My installer wasn’t very helpful and I’ll launch an inquiry with Vaillant. Some more things I found:
From a service manual, it appears the sensors are extrnal to the pipes (13 and 19):
Also, it appears the return temperature also slightly varies with outside temperature (higher than my inside measurements when it’s warmer outside). I have implemented a simple running fix in Home Assistant:
- Calculate the delta T between flow and return
- When the pump is idle (i.e. in case it’s cycling), take the Flow-Return delta at 10 minutes after it got idle and store this as an offset to subtract this number from the flow temperature. This will give a running calibration of the delta T to be zero in the idle phase as would be expected.
- Use the corrected flow temperature for power calculation.