I was wrong about these 
pocket sensors, I’ve heard from an internal source at Vaillant that the sensors they use are clip on sensors with tiny copped tipped NTCs, they clip onto the outside of the pipe. This is the exact part 
:
The fact these are clip-on sensors explains probably explains why some systems have significant temperature sensing errors, possibly the sensor is not fully clipped on?
The good news is that it should be a relatively easy fix to remove the side panel of the HP and check the sensors are clipped tightly to the pipe. Can anyone experiencing significant errors confirm if the issue is indeed a poorly clipped sensor? @Timbones @Andre_K @dfeist. Removing the side panel is quite straightforward, from memory you’ll need to remove the top cover first, which will then expose the screws on the side cover. You’ll need to remove the side cover closest to the flow/return connections. Please take some photos if you find any issues with how the sensors are attached,
It does seem that if you get lucky and have a system where the sensors are clipped on properly, the onboard monitoring can be pretty good. Here’s another data point: Comparison between the onboard monitoring and MID meter data from a 5kW Arotherm+ that I installed for my parents, the data from this system is on heatpumpmonitor, the onboard heat & elec data matches the MID meter within about 3-4%. However, I’m still very glad that I installed 3rd party monitoring, the data granularity has provided a lot of additional insight.
Interestingly, the Vaillant ‘Energy efficiency’ figure shown in the app had average 7.3% error compared to MID and was different to the calculated COP = (environmental yield + electrical energy) / electrical energy, I decided to use the calculated COP figure since it was more accurate but most users would probably just look at the ‘Energy efficiency’ number. Can anyone explain this discrepancy?
All these figures were taken from the MyVaillant app, I’ve not yet compared these figures with the working figure displayed on the Vaillant controller.