What is Opentherm?

EOGB recently became a member of the OpenTherm association, and with that the Sapphire Boiler became an OpenTherm certified product.

The Sapphire Fully Modulating Blue Flame Boiler is due for release this summer. One of the key features of The Sapphire is the use of OpenTherm technology, which works perfectly with our totally unique fully modulating burner control system.

Become an approved Sapphire Agent here


Here we look at how OpenTherm works, the advantages of it and how you can get the best out of it with use of smart controls.

What is OpenTherm?

OpenTherm is the name given to a form of communication of information between modulating central heating appliances and room thermostats.

The term “open” in the title means that boiler and controller manufacturers can easily bring their products together and work in partnership – something that has not been so easy in the past.

How does it work?

Most boilers currently have a simple on/off control system, often with a mechanical room thermostat and time clock which means that the boiler is switched on and off at intervals to either provide full heat power or no heat at all. On the other hand, modulating control means the heating power is increased or reduced based on the actual output needed.

Boilers which are the actual “Slave” are controlled by the “Master” which is the OpenTherm Thermostat. This calculates exactly what the system needs every few seconds and sends information to the “Slave” to either increase, decrease or stay where it is.

So to put it in simple terms if you only require a small amount of heat for a particular part of the property, the OpenTherm master control will tell the slave to run at minimum power or whatever power it calculated until it sees a change in the temperature. It then continues to regulate the heat output as required via load compensation, and if another zone comes on it will keep adjusting to suit the load to suit.

To get the best from OpernTherm a multi zone system such as Genius Hub or Honeywell Evohome is advised.

The Advantages

Opentherm has many advantages such as plug and play installation, fault code diagnosis, water temperature, outside temperatures, plus the add on features so Opentherm controls can integrate with other parts of the system such as solar thermal, if it were fitted to the system.

Also, fuzzy logic incorporated in Opentherm system such as Genius Hub and Honeywell EvoHome enable the system to self-learn and make the heating system work as efficiently as possible by optimizing heating up times in the most efficient way.

Other advantages include efficiency increase, increase of boiler life, use of smart controls, and remote appliance monitoring.

Even Smarter!

If Genius Hub controls are chosen to control your heating, you can incorporate a wireless room sensor that not only allows you to control the room temperature, it also monitors room occupancy. It self learns your occupancy levels and adjusts the heat up times according to how often you use the room.

Genius Hub

Any upfront installation overheads will soon be offset and you will find the investment is certainly worth it with reduced heating bills in the long run. Certified testing has proven a minimum of 15% fuel reduction using a single Thermostat and real world usage is as much as a 25% reduction fuel consumption using a multi zone installation.

This really is a fantastic addition to the heating system.

Martin Cooke, managing director has his say on why we should be looking at OpenTherm and Modulating Boilers in off gas grid areas – If we chose the best available technology such as modulating boilers with smart OpenTherm controls on new installations, the carbon emission reductions are significant and you will see a rapid return on investment due to the reduced running costs make this a truly cost effective alternative to the high costs of heat pumps or biomass”.


For any enquiries on The Sapphire, OpenTherm controls, HVO or any technical questions please contact us at technical@eogb.co.uk or give us a call 01480 477066.