So, cold temperatures are already here. I had not really touched the Node-RED PID controller after I lowered the derivative factor. It seems to be working properly. There is some overshot when adjusting the setpoint that could be corrected for, but the boiler clearly has it's own PID.

This is the current grafana output, including the gas consumption, but that is not very useful in this case because it also counts the shower (on for around 10 minutes at 9:45). The gas consumption is a rolling average and has some delay.

Note that both graphs have two Y axis. "ouside" and the gas m³/h are on the right side. The green spiky plot on the bottom is the heating circuit water temperature. It's LOW, which is the whole point; running it low to maximize the condensation boiler efficiency.