OpenTherm is a non-manufacturer-dependent system of communication between modulating HVAC heating appliances and room thermostats. It consists of a communication protocol and an interface specification. OpenTherm is a futuristic technology combining simple installation procedure with high functionality and future expansion possibilities.
Thermostats operated with a traditional on/off controller can be exchanged easily, regardless of whether they are regular thermostats or clock controllers. The same applies for the compatibility of central heating appliances operated by on/off controllers. This provides a wider choice to the fitters when determining the components of a heating system.
Yet when manufacturers of heating systems apply extra functions, more sophisticated communication protocols are needed. The various products are then no longer necessarily interchangeable. In this case, products (i.e. a central heating appliance and a controller) can only operate together provided that they use the same manufacturer-specific protocol.
On the other hand, if manufacturers of both central heating appliances and controllers all maintain a common standard protocol, their respective products can easily operate together. Hence, opting for a standard protocol means opting for a larger market and more freedom of choice for all market participants.
Standards for heating controllers
The communication protocol describes the way digital information is exchanged between central heating appliances and room thermostats. The way this protocol is implemented, and any extra functionality resulting from the use of OpenTherm, is determined entirely by manufacturers of central heating appliances and room thermostats.
Product documentations should state which system functions are available for particular combinations of products. In any case, using OpenTherm guarantees a minimum level of compatibility.
Future expansion possibilities include connecting OpenTherm controllers to a home bus system (yet to be determined).
The following applies to the OpenTherm protocol:
- OpenTherm is a multi point to point communication protocol for use in residential heating.
- OpenTherm requires a standard type of connection like in any modern installation practice, i.e. a 2-wire low voltage and polarity-free connection.
- OpenTherm can also be used as a wireless protocol.
Click the “Using the protocol” button to find further technical information on the OpenTherm protocol. The means of obtaining the Protocol Summary for evaluation purposes is also discussed here. No costs are involved but we must emphasise that possession of the OpenTherm Protocol Summary does not automatically mean that products in which OpenTherm is implemented may be brought onto the market.
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