KNX

What is KNX, and where does the standard come from?

KNX is an open international standard, published as ISO/IEC 14543, for controlling lighting, shading, heating and ventilation in houses and other buildings over one shared wired bus. Devices from different manufacturers can work together because they follow the same protocol, so a push-button from one maker can command an actuator from another. Choosing KNX is decided on the drawings, long before the first switch is fixed to a wall.

The standard grew out of several European installation bus systems from the late 1980s and 1990s, which were merged under one association so that products could share a common language. The KNX Association maintains it today, and certified devices carry the KNX logo.

How does a KNX installation work?

A KNX installation has three parts: one twisted-pair bus cable that runs through the house, devices that connect to it, and configuration software that decides which device reacts to which. The same cable carries data and the low-voltage power the bus devices run on. That power comes from a bus power supply mounted in the rozvádzač (distribution board), not from each wall box. Commands are not wired point to point. A wall push-button sends a telegram onto the bus, and any actuator programmed to respond to that group address switches its load.

Configuration happens in the ETS (Engineering Tool Software), which the integrator uses to give each device a physical address and to link switches to actuators through group addresses. The output is an ETS project file, and that file is the real record of how the house behaves.

Component	Role	Usual location
Bus cable	Carries data and bus power between all devices	Routed through walls and ceilings, apart from mains cabling
Bus power supply	Feeds low-voltage power onto the bus	DIN rail inside the distribution board
Push-button or sensor	Sends commands such as a press, a movement or a temperature reading	Wall box or ceiling
Actuator	Switches or dims a load, or drives a blind motor	Distribution board, or a flush box near the load
IP gateway or router	Connects the bus to the home network and to apps	Distribution board or network cabinet
Room controller	Reads room temperature and drives heating valves or actuators	Living space, away from heat sources

How is KNX different from wireless and proprietary smart home systems?

Wireless products such as Zigbee, Z-Wave, Wi-Fi bulbs or single-brand ecosystems are easier to add to a finished house. Most devices pair with a hub or a cloud service. The trade-off is that each ecosystem has its own rules, and the reliability of the whole system depends on radio coverage, battery health and the vendor staying in business. A KNX system is harder to start, but its backbone is a wire that does not depend on a single company.

Table 2 sets the two approaches side by side. Matter, a newer interoperability standard, narrows the gap for simple products, but a wired bus still gives the more predictable result for whole-house heating and shading.

Question	KNX (wired bus)	Wireless or proprietary
Retrofit into a finished house	Hard, because new cable must be routed	Easy, devices replace existing switches or bulbs
Mixing manufacturers	Designed for it, using certified devices	Usually limited to one ecosystem or hub
Reliability	Wired, not affected by radio conditions	Depends on radio coverage and battery state
Where the logic lives	Local controller and project file	Often a hub or the vendor cloud
Changes after handover	Needs the configuration software and skills	Often done by the owner in an app
Upfront labour	Concentrated in cable installation during construction	Lower in a new build, but many devices add up

Why must KNX be planned before plastering?

In a new build or deep renovation, the bus cable is pulled during the hrubá elektroinštalácia (first-fix electrical installation), at the same stage as the power and data cabling and before the walls are closed. Once plaster and screed are down, a missing cable run means cutting into finished surfaces, repairing them and repainting, which is slow and costly. The cable must also stay apart from mains-voltage wiring, so its route is a design decision that should not be improvised on site.

The bus cable is conventionally green-jacketed, but the route matters far more than the colour. Every switch position, sensor and shading motor needs a path back to the controller, so the designer should fix the location of each flush box and the cable path between them on the drawings. Lighting positions should follow the interior lighting plan, so that switches and fittings are placed once rather than patched later.

The distribution board is the second constraint. It needs room for the bus power supply, the DIN-rail actuators that follow, and the usual protective devices. Boards designed only for a conventional installation often lack that space, and the compromise is a second enclosure or a larger board.

What does integrator dependence mean for the owner?

Most residential KNX systems are configured by an integrator using ETS, and the full version of that software is a paid licence. The owner lives with the result but may not hold the knowledge needed to change it. A new scene, a relocated switch or a revised heating schedule may then require a visit, and the cost depends on how well the work was documented.

The exposure is qualitative rather than a fixed cost, but it is real, and it is decided at the contract stage. Three practices reduce it:

• Require the ETS project file and a written list of group addresses at handover, with copies kept by the owner.
• Choose certified products that other integrators can also support, so a later change of firm does not force a rebuild.
• Agree in the contract who holds the configuration licence and how another integrator would take over the project.

When is KNX the right choice for a house?

KNX fits best when the client wants lighting, shading and heating to work as one system, the house is new or deeply renovated, and the owner expects to keep the building for decades. Wireless is usually the better answer for a small flat, a rental, or an existing building where cable routes cannot be opened. The two can also meet: a KNX core can handle heating and shading while a few wireless devices cover spaces the bus does not reach. Single-room projects are often better served by smart lighting control or a smart thermostat.

Before committing, the architect and client should be able to answer yes to these questions:

• Is the house designed before the electrical first fix starts?
• Does the client want scenes that combine lighting, blinds and heating?
• Is there space in the distribution board for the bus power supply and modules?
• Will the owner accept a documented system and a specialist for later changes?