Thermostats
OpenTherm Thermostat (a room unit that talks)
In short
An OpenTherm thermostat is a room unit that talks to the boiler on two extra-low-voltage wires. Its terminals: OT1, OT2.
If this is your home: this page is written for heating engineers, and for learners under the supervision of one. It explains how the wiring works; it is not training to do the work. In England and Wales, electrical work in a home must meet the Building Regulations (Part P), and some of it must be notified to building control or done by a registered electrician; Scotland and Northern Ireland have their own rules. For the wiring, call a registered electrician or a heating engineer competent in controls. Work inside a gas boiler is for a Gas Safe registered engineer only. If you smell gas, leave the building and call the gas emergency number: 0800 111 999 in Great Britain, 0800 002 001 in Northern Ireland.
What it is
An OPENTHERM THERMOSTAT is a ROOM UNIT that TALKS to the boiler on two extra-low-voltage wires. OPENTHERM is the name of that way of talking. It does not switch 230 volts (230 V).
It asks the boiler for the WATER TEMPERATURE it wants, and whether heating is wanted. The boiler turns its flame up and down to give it: it MODULATES. This is LOAD COMPENSATION.
In England a gas combi fitted in an existing home must have at least one of four energy savers, and this is one (Boiler Plus).
The boiler POWERS it through the same two wires. This one has no battery and no mains supply, so with no boiler supply it is DARK.
OpenTherm is POINT-TO-POINT: one room unit on one boiler. The pair may be up to 50 metres long (the specification).
In this simulator it calls when the room is more than half a degree below its setting, and is satisfied half a degree above it. It asks for 40 °C just past that, rising to 75 °C when the room is 3 °C or more below its setting.
It comes set at 15 °C. Press - or + to change the setting by 1 °C, from 5 to 30 °C. The buttons work only while it is lit: dark, a press changes nothing, and the status line says why.
Its screen shows the setting, the room, and a small OT mark while it talks to the boiler. While it calls, it shows a flame and FLOW, with the water temperature it asks for.
DrawWarm's own design, wired the way the makers' OpenTherm room units are. Product and brand names belong to their owners; DrawWarm is not affiliated with any manufacturer.
Terminals
| Terminal | What it is for |
|---|---|
| OT1 | OpenTherm: one wire of the pair, to one of the boiler's OT terminals. It goes either way round: OpenTherm has no POLARITY. |
| OT2 | OpenTherm: the other wire, to the boiler's other OT terminal. It is EXTRA-LOW VOLTAGE: never 230 volts (230 V), never the boiler's switched live (SL), and never a volt-free input. |
What the meter reads
Live tests. On site, any reading taken with the supply on is work on or near live parts. Regulation 14 of the Electricity at Work Regulations 1989 allows it only when working dead is unreasonable, as when a fault shows only while the controls work, and only with suitable precautions: test leads made to the HSE’s guidance note GS38, fingers behind the guards. Regulation 16 allows it only for a competent person, or a learner under the supervision of one. Wiring and repairs are always done dead. In the simulator, nothing is live.
Wired to a DrawWarm boiler's two OT terminals, with the boiler supplied and the simulation running: it lights in under a second. With its wires crossed, it lights just the same.
With the simulation stopped it is dark, because the boiler powers it.
Its setting above the room: it calls, and its flame and FLOW show. The boiler heats to the flow it asks for, and the boiler's screen shows OT and that temperature.
Across its two terminals, on volts: a low direct current (DC) voltage, under 7 V, the specification's limit. In this simulator it lifts a little once a second while they talk. The other way round, it reads the same with a minus sign.
Taken off the pair: across the boiler's OT terminals the meter reads about 24 V DC in this simulator. That is the boiler's own supply for a room unit; the specification allows up to 42 V.
Where it connects
Lessons that use it
- What OpenTherm is
Intermediate · Bus controls
- Wiring an OpenTherm room thermostat
Intermediate · Bus controls
Practise it in the simulator.
Runs in the browser on a laptop or desktop. Cancel any time.