Intermediate · Unit 7 · Written and being checked
Bus controls
In short
OpenTherm: two extra-low-voltage wires between a room unit and a boiler. Polarity-free, one room unit per boiler, the boiler able to power the room unit, the room unit asking for a water temperature. This simulator's meter reading DC by itself, an OpenTherm stat wired to a combi, what OpenTherm cannot do, and two zones on a receiver. eBUS and EMS in words. This unit is written and being checked; it opens to learners when it is ready.
In this unit
What OpenTherm is
Two extra-low-voltage wires between a room unit and a boiler. They are polarity-free, they join one room unit to one boiler, and the boiler can power the room unit through them. The room unit asks for a water temperature. eBUS and EMS, in words.
How it opensUntil now, every room thermostat (stat) you have wired was a SWITCH. It sent the boiler 230 volts (230 V) to say "heat", or nothing to say "stop". That was all it could say.
The meter reads DC by itself
This simulator's meter shows AC or DC by itself on its volts setting. You read 230 V AC at the spur, then about 24 V DC across the DrawWarm Combi's empty OpenTherm pair, and a minus sign with the probes the other way round.
Wiring an OpenTherm room thermostat
An OpenTherm room thermostat wired to the DrawWarm Combi by a twin cable, crossed on purpose. The boiler powers it, the heating link stays in, and the flow temperature follows how far the room is below the setting.
What OpenTherm cannot do
This thermostat has no battery: with no boiler supply, it goes dark. One pair takes one room unit, so a second stat will not work. A boiler without OpenTherm cannot take an OpenTherm stat, and 230 V on the pair can destroy the electronics.
Two zones on an OpenTherm receiver
An OpenTherm receiver with two zone relays drives two zone valves, and asks the DrawWarm System Boiler for heat over OpenTherm.
In the simulator, you practise how to
- Say what OpenTherm is: two extra-low-voltage wires, polarity-free, one room unit on one boiler, the boiler able to power the room unit
- Say what an OpenTherm room unit sends: the water temperature it wants, and whether heating is wanted - the boiler modulates
- Read 230 V AC and a DC voltage with the meter, and say what a minus sign on a DC reading means
- Wire an OpenTherm room stat to a combi with a twin cable, leaving the boiler's own heating link in
- Say why a second room unit on one OpenTherm pair will not work, and what to check before fitting an OpenTherm stat to a boiler
- Say why 230 V must never reach OpenTherm, eBUS or EMS wires
- Wire an OpenTherm receiver to two zone valves, the oranges parked on block 10
- Name eBUS and EMS, and what each boiler maker's bus asks of the installer
- Isolate and prove dead before any wire is touched, and test the earth of what was changed before switching back on
Checks and credit
- A timed check at the end of the unit, its questions drawn at random. The score goes on your record.
- The unit earns the credential Bus Controls (simulator).
Certificates of completion and credentials are DrawWarm’s own. They record practice and tests in the simulator. They are not a formal (regulated) qualification, and they do not show that anyone is competent to work on a heating system.
Where it connects
The parts it uses
Intermediate
- Protection and extras
Unit 6, before this one
- Actuator anatomy
Unit 8, next
Back to the Programme.
Practise it in the simulator.
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