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 secretariat. 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 issue licenses to manufacturers and suppliers under certain conditions. See the section left 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 “Obtaining a licence“.

Implementation for applications

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

There is also the option of participating in the association’s Technical Commission which discusses any potential implementation issues. The Technical Commission is also responsible for maintenance and the expansion of the Protocol Summary with new functions.

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).

Testing the implementation yourself

A manufacturer will often want to carry out tests himself, preferably during the development phase, in order to ascertain that he is heading in the right direction. He may possess a test specification which outlines the demands the product has to fulfil in order to comply with the Protocol Summary. Additionally, a test tool, developed by a member of the association, is available which can be used to test the correct implementation.

A new version of the OpenTherm Test Tool is available. This tool is used by almost all members to test their products and used by Kiwa Gastec Certification for certification testing. The most important changes and improvements in this new Test Tool are:

– Improved and similar user interface for master and slave mode
– Improved response of slave according to settings
– Display of master request in slave mode
– Option to give no response on request in slave mode
– Log files are improved for easy import in excel
– Occasionally incorrect detection of midbit error repaired
– Existing OpenTherm Test Tools can be upgraded by new firmware and downloading the latest verion Windows program – For more information contact info@hccp.nl

For more information and prices please contact the website: http://www.hccp.nl (select: Products)

Testing by an independent testing body

To receive a licence for an OpenTherm product, a prototype of the product has to be tested for conformity. These tests are carried out by an independent testing body.

The OpenTherm Association has appointed Gastec Certification as a specialised and independent body to carry out conformity assessments for each new product type. These tests are made according to a specification drawn up by Gastec Certification in consultation with the association. The same test specification is made available to association members (see “Testing the implementation yourself“).

In consultation with Gastec Certification, the information to supply must be determined, such as for instance the diagrams, technical descriptions and any results derived from tests carried out by the manufacturer himself.

On the basis of the technical study , Gastec will issue a test certificate and report to the client. The tests carried out by Gastec Certification are restricted to the compulsory elements of the OpenTherm protocol. The client is free to make further agreements with Gastec regarding the testing of other functions as well as carrying out other studies, e.g. within the scope of electrical safety or EMC. The costs of testing are to be settled by the client.

Manufacturer’s self-certification

On the basis of a positive test report from Gastec Certification, the manufacturer himself then draws up a certificate in which he declares that the tested product complies with the demands of the Protocol Summary, and guarantees that the series of products which are manufactured on the basis of the tested prototype will be manufactured to the same technical standard as the prototype. The manufacturer sends this declaration to the secretariat of the OpenTherm Association with a request to issue a licence for the products in which the certified version of the OpenTherm implementation is to be applied.

Obtaining a licence

Once all the preceding steps have been completed successfully, the association issues the requested licence to the manufacturer. This bestows upon him the right to market the stated product under the name and logo of OpenTherm.

These agreements and procedures must guarantee that products marketed as OpenTherm are actually able to operate with other OpenTherm products.

Expanding the functionality

The OpenTherm communication protocol is primarily designed to control modulating heating appliances. As stated above, the protocol offers expansion possibilities which enable the manufacturer to build extra functions into their control devices or appliances as well as the basic function which is compulsory for all OpenTherm implementations. As such they can distinguish themselves from other suppliers and increase the freedom of choice for installers and consumers.

Of course, one must ensure that these extra functions – which are not included in the OpenTherm Protocol Summary’s minimum specifications – are also communicated in a standardised fashion. The Technical Commission of the OpenTherm Association monitors this and ensures that standardised functions are added to the Protocol Summary.

Here are some functions which have already been standardised in this way:
– setting and reading tap water temperature
– switching tap water on and off
– reading the outdoors temperature
– reading malfunctions
– weather-dependent control