Homeowners Professionals

Understanding OpenTherm systems

Many of us are trying to cut down our energy usage, and therefore the cost of our energy bills, at home. There are multiple ways we can do this, from reducing the thermostat by one degree to taking short showers instead of baths. But did you know that an OpenTherm or weather compensation controller could also help.

What is OpenTherm?

OpenTherm is a type of communication system that allows your gas boiler and room thermostat to interact with each other more effectively.

Many homes that have a gas boiler will usually have a thermostat to help control the heating. This is so that the boiler understands when the set temperature has been reached and when it no longer needs to heat your home. Without a thermostat, a boiler could just continually produce heat for the programmed time, making your house unnecessarily hot and wasting a lot of energy in the process.

With this kind of regular thermostat, a boiler will turn itself on to reach the set temperature and off to stop heating for a while. In fact, most boiler systems in the UK work with a simple on/off switch and a thermostat. When the heating system has warmed the house to the correct temperature it will turn itself off completely.

Then, as your home gradually begins to lose heat and cool down again, the boiler will start up to once again reach the set temperature.

However, this on-off programming can actually waste a lot of energy and isn’t the most efficient way to heat a house. The temperature in your home would look a little like this:


You can see as the boiler comes on, it overheats your home, taking the inside temperature past the set 20°C mark (as an example). When this temperature is reached, the boiler turns off altogether until the temperature dips below the set mark, and kicks in again to reheat. This process uses a lot more energy than a boiler that has OpenTherm technology.

OpenTherm systems work in a much more efficient way. Instead of the boiler simply turning on or off when heat is or isn’t needed, the system can adjust the boiler’s water temperature to a set point based on the temperature predetermined at the thermostat. This means the boiler can adjust the heat of the water as required, helping the boiler become more efficient.

As you can see from the graph below, this results in a much more steady heat output, which is why OpenTherm is much more accurate than regular boiler thermostat connections.

When a boiler uses the first method, people generally make the mistake of turning their thermostat up higher because they feel like their home isn’t being heated enough for their liking. This is just another reason why OpenTherm could save you money on your heating bills and reduce the amount of energy you use.

Other benefits of this type of system include:

Is my boiler OpenTherm-compatible?

Your boiler will require a pre-installed OpenTherm interface in order to be compatible. Most older boilers, therefore, won’t work with OpenTherm, but you also shouldn’t assume that your newer boiler is automatically compatible, either. Some newer boilers can have an OpenTherm mode activated if it isn’t already, but not all manufacturers offer this kind of technology with their appliances.

For this reason, you should contact your boiler’s manufacturer to find out if OpenTherm will work with your particular model and make of appliance. Another technology which satisfies Boiler Plus Regulations is weather compensation.

Find out more about Viessmann weather compensation

As well as your boiler being compatible, your thermostat also needs to be compatible. You will be able to check by finding the OpenTherm logo on the packaging. The logo is a small green rectangle with a circular ring around it.

If you’ve confirmed that both your boiler and thermostat are compatible, a heating engineer may be able to connect the two for you. 

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