OpenTherm
OpenTherm (short: OT) is a protocol which is used to control of central heating by room temperature controller was developed (thermostats). OpenTherm is a "point to point" communication, whereby the thermostat is usually the "master" and the heat generator is the "slave". Several devices can be connected to one another using multi-point-to-point.
OpenTherm is manufacturer-independent. A controller from manufacturer A can communicate with a heater from manufacturer B.
functionality
A digital, bi-directional data exchange takes place between the thermostat (master) and heating (slave). It is possible to send status reports and request information between the two devices. The most common command, however, relates to the control of the boiler temperature. If the heating receives the target temperature request, it begins to modulate (increase or reduce the heating output) in order to reach and maintain the target temperature. The room thermostat constantly recalculates the boiler temperature in order to keep the room temperature at the setpoint. This leads to higher energy efficiency.
Communication medium
OpenTherm uses a 2-wire connection. Existing lines can therefore mostly be used. OpenTherm is reverse polarity protected, i. H. the polarity of the cables does not matter when connecting.
The maximum cable length is 50 m with a maximum resistance of 2 × 5 ohms.
OpenTherm / Plus (OT / +)
The two wires are used both for data exchange and for power supply. With this "point-to-point" connection, the room controller is the master and the heater is the slave. The master requests by changing the voltage level and the slave answers by changing the current strength.
The power supply for the controller is provided by the slave. The minimum available power is 40 mW (LowPower 5 mA / 8 V). OpenTherm Smart Power can provide 136 mW (MediumPower 17 mA / 8 V) or 306 mW (HighPower 17 mA / 18 V) upon request by the master. If the OpenTherm connection on the heater is briefly closed, the heater starts.
OpenTherm prescribes a minimum data exchange interval of 1 second. The data in the communication package are functionally specified and are called OpenTherm-ID (OT-ID). 256 OT IDs are available, 128 are reserved for OEM applications. Of the other 128 IDs, 90 are specified as a function (OT specification v3.0)
OpenTherm / Lite (OT / -)
If OT / - is used, the master generates a PWM voltage signal for the set boiler water temperature. The current signal of the heater shows the status of the heater: error, no error. If the OpenTherm connection is short-circuited, the heating starts. Due to the limited possibilities, OT / - is rarely used.
OpenTherm Smart Power
On June 16, 2008, the OpenTherm Specification v3.0 was officially adopted by the OpenTherm Association. With this new specification, OpenTherm Smart Power was introduced. The master can request low, medium or high power from the slave. This enables the manufacturers of room thermostats to equip the devices with more functionality, such as B. Backlight, large touch panels, additional sensors, etc. equip.
Multi point to point
Specification v3.0 also describes how more than 2 devices can be connected to OpenTherm. Although OpenTherm is a point-to-point connection, additional devices (gateways) can be integrated between master and slave. A gateway has 1 slave and 1 (or more) master interface. The gateway regulates which data is allowed through to the respective slave. One application example is a room temperature controller that controls a heat recovery system that is in turn connected to a heater. The heat recovery system works like a gateway.
Certify products
Manufacturers are allowed to bring OpenTherm products onto the market, provided they comply with some of the requirements of the OpenTherm Association. Above all, the manufacturer must be a member of the OpenTherm Association and the product must be tested by an independent laboratory. After the test report and a declaration of conformity have been submitted to the OpenTherm Association, the manufacturer is permitted to use the OpenTherm logo.
history
OpenTherm was founded because various manufacturers needed a simple system for data exchange between room controllers and heating. Installation and commissioning should be as easy as with conventional ON / OFF room temperature controllers. Required features were: Reverse polarity protected 2-wire technology with power supply (without batteries) via the bus.
In November 1996, Honeywell sold the first specification protocol to the OpenTherm Association for £ 1. Shortly afterwards, the first OpenTherm products were introduced. The OpenTherm Association has since grown to 42 members (2008). Since then, the specification has been regularly adapted and expanded to meet the increased requirements. Furthermore, the OpenTherm Association has the task of promoting the dissemination of this open standard, for example through specific lobbying work, and being present at international trade fairs such as the ISH (Frankfurt) and the Mostra Convegno (Milan).
Documentation about the specification
The protocol specification : Protocol specification .
Documentation for certification: Test Specification .
The Application Functional Specification document describes various applications and how the OpenTherm IDs should be used. In addition, instructions for implementation are given.