OpenTherm is the name given to a form of communication of information between modulating (condensing) central heating appliances and room thermostats. The OpenTherm protocol sets the rules as to how the appliance and room thermostat should communicate with each other.
When installed, an OpenTherm compatible boiler using Honeywell OpenTherm compatible thermostat (Honeywell evohome and an R8810A1018 OpenTherm Bridge for example) will control your boiler’s water temperature, modulating the appliance to the most efficient point at all times. When compared to basic ON/OFF heating systems (and even TPI controlled heating systems) the boilers water temperature is always controlled by a fixed preset value set at the appliance – which isn’t the most efficient way to run the boiler.
The standard way to control a heating system is to switch the boiler on and off at certain intervals with On/Off or TPI controls. OpenTherm provides more precise control on the boiler through the control of the gas valve. It allows the amount of heat provided by the boiler to be controlled to match the varying demand requirements. By reducing the flow temperature to a minimum as it leaves the boiler, OpenTherm modulation helps the boiler efficiency by running at lower flow temperatures for longer periods; this ensures more of the heat exchanger is at condensing temperatures.
To give you a point of reference and an easier way to understand how OpenTherm works here is a real life example:
Imagine two cars at a set of traffic lights. When the light turns green, one of them speeds off as fast as possible while the other drives steadily and sensibly. By the time they reach the next set of lights the sensible driver has caught up and has consumed far less fuel and therefore saved money. Also the fast car has not benefited at all because both cars are now in the same position. In this example the inefficient speedy driver is similar to an ON/OFF control system where as the steady, efficient driver is similar to OpenTherm.
OpenTherm is not compatible with all boilers and it is advised that you check with the boiler manufacturer if you are unsure. Another thing to take note of is that OpenTherm is not compatible with non-condensing boilers. You can download a complete list of OpenTherm boilers here
OpenTherm cannot work with mains voltage. This is because OpenTherm room thermostats don’t have switches within them; instead they have a sensor that sends a variable signal, which is received in the boiler. There it can be acted upon to make the boiler operate efficiently. Therefore OpenTherm controls can’t work with traditional (non OpenTherm) boilers.
We sell a range of OpenTherm compatible thermostats which you can find here