Many gas boilers are oversized which means it's more difficult to run them efficiently. Also, many gas boilers have capability to run more efficiently but are not configured to.

This particular boiler does not seem to respect the MM=0 command (despite OpenTherm's such specification requirement) which is not uncommon. SAT sends the MM=0 command for the boiler to work at its minimum capacity. CS= minimum_setpoint and then PWM controls the ON/OFF times of the boiler. However, I have a workaround which ensures that most of the time, the boiler is indeed working at its minimum modulation, unless the target flow (heating curve + PID) demand is high [higher than current flow temperature] during warm up.

The boiler supplies heat to the central heating and domestic hot water tank, but not at the same time. The hot water tank capacity is 255 litres. When DHW call is made, boiler ignores OpenTherm instructions and runs at 80 C degrees. There are three zone valves in the house: Hot Water, Upstairs and Downstairs (not talking about TRVs here). When DHW call is made, Hot Water valve opens, and Upstairs & Downstairs close; then if CH call is made, Hot Water valve closes and the other two open.

The hot water system is also fitted with "Waste Water Heat Recovery System". In essence, the heat energy extracted from showers and baths is used to preheat cold water that is then sent to the hot water tank. The extraction recovers about 60% of heat normally lost. You can watch examples of how it works here - https://www.youtube.com/watch?v=G-jKlCoRvEM

SAT project - the brain behind the efficiency https://github.com/Alexwijn/SAT

OpenEnergyMonitor - for making possible to share real time boiler data https://community.openenergymonitor.org/t/my-gas-boiler-data...

Any questions, let me know :)