Hi JGJ,
your setup is similar to mine, I have the 105/6. Can you provide details on your house/emitter setup, e.g. expected heat loss at design temperature, type of emitters (radiators, UFH,…), weather compensation mode, design flow temperatures, flow rate etc.?
How do you get to the conclusion that efficiency is lower than expected? COP data from the myVaillant app?
2 degrees seems very low. Vaillant does not target a specific delta but is purely driven by achieving a certain flow temperature. If the flow temperature is above the target flow for a specific time (i.e. the energy integral rises >0), the system will shut down the compressor and just pump the warm water around until it has sufficiently cooled (i.e. energy integral is below your set threshold of 90°min).
Vaillant has notoriously bad temperature sensors and the whole efficiency calculation hinges on an accurate flow-return temperature delta. Please check what flow and return temperatures are when the compressor is off, e.g. between cycles when the energy integral is decreasing. If they are working as expected, both temperatures should be equal. If you see the flow temperature is lower than the return when the compressor is off, efficiency will be underestimated. In my system, flow is always 1.2K above the return, hence my efficiency is vastly overestimated.
The easiest route is getting the data from myVaillant into a local emoncms instance, but this will only give a very limited view as important metrices (like return temperature) will be missing.
Integrating MyVaillant app with EmonCMS - Hardware / Heatpump - OpenEnergyMonitor Community
The next step up is to set up a Raspberry Pi with an emonsd image and an ebus USB adapter that you can attach to the ebus data lines of your heat pump. This will allow monitoring all the data coming out of the heat pump, store it and graph it.
SD card image: emonSD Download — OpenEnergyMonitor 0.0.1 documentation
ebus adapter (unsure when you can get one): Reserve-my-adapter – ebusd
alternative ebus adapter (immediate availability): eBus heating bus couplers and gateways (esera.de)
You can then perform a simple calibration of the temperatures by adjusting for the wrong offset between flow and return within emoncms.