Heating · Glossary
Open Therm
A communication protocol between modulating boilers/heat pumps and smart thermostats, enabling load-modulating control rather than simple on/off.
Open Therm (OT) is a 2-wire bidirectional communication protocol between a modulating heat source (boiler or heat pump) and a compatible thermostat (Honeywell evohome, Nest 3rd gen, Tado, Ecobee, Drayton Wiser). Instead of simple on/off, the thermostat tells the heat source the required modulation level and the heat source reports actual output, modulating the burner or compressor smoothly.
The result is more comfortable rooms (no temperature swings), longer equipment life (no rapid cycling), and 5-15% lower fuel bills. For boilers, OT is well-established. For heat pumps, OT-on-heat-pumps is a newer standard that pairs natively with Vaillant aroTHERM, Viessmann Vitocal and some Bosch/Worcester units.
If you are upgrading a boiler in Cornwall and value comfort and efficiency, ask CCS to specify an OT-compatible thermostat from new. See our boiler installation page.