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.
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.
When referring to OpenTherm/Plus (OT/+) most of the time the “Plus OT+” part is left out.
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.
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).
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.)
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.
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.
The protocol specifications document: Protocol specification.
