Smarthome devices are not just about making our homes more practical. Products such as smart thermostats and radiator valves also help homeowners to save money on their energy bills by using the latest protocols and technology to tackle the energy efficiency problem in many homes. Here at Ovon, we are experts in a variety of protocols, but this time we’re going to explore OpenTherm.


What is OpenTherm?

OpenTherm is a standard communications protocol used in central heating systems for the communication between a central heating boiler and a room thermostatic controller, such as the ones you find on the wall. It enables thermostats to control heating and domestic hot water systems. As a standard, OpenTherm is independent of any single manufacturer.

With traditional heating systems, a simple relay is used to turn the boiler on or off. When the boiler is on, it heats water and your home to the pre-set temperature until the thermostat tells it to turn off. However radiators turned on to full power can often overpass the temperature set. As a result, the thermostat waits until the temperature falls, before starting the boiler at maximum power again. This is not the most efficient way to keep a comfortable home. 

Thanks to OpenTherm, a user can get a real and accurate idea about what is happening to the boiler. In fact, it gives a digital and friendly interface that allows the thermostat to change the temperature of the water used for heating. Using a process called modulation OpenTherm boilers can reduce water temperature to balance out natural heat loss through windows, walls, roofs and doors.

As almost all smart thermostats understand how long a house takes to warm up completely, they can measure the heat required to get a house up to the ideal temperature. Therefore, a thermostat can set the boiler to the most efficient water temperature to achieve the comfort needed. 

Even more, in many cases, thermostats may set the water temperature to a lower level, using longer heating times to reach the set temperature. Despite how this sounds, longer heating times do not necessarily mean more energy is being used, in fact, this type of heating control can save more energy and be more efficient overall.

OpenTherm communication is digital and bi-directional between the controller (called master) and the boiler (called slave) so how does it work? 

Master controllers can come from different sources, but at this point we are going to stop on two:

  1. A Thermostat. It will use its inbuilt smart algorithms to tell the boiler when to change its target water temperature so that it will use more or less energy as needed.
  2. A Gateway or multi point to point. This is a new feature from specification 2.4 that also describes how more than two devices (boiler-thermostat) can be connected by OpenTherm.
    Whilst OpenTherm is a point-to-point connection, an extra device (gateway) is added between the master and the slave. This gateway has 1 slave and 1 (or more) master interfaces. The gateway controls which data is passed to the slave.  


Does OpenTherm save money on the energy bill? 

It is really difficult to give an exact figure as it will depend on the home, the number of radiators, heating temperature and the existence or not of smart radiator valves. There are studies that confirm that it can save around 6% energy.

The fact is that by using OpenTherm a boiler only uses the energy it needs to provide the heat required at that point to save money.


Is my current heating system OpenTherm compatible?

The simplest way to know whether an existent heating system is OpenTherm compatible or not is to look for the OpenTherm logo on the boiler or thermostat.  If the boiler isn’t easily accessible, the best way is to check with the boiler and thermostat manufacturers to see if OpenTherm.

If you’ve got a compatible thermostat and boiler, it’s worth checking with a heating engineer if the OpenTherm connection is enabled as thermostats that support it have a specific wiring for this standard.

Finally, an OpenTherm controller from manufacturer A can in principle be used to control a boiler from manufacturer B. However, OpenTherm controllers and boilers do not in fact always work properly together. The OpenTherm standard comprises a number of optional features and some devices may include manufacturer-specific features. The presence or absence of such features may impair compatibility with other OpenTherm devices. This is a rare scenario but could be the reason why a compatible OpenTherm system is not working properly.


Alternatives to OpenTherm

OpenTherm is the most popular modulating interface, but some boilers have their own digital interface. There are other standards/protocols such as eBUS and EMS and here at Ovon we work with all of them when developing our smart devices. We promise to go through them and their advantages and inconveniences in the following blogs. Thanks for reading and we hope you found this useful!