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What is OpenTherm?

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 ECO RF) 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.

How does OpenTherm work?

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 temperaure set point, the thermostat turns the boiler OFF.  The result of this is shown in the graph below.

ON/OFF Thermostat Graph

As you can see, the graph above shows that the setpoint of 19 degrees Celcuis is not accurately controlled and the temperature in the room both 'overshoots' and 'undershoots' the required tempertaure in the room.  It does 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 continiously adjusting the boiler's water temperature setpoint (control setpoint). OpenTherm always assumes that the room thermostat calulates 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 setpoint as it leaves the boiler, OpenTherm not only maintains the correct room temperature setpoint 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.

OpenTherm Controls Graph

The OpenTherm thermostat graph above shows how once the temperature of the room nears its temperature setpoint, the boiler is told to reduce its output (control setpoint) to prevent the room temperature overshooting. Once the room sensor sees that the temperature setpoint has been achieved, it will tell the boiler to change its temperature setpoint to a lesser value, dependant on the feedback the room sensor is receiving.

This control setpoint change can be seen on the graphs below. We have an Intergas ECO RF 24 and Honeywell evohome multi zone system at our training/testing facility in Mid Wales.

Intergas ECO RF & evohome OpenTherm

What are the benefits of using OpenTherm?

  • Increases the efficiency of the heating system
  • Reduces boiler 'start/stops' and therfore boiler wear and tear is decreased
  • Ability for remote control and diagnostics of the boiler from the controls (bringing future advantages in remote monitoring of the heating system)
  • Extra functions (information sent to room thermostat from the boiler, such as fault codes)
  • No batteries required (thermostat can be powered by the OpenTherm connection)
  • Increases the life of the boiler
  • Devices are very simple to install (two wire, extra low voltage cables and in many cases simpler to wire than current ON/OFF controls)
  • No need to configure
  • Can provide status of the boiler on the display
  • Lowers fuel bills

Is OpenTherm compatible with all boilers?

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

Can I use OpenTherm controls without an OpenTherm boiler?

OpenTherm cannot work with 230v controlled 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 signal, which is received in the boiler. Therefore OpenTherm controls can’t work with traditional (non OpenTherm) boilers.

What OpenTherm thermostats do we sell?

We sell a full range of Honeywell OpenTherm Compatible Thermostats, and you can find these here.

Any questions on OpenTherm?

Please look at the OpenTherm section on The EVOHOME Forum, if you have any questions about OpenTherm or its integration.