Smart Thermostats have started to become a mature product since the introduction of the Nest Thermostat in 2012. They have bought attention to internet connected controls and smart intelligent controls in general. A Smart Thermostat is any device that offers more intelligence and connectivity than your usual simple analogue or digital thermostats
Currently the market leaders in the UK are Nest Thermostat, Honeywell Evohome and Lyric and British Gas Hive followed by few smaller companies such as Tado, Netatmo, Climote, Heat Genius, Heatmiser and a few more.
The technology used by these companies ranges from simple web control all the way to advance modulating systems that feed requirements to compatible boilers using open standards such as OpenTherm and proprietary systems such as Vaillant vSmart that interfaces with Vaillant eBus.
We are proponents of open standards as this prevents lock in to a particular system. So where possible it is usually a better idea to go with a technology such as OpenTherm. see OpenTherm Compatible Controls for more info