OpenTherm is a method of communication between the central heating boiler and the thermostat. With a standard on/off setting, the thermostat can only instruct the boiler to switch 100% on or 100% off. With OpenTherm, the central heating boiler can be operated using modulation, among other things. That means the central heating boiler can also be instructed to turn on at 10% or 30%.
This means it is possible to compensate for heat loss in your home with heat from the central heating boiler. This provides a constant internal temperature. That is not only comfortable it is also good for the environment. This means a surplus (a deviation in the temperature above the desired temperature) and the corresponding heat loses (which increase at higher temperatures) are things of the past. OpenTherm ensures your comfort increases and your gas bill decreases.