a very good indicator of room temperature, slightly to the side it will do nearly the same, and the energenie TRV uses two sensors one water the other air and the air temperature is compensated to allow for water temperature and it works well, not sure on Wiser but I know Wiser works out time to move to new temperature where the energenie anti hysteresis software means that one is slow to change temperature.
Each TRV head is different, the cheap eQ-3 for example has a open window detect and can have an offset to correct temperature, unfortunately in the main you need to buy them to find out all features. By which time it is a little late.
In the main the base for the smart thermostat replaces the programmer not the original thermostat, it does seem daft that anyone still fits a programmer when you can get programmable thermostats, but they do, it also seems daft to have motorised valves to form zones when you can get programmable TRV heads, but again it is often done.
So if you have a programmer then likely you can replace that with the Drayton Wiser, and if not the Drayton Wiser will go very close to the boiler, not too close as the fans can produce RF interference, but only needs to be a meter away likely even less, personally I would test first and if OK then fit trunking to make it look good.
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