KNX

KNX is an open international standard (ISO/IEC 14543-3) for building automation, and it is the protocol a European electrician is most likely to have wired into a distribution board. Devices sit on a twisted pair line that carries both data and 29V bus power, they are configured once in ETS, and from then on they talk to each other directly. A switch actuator does not ask a hub whether the light should turn on; it listens for a group address and acts.

That is the property that sets KNX apart from every radio protocol on this site: the logic lives in the devices. Pull the controller out of a KNX house and the lights, blinds, and heating carry on exactly as commissioned.

Strengths

• No single point of failure. Group addresses are wired into the devices themselves, so lighting and shading keep working with the server, the network, and the internet all down. This is the reason KNX is specified on projects where a failed hub is not an acceptable outcome.
• Fully local and unencumbered. No cloud, no account, no vendor API to be deprecated. The bus is the bus.
• Genuinely vendor neutral. Certified devices from different manufacturers interoperate on the same line, which is why a KNX board routinely mixes ABB, MDT, Gira, Schneider, and Theben in one project.
• Built for scale. Lines couple into areas over IP, and a project runs to thousands of addresses without the routing and range problems a mesh radio hits. Wired also means no batteries and no coverage survey.
• Long service life. Devices commissioned twenty years ago still work on a current line, and the ETS project file documents the installation in a form the next contractor can pick up.

Trade-offs

• It has to be wired. Bus cable to every switch, actuator, and sensor position is a first-fix decision. KNX RF exists but is rare, so in practice KNX is for new build and full renovation, not retrofit.
• ETS is mandatory, and it is not free. Every device is commissioned in ETS, licences are paid, and the resulting project file is the installation. No ETS project, no changes; a job handed over without one is a job the next contractor has to reverse engineer.
• Commissioning is a trade skill. Group address structure, flags, and filter tables are not something to learn on a client’s board, and the cost of the design work is a real part of the project.
• Weak at the modern layer. KNX visualisation, voice control, phone notifications, presence detection from non-KNX sources, and integration with everything from a doorbell camera to an EV charger are where KNX ages fastest, and where the manufacturer answers are expensive and closed.
• Hardware costs more per point than Zigbee or Z-Wave equivalents, though on a project of any size the wiring and commissioning usually dominate the bill anyway.

KNX in Home Assistant

Home Assistant reaches the bus through the KNX integration, which needs a KNX IP interface or IP router on the line. Three connection types are available:

• Tunnelling over UDP or TCP to one IP interface. This is the default and the one to use first. Secure Tunnelling is the KNX Secure variant, and UDP supports NAT traversal through the route back option.
• Routing over IP multicast, at 224.0.23.12:3671 by default. This requires a dummy device created in ETS at the same topology level as the routers, and a network that actually passes multicast between the Home Assistant host and the board. On a managed switch or across a VLAN boundary, prove the multicast path before relying on it.
• Automatic, which scans for a gateway and connects by tunnelling.

Everything is local: Home Assistant sits on the bus as another participant, reading and writing group addresses. The integration covers sixteen entity platforms, including light, cover, climate, fan, sensor, binary sensor, scene, number, select, text, date, time, and notify, so a commissioned KNX house maps onto Home Assistant entities without custom code.

Two details worth knowing before the first job. KNX IP Secure and KNX Data Secure both need a .knxkeys keyring exported from the ETS project, so export it during commissioning rather than scheduling a return visit; Data Secure makes the keyring mandatory. And KNX covers invert Home Assistant’s convention, where 0 percent is closed and 100 percent open, which is the single most common surprise on a first KNX integration.

Where KNX and Home Assistant fit together

KNX is excellent at the layer it was designed for and poor at the layer that has grown up around it. Home Assistant is the reverse. A KNX bus keeps the building running and Home Assistant supplies the visualisation, the automation logic that spans systems, the voice and app control, and the integrations with everything that will never speak KNX. Because the bus does not depend on Home Assistant, adding it carries none of the risk that adding a controller normally does: if the Selora Hub is off, the building is still a working KNX installation.

That makes KNX one of the few protocols where a Home Assistant layer is an addition rather than a dependency, and it is why a KNX project is a good candidate for a managed Home Assistant install rather than a proprietary visualisation server.