Range of application

Though there are ‘high-end’ solutions available involving bus systems as part of building automation systems, a single unequivocal standard has not yet emerged. Moreover, such solutions are relatively costly for the point-to-point connections which are common in the intended market segment. Many manufacturer-specific systems have since been developed but these do not offer the option of exchanging products from different manufacturers.

The need has arisen for a standard which enables the straightforward coupling of controlling devices like room and clock controllers to heating appliances such as central heating appliances and air heaters with low costs. The OpenTherm communication protocol fulfils this requirement.


A brief introduction to this Protocol Summary including an overview of its main features and a system list can be viewed by clicking the following link (English version). Here it is also stated that, in addition to the full implementation of the protocol, designated OpenTherm/plus or OT/+, there is also a ‘light’ version, OpenTherm/light or OT/-. The OT/- is easier to implement but offers fewer functions.

The basic function which every OpenTherm controller must provide is the communication of the heat requirement (Control Setpoint) to the heating appliance. This heat requirement is calculated by the controller on the basis of the difference between the set temperature and the desired one, or another form of control such as outside temperature control. The protocol also supports many other non compulsory functions offering the manufacturer the opportunity to develop multiple product variants. Further information on implementation can be found in the Protocol Summary.

The availability

Manufacturers of heating boilers, controlling devices and associated products who are interested in developing and producing OpenTherm-compatible products, and therefore require a description of the communication protocol, can request a data sheet from the secretariat of the OpenTherm Association. This version is intended to enable manufacturers to evaluate the OpenTherm protocol’s compatibility with their systems.

The data sheet can be obtained for free by completing the request form under the link below and returning it to the OpenTherm Office. The data sheet will only then be sent to you. The original protocol is only available for members.

  • Data Sheet Specification Request Form: click here

Note: The OpenTherm Association emphasizes that possession of the evaluation version of the Protocol Summary does not convey the right to apply it to commercial products. Rights to use the protocol are reserved for the OpenTherm Association who issues licenses to manufacturers and suppliers under certain conditions. See the section for information on obtaining a license.

Who uses this standard?

The OpenTherm protocol must be implemented in the control electronics of the appliance or controller which communicate via the OpenTherm protocol. Appliance manufacturers often obtain the controlling elements of their appliances (burner automation and control electronics) from specialised suppliers. If the OpenTherm protocol is implemented with the aim of bringing a commercial product onto the market, then one of the parties stated must take responsibility for the accuracy of the implementation.

This party will be the mouthpiece of the association regarding the procedures as described below. It must also be a member of the OpenTherm Association. This party needs not necessarily be responsible for bringing the appliance or controlling device onto the market. See the section “

Implementation for applications

Manufacturers who opt to implement the OpenTherm protocol for OpenTherm compatible applications must be members of the OpenTherm Association. During implementation, application notes and an application functional specification may be used. These are issued by the Association.

All members of the OpenTherm Association receive a unique identification code they can use in the applications developed under their responsibility. This makes it possible for products to identify each other in order to harmonize on compatible functions (handshaking).