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OpenTherm Explained

OpenTherm is a communication protocol between heating controls and the boiler to modulate the temperature flow through a heating system. This can increase the energy efficiency of a heating system whilst maintaining the desired set point temperature in the home.

Here we explain how three types of heating systems operate.

Heating systems with basic controls

Basic heating systems control the temperature of a home by heating the water running into the radiators at one temperature, then switching the boiler off when the room reaches its set point. The boiler will then switch on again when the room temperature drops below set point.  This standard on/off operation often over/under shoots the set point when used with basic heating controls. 

Heating systems with ON/Off modulating 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 low load conditions. When compared with basic On/Off thermostats this uses less energy to maintain more comfortable conditions.

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

OpenTherm heating system with OpenTherm modulating 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 set point. The important difference here is that the boiler will run for longer periods but it will be producing water at lower temperatures, resulting in less energy being used and maximising the time spent in the higher efficiency condensing mode.

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

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

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