OpenTherm

OpenTherm (short: OT) is a protocol used in central heating systems between a central heating boiler and a thermostat or controller. It is a point to point protocol where one device (thermostat) is the master and the other the slave (boiler). Multiple devices can be linked by using the Multi Point to Point specification.

OpenTherm is manufacturer independent. A controller from manufacturer A may be used to control a boiler from manufacturer B. However, interoperability between two Opentherm devices cannot be guaranteed without testing them specifically. Opentherm contains a number of optional features and some devices may include manufacturer-specific features. The presence of such a feature on only one of the devices of course will not allow it to work as intended. In the worst case, the presence of a manufacturer-specific feature may prevent successful operation altogether.

How OpenTherm works

Communication is digital and bi-directional between the thermostat (master) and the boiler (slave), many different commands, status reports and requests for information between the two devices is possible. However the most basic command is to control the boiler water temperature. The boiler when it has received a temperature control setpoint command will modulate (reduce or increase the heating power) to maintain this temperature setpoint. The thermostat constantly calculates what temperature the boiler water should be to maintain control of the room temperature, this results in a greater energy efficiency.

Communications media

Physically OpenTherm is a 2 wire connection allowing the existing wiring to be re-used. OpenTherm is not polarity sensitive: wires can be swapped. The maximum wiring length is 50m up to maximum 2 x 5 ohm resistance.

OpenTherm/Plus (OT/+)

When referring to OpenTherm/Plus (OT/+) most of the time the “Plus OT+” part is left out.

The two wires are used for both communications and power supply. In this point-to-point connection the controller is the master and the boiler the slave. The master requests by changing the voltage level, and the slave responds by changing the current. Power supply for the controller is supplied by the slave. The minimal 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).

When short circuiting the OpenTherm connection on the boiler, the boiler will start heating.

OpenTherm specifies a minimal 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 use. The other 128 are reserved, 90 of them are functionally specified. (OT specification v3.0)

OpenTherm/Lite (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.

If the wires forming the OpenTherm connection to the boiler are connected together (short-circuited), the boiler will start and heat up to its existing setpoint. This is a useful means of localising a fault on a system using Opentherm: if the boiler fires with the Opentherm connection short-circuited, this proves that it is working, at least to some extent.

Due to the limited possibilities OT/- is hardly used.

OpenTherm Smart Power

On June 16 2008 OpenTherm specification 3.0 is 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).

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.)

Certifying products

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.

History

OpenTherm was founded 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, Honeywell sold the first specification to the OpenTherm Association on November 1996. Shortly after the first products appeared on the market. The Association has grown since then to around 42 members (2008) 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) and the Mostra Convegno (Milan)

Specification documents

The protocol specifications document: Protocol specification (v2.2).

Document used for certification: Test Specification.

The document Application Functional Specification describes different applications and how the OpenTherm ID’s are to be used. In addition implementation tips are given.

See also

• Building automation
• Central heating
• Condensing boiler
• HVAC
• HVAC control system
• KNX (standard)
• Programmable thermostat
• Radiator

External links

• OpenTherm website