KNX or wireless: when a wired system is still right

A KNX installation starts as a length of two-core cable, before it is anything else. The same pair carries the 29 V DC that powers each device and the data telegrams that tell a relay to close or a dimmer to fade, at a fixed 9,600 baud, according to the system-media documentation published by Weinzierl Engineering, a KNX Association member manufacturer. Nothing in that description involves a hub, a mobile app or a cloud account.

Wireless automation solves a different problem: getting a command to a device with no cable run to it at all. KNX itself sells a radio version built for that case, KNX RF, and the KNX-certified manufacturer Theben states its indoor range without much padding: up to 30 metres under favourable conditions, and only a few metres where conditions are not. That figure, published from inside the KNX ecosystem rather than by a wireless rival, is a fair place to start comparing the two.

The bus, in specific terms

KNX TP1, the twisted-pair medium most installers mean when they say KNX, runs at a nominal 29 V DC on the bus itself. ABB's power supplies for it are rated at 30 V DC plus 1 minus 2 volts with the line choke fitted, sitting a little above nominal to allow for voltage drop over the length of the cable. The bus is Safety Extra-Low Voltage throughout. A single 640 mA supply covers a fully loaded 64-device line; a 320 mA model suits a smaller one.

Communication runs peer to peer rather than through a hub. A telegram carries the sending device's own individual address and a group address, not an instruction aimed at one named device, and any actuator subscribed to that group address acts on it. A push-button sensor and a dimmer actuator, once commissioned, keep working with no controller running in the background, no gateway, and no internet connection required for the light to turn on.

Lines, areas and a backbone

KNX Association's own Project Design Guidelines fix the topology in numbers, not rules of thumb. A line takes up to 64 devices and needs its own power supply. Fifteen lines, plus one main line, form an area. Fifteen areas can be joined to a backbone line by area couplers to make one installation. The guidelines are blunt about what that means for hardware: the correct number of power supplies in a project is the number of line couplers plus one.

The same document gives the rule installers actually use. One line is often enough for an entire single-family house, or one per floor at most. Commercial buildings only differ because device counts and the cable-length limit force the structure into areas sooner: one area per floor, one line per energy zone, is the pattern the guidelines suggest, not an arbitrary one.

Commissioning is a project file

Every KNX device, whichever of the more than 500 member manufacturers made it, is configured in the same tool: ETS, the Engineering Tool Software that KNX Association publishes and maintains itself. It imports certified product data into one project, and one workflow covers system layout, device configuration, group-address linking, parameter download and documentation. A free demo version covers training and small jobs; unrestricted project sizes need a paid professional licence, sold through KNX's own store rather than through any one manufacturer. The current release is ETS6.4.

That project file is the asset a wireless retrofit rarely produces. Pairing a bulb to a phone app records almost nothing outside that app's own account. Commissioning a KNX line through ETS produces a document naming every device, every group address and every line, and that document outlives the installer who wrote it.

KNX's own radio, and where it stops

KNX RF runs at a fixed 868.3 MHz using frequency-shift keying, at 16,384 baud, in packets of 8 to 23 bytes, transmitting at between 0.5 and 25 milliwatts. Theben's own system manual for the standard puts the achievable range at up to 30 metres indoors in favourable conditions, only a few metres where reflection, absorption or a reinforced-concrete wall gets in the way, and up to 100 metres in open air. A media coupler bridges an RF segment back onto a TP line, commissioned from the same ETS project either way, and individual RF devices can be set as repeaters, though the manual warns against turning that on for every device in a system.

What that tells you about other wireless mesh systems

KNX RF is a useful comparison precisely because a manufacturer inside the same standards body publishes its indoor range without marketing gloss: thirty metres, then a few, depending on the wall. Consumer mesh protocols tend to describe only the upside. The Connectivity Standards Alliance describes Zigbee as scalable to thousands of nodes on a self-organising, self-healing mesh, and the Thread Group makes the same self-healing claim for Thread, adding that it needs no proprietary gateway to reach an IP network. Those claims describe the mesh layer honestly enough. They say nothing about the single hub, single app and single cloud account most consumer installations still route through underneath it.

Cost and lifespan sit on the same ledger

A wired bus has to be run before the walls close. That is a scheduling cost as much as a materials one, and it is real, not theoretical, in a finished building with no accessible voids. Against that, KNX's hardware count scales in a way a specifier can plan for in advance, using the line-coupler-plus-one rule above, rather than discovering after handover that one hub has quietly become the single point of failure for forty devices.

• New-build or a renovation stripped to the plasterboard, where containment can be run before finishes close.
• Device counts per floor that already push toward the line and area structure KNX's own guidelines describe, rather than sitting comfortably under it.
• A requirement that the system keep working even if one manufacturer discontinues one hub model or one cloud account.
• A building large enough that KNX's published one-area-per-floor pattern is the natural way to organise it, not a workaround forced on a smaller job.

KNX has stayed one open standard since the KNX Association introduced its specification in spring 2002, built on the earlier EIB specification. The same protocol was ratified as European standard CENELEC EN 50090 in December 2003, and as international standard ISO/IEC 14543-3 in November 2006, across all four of its transmission media. No single company owns it, and no single company can discontinue it. A wireless smart-home hub is usually one product line owned by one company.

Where the crossover sits

None of this makes wireless the wrong answer everywhere. A rented flat, a listed building where chasing a wall is not permitted, or a handful of devices added after the fact, are all reasonable candidates for wireless, KNX RF included, within the range figures above. The threshold KNX's own guidance points to is structural rather than a figure in square metres. Once a project's device count or cable run pushes past what one line and one power supply carry, or the building has enough floors that KNX Association's own guidance calls for one area per floor, the planning discipline of the bus starts paying for itself rather than getting in the way.

Before choosing a radio-based system over a wired one, get the manufacturer's own range and attenuation figures for the exact building fabric involved, not the headline range printed on the box. Theben's own manual shows how far those two numbers can drift apart.