OpenTherm is a point-to-point communication system and connects gas central heating (condensing) boilers with room thermostats. The OpenTherm 'protocol' sets the rules on how the boiler and the room thermostat communicate with each other.
When installed, an OpenTherm compatible boiler (such as an Intergas Xclusive 30) using OpenTherm compatible controls (Honeywell evohome paired with a Honeywell R8810A1018 OpenTherm Bridge) will control your boiler’s water temperature and modulate the appliance to the correct 'Control Setpoint' at all times.
Basic ON/OFF heating controls (including Time, Proportional & Integral (TPI) controlled heating systems) have no affect on the boilers water temperature and this is always controlled by a fixed preset value (normally set at the boiler). This is not the most efficient way to run the boiler, especially during Spring & Autumn months when heating demand is less than in Winter months.
In the UK, the traditional way to control a central heating system is with a basic ON/OFF or TPI room thermostat. The room thermostat is set to a temperature and will switch the boiler ON and OFF; when the room temperature falls below the temperature set point, the thermostat switches the boiler ON and then when the room temperature goes above temperature set point, the thermostat turns the boiler OFF. The result of this is shown in the graph below.
As you can see, the graph above shows the set point of 19°C is not accurately controlled and the temperature in the room both 'overshoots' and 'undershoots' the required set temperature in the room. This because the thermostat is 'dumb' in the way it operates (i.e. no temperature feedback to the boiler). This can feel uncomfortable and normally leads to occupants setting the thermostat higher than required.
OpenTherm provides much more precise control of the boiler, by continuously adjusting the boiler's water temperature set point (the 'Control Setpoint'). OpenTherm always assumes that the room thermostat calculates the amount of heat required from the boiler and this is matched to the varying demand requirements. By setting the boiler's water temperature to the correct temperature set point as it leaves the boiler, OpenTherm not only maintains the correct room temperature set point but also helps the boiler's efficiency by running at lower flow temperatures for longer periods; this ensures more of the boilers heat exchanger is at condensing temperatures, which is the boilers most efficient point.
The OpenTherm thermostat graph (above) shows how once the temperature of the room nears its temperature set point, the boiler is told to reduce its output to prevent the room temperature overshooting. Once the room sensor sees that the temperature set point has been achieved, it will tell the boiler to change its temperature set point to a lesser value, dependant on the feedback the room sensor is receiving.
This 'Control Setpoint' change can be seen reducing the flow temperature of the boiler on the graphs below. We have an Intergas Xtreme 36 and Honeywell evohome Connected Modulation Pack using OpenTherm at our training/testing facility in Mid Wales.
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 not all boilers using OpenTherm are certified to do so, which can lead to issues. We recommed if you are replacing your boiler, you research OpenTherm boilers well before purchase. We personally recommend Intergas boilers as well proven and certified OpenTherm boilers. We also have a list of OpenTherm compatible boilers on The EVOHOME Shop Forum.
OpenTherm cannot work with 230v switched boilers and non-condensing boilers. This is because OpenTherm room thermostats don’t have switches within them; instead they use the sensor in the thermostat that sends a variable demand signal along a digital BUS cable, which is received in the boiler. Therefore, OpenTherm controls can’t work with traditional (non-OpenTherm) boilers.
Please look at the OpenTherm section on The EVOHOME Forum, if you have any questions about OpenTherm or its integration.