Thermostats with an OpenTherm protocol are the most standard modulating thermostats. OpenTherm is the language with which an OpenTherm thermostat communicates with a modulating central heating boiler. Not all modulating central heating boilers are equipped with an OpenTherm protocol. Some are brand-specific, which means that they only communicate with a brand-specific thermostat. With the combination of an OpenTherm thermostat and OpenTherm central heating boiler you have much more freedom of choice than with a brand-specific central heating boiler.