OpenTherm (abbreviated OT ) is a protocol that was originally developed for the control of central heating by room thermostat (room thermostat ). In OpenTherm is a " Point to Point " communication, while the thermostat is usually the "master" and the heat source is the "slave". Using multi-point -to-point, multiple devices can be connected together.

OpenTherm is vendor independent. A regulator from manufacturer A can communicate with heater manufacturer B.


Intermediate thermostat (master) and heating ( slave) takes place a digital, bi -directional data exchange. The sending status reports and requests for information between the two devices is possible. However, the most common command refers to the control of the boiler temperature. Receives the heating set temperature request, it starts to modulate ( increase or reduce the heating power ) to achieve the desired temperature and keep. The room thermostat constantly recalculated the boiler temperature to keep the room temperature at the setpoint. This leads to a higher energy efficiency.

Communication medium

Physically OpenTherm a 2-wire connection. Often already existing lines can be used. OpenTherm is polarized, that is, the polarity of the cable does not matter when connecting.

The maximum cable length is 50 m with a maximum resistance of 2 × 5 ohms.

OpenTherm / Plus (OT / )

In OpenTherm / Plus (OT ) the " " at "OT " usually is omitted. The two wires are used both for data exchange as well as for power supply. In this " point- to-point " connection of the room controller is the master and the heating is the slave. the master queries by changing the voltage level and the slave responds by changing the current.

The power supply for the control is performed by the slave. The minimum available power is 40 mW ( 5 mA LowPower / 8 V). OpenTherm Smart Power can provide upon request by the master 136 mW ( Medium Power 17 mA / 8 V) or 306 mW (High Power 17 mA/18 V ) are available. If the OpenTherm connection to the heater shorted, start the heater.

OpenTherm prescribes a minimum data exchange interval of 1 second. The data in the communications package are specified functional and are OpenTherm ID ( OT- ID) called. 256 OT IDs are available, 128 are reserved for OEM applications. Of the other 128 IDs are specified as a function 90 (OT specification v3.0 )

OpenTherm / Lite (OT / -)

If OT / - is used, the master generates a PWM voltage signal for the boiler water temperature. The current signal of the heater indicates the status of the heating error, no error. If the OpenTherm connection shorted, start the heater. Due to the limited possibilities is OT / - hardly used.

OpenTherm Smart Power

On 16 June 2008, the OpenTherm specification v3.0 was officially adopted by the OpenTherm Association. With this new specification OpenTherm SmartPower has been introduced. The master can the slave requesting low-, medium - or high power. This allows the manufacturer of the room thermostat device with more functionality, such as Backlight, large touch panels, additional sensors etc. equip.

Multi Point to Point

The specification v3.0 additionally describes how more than 2 devices with OpenTherm can be connected. Although OpenTherm is a point- to-point connection, additional devices can be connected between the master and slave ( gateways). A gateway has 1 slave and 1 ( or more ) master interface. The gateway controls what data is passed to the respective slave. An application example is a room temperature controller of a heat recovery system controls the turn is connected to a heater. The heat recovery system works like a gateway.

Certify products

Producers to place on the market OpenTherm products provided they comply with several requirements of the OpenTherm Association. The manufacturer shall be and the product must be tested by an independent laboratory, especially a member of the Association OpenTherm. After the handover of the test report and a declaration of conformity to the OpenTherm Association, the manufacturer is allowed to use the logo OpenTherm.


OpenTherm was established because different manufacturers needed a simple system for data exchange between controller and room heating. The installation and commissioning should be like in the conventional ON / OFF room temperature controllers so easy. Required characteristics were: reverse polarity protected 2-wire technology with power supply ( without batteries) via the bus.

In November 1996, Honeywell sold the protocol with the first specification for 1 pound to the OpenTherm Association. Shortly after the first OpenTherm products have been introduced. The OpenTherm Association has since grown to 42 members (2008). The specification has since been regularly adjusted to the increased requirements and expanded. Furthermore, the Association OpenTherm the task of promoting the spread of this open standard, for example, by specific lobbying and presence at international fairs such as the ISH (Frankfurt) and the Mostra Convegno ( Milan ).

Documents relating to the specification

The protocol specification: Protocol specification.

Documents for Certification: Test Specification.

The document Application Functional Specification describes various applications and how the OpenTherm IDs to be used. In addition, instructions for implementation are given.