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OpenTherm Explained

A key benefit of Wiser Smart Heating Systems is that they support OpenTherm modulation.


Essentially, this is the set of rules used to communicate information between smart heating controls and your combi boiler. It does this in order to modulate the temperature flow through your heating system. It can increase energy efficiency in a heating system whilst maintaining the desired setpoint temperature.

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 remains fixed.


The On/Off period comes from several factors but is primarily based on the work rate your heating requires to reach a target temperature. 


For example, say you’re heating up a cold room. The boiler is ‘On’ for longer periods here as 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 this or how Wiser works with your OpenTherm boiler, please check out our  You can also find more information on your Wiser smart heating system in our Beginner’s Guide!