OpenTherm: Universal protocol for smart home heating systems (opentherm.eu)

"The original protocol is only available for members."

Not "open" IMHO.

The only open source implementation of OpenTherm (and EMS bus too now) that actually works (unlike commercial thermostats) and provides low load and weather compensation at the same time is:

https://github.com/Alexwijn/SAT

PID is used to smooth out weather compensation errors.

See my grafana for realtime boiler and cost data:

https://gasboiler.grafana.net/public-dashboards/8d44381aafa9...