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...
