OpenTherm (OT) is a standard
communications protocol
A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any kind of variation of a physical quantity. The protocol defines the rules, syntax, semantics and synchr ...
used in
central heating
A central heating system provides warmth to a number of spaces within a building from one main source of heat. It is a component of heating, ventilation, and air conditioning (short: HVAC) systems, which can both cool and warm interior spaces.
...
systems for the communication between a
central heating boiler and a
thermostat
A thermostat is a regulating device component which senses the temperature of a physical system and performs actions so that the system's temperature is maintained near a desired setpoint.
Thermostats are used in any device or system tha ...
ic 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
A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, central h ...
from manufacturer B. However, OpenTherm controllers and boilers do not 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.
History
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
Sterling (abbreviation: stg; Other spelling styles, such as STG and Stg, are also seen. ISO code: GBP) is the currency of the United Kingdom and nine of its associated territories. The pound ( sign: £) is the main unit of sterling, an ...
,
Honeywell
Honeywell International Inc. is an American publicly traded, multinational conglomerate corporation headquartered in Charlotte, North Carolina. It primarily operates in four areas of business: aerospace, building technologies, performance ma ...
sold the first specification to the OpenTherm Association in 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
Frankfurt, officially Frankfurt am Main (; Hessian: , "Frank ford on the Main"), is the most populous city in the German state of Hesse. Its 791,000 inhabitants as of 2022 make it the fifth-most populous city in Germany. Located on its na ...
) and the Mostra Convegno (
Milan
Milan ( , , Lombard: ; it, Milano ) is a city in northern Italy, capital of Lombardy, and the second-most populous city proper in Italy after Rome. The city proper has a population of about 1.4 million, while its metropolitan city h ...
). , the association has 53 members from around the world.
Design
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.
Multi Point to Point
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.)
Variants
OpenTherm/Plus (OT/+)
The two wires are used both to supply power to the controller and for bidirectional digital communication between the controller and the boiler. The minimum available power is 35 mW. When using OpenTherm Smart Power this can, by master request, also be 136 mW (medium power) or 255 mW (high power). The controller transmits to the boiler by sending a
Manchester-encoded sequence in the Voltage domain. The boiler transmits data back to the controller in the current domain. OpenTherm specifies a maximum communications interval of one second. The data in the communication packet is functionally specified and is called OpenTherm-ID (OT-ID). 256 OT-IDs are available, 128 are reserved for
OEM
An original equipment manufacturer (OEM) is generally perceived as a company that produces non-aftermarket parts and equipment that may be marketed by another manufacturer. It is a common industry term recognized and used by many professional or ...
use. The other 128 are reserved, 90 of them are functionally specified. (OT specification v3.0)
OpenTherm/Light (OT/-)
When OT/- is used the master generates a
PWM voltage signal, representing the boiler water temperature set point. The boiler current signal indicates the status of the boiler: error, no error. Due to the limited possibilities OT/- is rarely used .
OpenTherm Smart Power
On June 16, 2008, OpenTherm specification 3.0 was approved by the association. This version introduces OpenTherm Smart Power. The master can request the slave to change the available power to low, medium or high power. With this master manufacturers can add more functionality to their products (backlight or extra sensors).
Certification
Manufacturers are allowed to market OpenTherm products when they comply with some rules of the OpenTherm association. Most importantly the manufacturer has to be an OpenTherm member, and the product must be tested by an independent testing body. By handing over the test report and a Declaration of Conformity to the association, the manufacturer is allowed to use the OpenTherm logo.
See also
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Building automation
Building automation (BAS), also known as building management system (BMS) or building energy management system (BEMS), is the automatic centralized control of a building's HVAC (heating, ventilation and air conditioning), electrical, lighting, ...
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HVAC
Heating, ventilation, and air conditioning (HVAC) is the use of various technologies to control the temperature, humidity, and purity of the air in an enclosed space. Its goal is to provide thermal comfort and acceptable indoor air quality. HV ...
*
HVAC control system
HVAC (Heating, Ventilation and Air Conditioning) equipment needs a control system to regulate the operation of a heating and/or air conditioning system. Usually a sensing device is used to compare the actual state (e.g. temperature) with a target s ...
*
KNX (standard)
KNX is an open standard (see EN 50090, ISO/IEC 14543) for commercial and domestic building automation. KNX devices can manage lighting, blinds and shutters, HVAC, security systems, energy management, audio video, white goods, displays, remo ...
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Programmable thermostat
A programmable thermostat is a thermostat which is designed to adjust the temperature according to a series of programmed settings that take effect at different times of the day. Programmable thermostats are also known as setback thermostats or ...
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Radiator
Radiators are heat exchangers used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in cars, buildings, and electronics.
A radiator is always a ...
References
External links
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Protocol specification (v2.2)
{{HVAC
Heating, ventilation, and air conditioning
Home automation
Residential heating
Temperature control