Modbus, KNX and Zigbee: how they work together in a Brick system

Walk into a plant room today and you often find three different worlds: a heat pump speaking Modbus, an electrical installation with a KNX bus, wireless sensors and actuators in the rooms. Each has its own cables, tools and specialists.

For years the answer was to pick one world and stay in it, with all the limits that brings. With an open controller and Home Assistant there is no need: Home Assistant can run Modbus over RS485, KNX and Zigbee together, each where it works best. Let’s see where to use each one and how to avoid the most common wiring mistakes.

Modbus: the protocol of the panel

Modbus was created in 1979 for industrial controllers and is still the most widespread protocol in building services. Heat pumps, chillers, energy meters, solar inverters, room controllers: almost all of them have a Modbus port.

The version most used in building systems is Modbus RTU over an RS-485 line: two twisted wires plus a common reference, daisy-chaining up to a few dozen devices over hundreds of metres. There is also Modbus TCP, which runs over Ethernet.

It works simply: the controller polls one device at a time and reads or writes registers, numbered slots. Register 100 might be the flow temperature, 200 the set point. That is why, with Modbus, the most important document is the manufacturer’s register map.

Home Assistant includes a Modbus integration: you declare the devices and the registers to read, and they become entities like any other.

KNX: the building bus

KNX is a European standard for building automation, widely used in high-end electrical installations. A bus cable reaches switches, actuators, sensors and controllers, and they all exchange messages.

The logic differs from Modbus: devices are not polled one by one, but send messages to group addresses. A push button sends “on” to the living room lights group address and every actuator in that group responds. Configuration is done with dedicated software, ETS.

Where a building already has KNX, it makes sense to use it: KNX room controllers, such as Siemens RDG, integrate without new cables. Home Assistant speaks KNX through an IP interface or a bus connection, and reads and writes group addresses.

Zigbee: wireless in the rooms

Zigbee is a low-power radio protocol designed for sensors and small actuators. It is the standard for many temperature and humidity sensors, thermostatic radiator valves and battery-powered room controllers.

The feature to understand is the mesh network: mains-powered devices, such as sockets and relays, act as repeaters for the others. A battery sensor at the end of the corridor reaches the coordinator via a relay in the kitchen. The more powered devices there are, the stronger the network.

In upgrades, Zigbee is often the best choice for the rooms: no cables, sensors you can move in a minute, batteries that last for years.

A mixed system, step by step

Take a realistic example: a detached house with a heat pump, underfloor heating on the ground floor, fan coils upstairs and a KNX electrical installation.

1. The heat pump connects over Modbus RTU. Brick reads temperatures, power and alarms, and writes the flow set point according to the heating curve.
2. An energy meter on the same RS-485 line, at a different address, measures the heat pump’s consumption.
3. The ground-floor controllers are KNX, already specified by the electrician. Brick reads their temperatures and drives the manifold actuators.
4. The upstairs fan coils have Zigbee controllers in each room. Brick reads temperature and demand and drives the valves and fan speeds.
5. Home Assistant brings it all into one app, and Node-RED decides the logic: how many zones are calling, what flow temperature is needed, when to stop the heat pump.

For whoever uses the app, the protocols disappear: there are only rooms and temperatures.

Common wiring mistakes

On RS-485.

• Star wiring. The line must be daisy-chained from one device to the next. Long spurs cause reflections and communication errors.
• Termination resistors forgotten or fitted everywhere. They go only at the two ends of the line, usually 120 ohms.
• A and B swapped. Manufacturers do not all use the same convention: if a device does not respond, it is the first thing to try.
• Mismatched settings. Baud rate, parity and address must match across the line. A device left on a factory address that clashes with another blocks both.

On KNX. Respect bus polarity, stay within the maximum line length, and document the group addresses used for heating control so the electrician knows not to touch them.

On Zigbee. Do not shut the coordinator inside a metal panel, which screens the signal: bring it out with an antenna or an extension. And choose a Zigbee channel away from the Wi-Fi one, since both use the 2.4 GHz band.

Which to choose in a new system

In upgrades you start from what is there. In a new system you can choose, and a simple rule works almost every time.

• Modbus for everything in the plant room: heat sources, meters, inverters, chillers. It is the protocol equipment manufacturers support most, and an RS-485 cable in the panel costs little.
• KNX if the electrical installation is already KNX, or if the client wants advanced switches and scenes throughout the house. Then KNX room controllers are the natural choice.
• Zigbee for the rooms when you don’t want cables, or when rooms often change use. In new builds, if walls are being chased anyway, wired sensors or Modbus controllers remain more robust.

Either way, avoid adding a fourth proprietary protocol just for one component. Every extra protocol is another tool to learn and another place where something can break.

Configurations C0 and D0

For mixed systems with underfloor heating and fan coils, Brick has two ready-made configurations. C0 uses Modbus room controllers, such as Siemens RDF, and handles three-speed or EC fan coils. D0 uses KNX controllers, such as Siemens RDG, and also handles terminals in sequence, multi-stage heat sources and twin pumps.

Both start from the principle described in Home Assistant in a professional installation: open software, panel-grade hardware and documentation that lets any integrator take over the system. For fan coil details, see the dedicated article.