I have to admit the way gas boilers connect to the electrics does with some makes seem daft, we have new landlord laws which require the inspector doing an EICR to open the boiler connections to check on if the correct cable retention etc has been employed, and then gas safe rules saying unless gas safe registered you should not remove covers.
I personally feel if one needs to be gas safe to remove covers it should be clearly marked on the covers to say so. I know since I did remove covers on my mothers Bosch there were no seals on it, however on my old house, there were seals, and removing covers could allow combustion gases to leave the boiler and enter the garage where it was housed, and there was no warning on the covers.
However in the flat where my boiler is now situated I have a CO² alarm to alert should combustion gases escape, and I see no real difference using oil or gas as far as combustion gases go. And the covers on my oil boiler simply clip into place.
OpenTherm as far as I am aware it not available for oil boilers, it is a gas only feature? The whole idea of a boiler modulating (turning up and down for output) seems to be a gas only feature. And with my late mothers boiler there was no way to electrically alter the boilers output. The only control she had was the temperature of the return water.
I walked into her living room and the heat hit me, her wall thermostat had failed to contact the base unit and the heating had failed to turn off.
But it should not have mattered, the main problem was when the heating system was installed no one had set the TRV's and lock shield valves, and to be frank I could have not done it if I had not been living in the house.
We zone our heating systems, in the main we use TRV's, I have both TRV's and zone valves, the latter controls if main house or flat is heated, rather pointless having up and down stairs split, as upstairs we have repurposed the bedrooms into craft and office rooms, so zone valves upper and lower would be a bit pointless, using the 9 programmable TRV's works better, but many homes are split upper and lower floor within the same house, and the EPH thermostat allows one to work with that split.
I note the Drayton Wiser system also has a three zone thermostat and that one does link to TRV heads, not read up how it works, but the EPH does not connect to TRV's only connects to the zone valves. And one can select master or slave. So some connections are hard wired and some are wireless connections.
So it combines digital and analogue control. So it has analogue control of boiler, and digital control of zone valves.
and the wiring diagram 
and the main point is the temperature sensing part is wireless connected to the units, and the units have connections for the motorised valves and independent connections for the OpenTherm to the boiler. So they are not direct replacements for a unit which combines all in one unit and does not require batteries or mains to power it.
