OpenTherm

OpenTherm (OT) is a standard used in central heating systems for the communication between a central heating boiler and a thermostatic controller.

As a standard, OpenTherm is independent of any single manufacturer. A controller from manufacturer A can in principle be used to control a boiler from manufacturer B. However, OpenTherm controllers and boilers do not in fact always work properly together. The OpenTherm standard comprises a number of optional features and some devices may include manufacturer-specific features. The presence or absence of such features may impair compatibility with other OpenTherm devices.

OpenTherm was founded in 1996 because multiple manufacturers needed a simple-to-use communicating system between room controller and boiler. It had to run, like the existing controllers, over the existing two wires, not polarity sensitive, without the use of batteries.

For one British Pound, Honeywell sold the first specification to the OpenTherm Association on November 1996. Shortly after the first products appeared on the market.

By 2008 the Association had grown to around 42 members and has regularly updated and improved the specification. Furthermore, the Association is also active in lobbying for the interests of its members and is also present at exhibitions like the ISH (Frankfurt) and the Mostra Convegno (Milan)

As of 2016^[update], the association has 53 members from around the world.

Communication is digital and bi-directional between the controller (master) and the boiler (slave). Various commands and kinds of information can be transferred; however, the most basic command is to set the boiler's target water temperature.

OpenTherm makes use of a traditional untwisted 2-wire cable between controller and boiler.

OpenTherm is not polarity sensitive: wires can be swapped. The maximum wiring length is 50 m up to maximum 2 x 5 ohm resistance.

For backward compatibility with traditional switching thermostatic controllers, OpenTherm specified that if the two wires are connected together then the boiler will switch on.

Specification 3.0 also describes how more than two devices can be connected by OpenTherm. Whilst OpenTherm is a point-to-point connection, an extra device (gateway) is added between the master and the slave. This gateway has 1 slave and 1 (or more) master interfaces. The gateway controls which data is passed to each slave. An application example is a room temperature controller connected to a heat recovery unit, which is connected to a boiler. The heat recovery unit is then functioning as gateway. In another possible configuration, a thermostat or room controller is connected to a sequencer with further Opentherm interfaces connected to more than one boiler. The room controller can be a standard unit, since it only 'sees' one heat-producer. The sequencer includes additional software to increase or decrease the number of running boilers to match the actual heat demand. The sequencer also needs a sensor to measure the temperature of the combined output from the boilers and usually would also control a main circulation pump. What happens after a fault occurs (resequencing remaining units, passing fault messages through for display on the room controller, etc.) is also part of the sequencer functionality. (The hydraulic design of such a system must also take account of different combinations of boilers running at the same time: a Low Loss Header / Hydraulic Separator is usually included to combine the flows from the boilers.)

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