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OpenTherm Explained

Great news! Wiser supports OpenTherm modulation. To help you further understand OpenTherm we’ve explained it in this blog and compare it to other ways heating systems operate.

OpenTherm is the set of rules used to communicate information between heating controls and your combi boiler to modulate the temperature flow through your heating system. This can increase the energy efficiency in a heating system whilst maintaining the desired set point temperature in the home.

How most heating systems operate with digital heating controls 

ON/Off modulating heating controls adjust the average water temperature within the heating system by cycling your boiler On and Off periodically. In this setup the water temperature produced by the boiler itself is still fixed.

The On/Off period is determined by several factors but is primarily based on how hard your heating needs to work to reach the target temperature. For example, when heating up a cold room or during cold conditions where rooms are cooling quickly, the boiler will be On for longer periods – these are ‘high load’ conditions. In ‘low load’ conditions, such as maintaining a temperature, the boiler may only be on for a few minutes at a time. This translates to high load conditions generating, on average, higher water temperatures than in low load conditions. When compared with basic On/Off thermostats this uses much less energy to maintain more comfortable conditions.



Note: may or may not overshoot, but there will be a small ripple around the setpoint


How an OpenTherm boiler operates with OpenTherm controls 

OpenTherm modulation operates on a similar principle but is achieved more directly by setting the desired water temperature from the boiler rather than by cycling it On and Off.

For ‘high load’ conditions a high water setpoint can be requested from the boiler. The water temperature being requested will then reduce over time as the room temperature approaches its target. The important difference here is that the boiler will run for longer periods but producing water at lower temperatures resulting in less gas being used and maximising the time spent in the higher efficiency condensing mode.

One of the main benefits of full OpenTherm modulation is more stable and accurate control of the room temperature, even when compared with On/Off load compensation controls.


See graph below.

Note: May or may not overshoot but will have a stable control (Very minor fluctuations – but better than On/Off load compensation).

If you have any further questions about OpenTherm and how Wiser works with your OpenTherm boiler, please check out our frequently asked questions here.