Two things you can do about DHW efficiency.
- The higher the charging hysteresis, the higher efficiency. You can set that on your controller - maximum is 20 K. Downside is that you will have more fluctuation on how hot your hot water really is.
- Set che charging mode on the VWZ to “eco”. This uses a lower compressor power for higher efficiency at the cost of longer runtime. There’s an excellent article from @Zarch, check the first posts of the Arotherm owner’s thread: Vaillant Arotherm Owners Thread
The charging offset setting is ignored and doesn’t have any effect.
Unless you have very hot weather and a very low DHW temperature, I would be skeptical of the COP you’re seeing. My Arotherm tells me similar values but more realistic is a COP of 4 at 25°C outdoor temperature and a 47°C target and a 20K charging hysteresis (that’s measured with correct temperature sensors instead of Vaillant’s).