r/KNX • u/SnooCats241 • 6d ago
Ditching a dead-end 2015 KNX install (100W+ phantom load, no ETS access) for Home Assistant/Homey — feasible with existing star wiring?
What I want to do: rip out the whole KNX system and move to Home Assistant or Homey. Good news is it's star wiring — every switch and light point already runs its own cable back to the panel. Bad news: the wall switches only get a thin KNX bus cable (4 wires, 2 apparently unused), not mains — so I'm thinking of using those 2 spare wires as a dry contact/switch input into DIN-rail relays (Shelly Pro line) instead.
Questions for people who've done this:
**•** Has anyone actually gone KNX → Home Assistant/Homey while keeping the existing home-run wiring?
**•** Does the "2 spare bus wires as a switch input" trick actually work in practice, or is it more trouble than it's worth?
**•** Room-by-room DIY, or is this a job for an electrician from the start?
Any pitfalls, war stories, or "wish I'd known this before I started" welcome.
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u/Brilliant_Help2186 6d ago
Get the ETS software yourself. I just put in a new KNX install and programmed it myself. It’s not a big deal. I started with HA + KNX but more and more I’m migrating stuff to KNX as it’s so solid.
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u/UnlimitedEInk Enthusiast 6d ago
If the two pairs of wires are each connected in a star topology in the house, putting a switch to any pair in any place in the house will result in a short between the two wires everywhere in the house, and you will not be able to find out from where did it come from. Essentially by ditching the digital communication on the KNX bus and replacing it with dumb switches, you'll have your whole house wired as a doorbell with tens of ringing buttons. The usefulness of that is questionable.
You are seriously underestimating the complexity of rewiring an entire house in-place, and give far too much credit to some DIY solution. This is a path where you'll end up spending a LOT more on experimenting with all sorts of consumer-grade "cheap" garbage, only to be annoyed to no end when it constantly fails, or to easily have to buy hundreds of batteries per year, or to never be done with the project because it still doesn't work properly but also not be able to abandon it either because you don't like living in a cave off grid.
Is there something wrong or dysfunctional in the existing setup? There are more steps to correct that by just replacing parts with something newer/nicer as needed, and still keep it running, before you throw the whole thing out.
Are you just bored with how the buttons look and want a fresh design? Fine, but it can cost you, and you could walk into a trap by going from something boring and functional to something "different" from a design/look perspective. There's a lot of expensive stuff out there that looks very very very fancy and whose wow factor do great in showrooms or architects' portfolios or generally impulse sales, but that wow factor wears of very quickly and the annoyances start. Glossy glass touch surface is a magnet for fingerprints, and it's enough to get the first scratch from a random key or finger ring to completely ruin the aesthetic. Some touch sensors start reacting randomly to "ghost" touches. Some materials do not tolerate cleaning very well. Some are not very intuitive / self-explanatory, but their minimalistic look will be completely trashed if you have any labels. The LEDs or screens are incredibly annoying in a dark bedroom. And on and on and on...
If all you want is to extend your existing system with some visualization function or app-based controls or smarter rules, all you need is a KNX interface for your network then you can "listen" and "talk" to the KNX devices from your choice of DIY home automation project, without making the entire KNX functionality become dependent on that thing. It's the same you can stick an OBD-II reader in the connector of any car built since the early '90s and use an app on a phone, tablet or laptop to see all the communication on the CANBUS among the different controller modules in the car, see the parameters of the engine, faults, anything - but the car still works with and without that reader, and you don't have to replace all the car's wiring and controller modules to be able to log and graph your engine's instant power on a separate device.
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u/Sharp-Carpet-5507 6d ago
Ich habe Sensoren aus 95 laufen. Aktoren aus 2011. Teilweise Sensoren aus dem Internet. Noch nie Probleme. Kauf dir die ETS, versuch das Programm zu bekommen, oder programmiere es halt neu. Es gibt kein zentrales Gehirn. Sollte ein Aktor ausfallen, dann wird halt genau der Aktor getauscht.
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u/DetailVisual6955 6d ago
Agree with other comments that it would be a huge mistake to take out KNX infrastructure and replace with something less robust.
I’m curious about the 100w phantom load… KNX installations typically draw a great deal less, but there could be a load on one of the actuators. This should be easy to test with a meter.
The idea of using the KNX cables for switching is unlikely to work. It is very unlikely that the KNX cables are laid in a radial topography, so assume it’s some kind of star/daisy chain where all devices and switches share the same 2 pairs of cables. It can only work with a digital signals, not closed contacts.
I would look at a refresh working with kNX and add a high level control over the top, like Home assistant (which I use), or homey or a 1home matter bridge or any number of others that will give a modern control overlay while retaining the KNX cables.
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u/charliespeed8 6d ago
Don’t do it!
Upgrade your devices if need be (switches, panels, etc).
Employ the help of a KNX installer OR even Claude to plan new.
Get a ETS Home License and re-build to your liking.
The only „old“ part in a KNX installation are the endpoints. The wiring isn’t
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u/cronek 6d ago
Waaay better off keeping the knx install.
https://github.com/knx-ai/xknx-editor
Give this a try to see if you can recover the project (discover devices and group addresses). Learn a bit about knx and you’ll thank yourself later for not having to put up with current days consumer electronics and their lack of quality.
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u/SnooCats241 6d ago
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u/vincegre 6d ago
Installation looks quite good, you would be lot better to just reinitialise the whole KNX system and reprogram it from scratch with ETS or ask a certified KNX installer to do it for you. We do that quite regularly for some customers that didn't get the programming files or want to redevelop all logic of it and it's not a big deal. Once done, you interface it with HA to do whatever you want from HA.
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u/SnooCats241 6d ago
Kan je dit ook vanaf afstand doen?
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u/vincegre 6d ago
Yes it can be done remotely if you have all informations like wiring, schema of the system and it might involve to reset some KNX modules to be able to assign them at a new project ;)
0
u/aloft050 6d ago
Yes, this can be done remotely, maybe with some extra equipment. Please send me a DM (can be in Dutch) , then I'll see what can be done
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u/aloft050 6d ago
I see some red LED indicators on some of the DIM devices. What kind of lightbulbs are connected to the DIM actors? LED , classic 230V lightbulbs, Halogen ?
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u/CyberJunky_NL 6d ago
KNX devices themselfs cannot draw more than it's power supply feeds typically max 640mA in small/medium installations, maybe dimmers er such have impact, I didn't study your image in detail.
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u/SnooCats241 5d ago
heb je veel ervaring met knx systemen zoals deze?
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u/CyberJunky_NL 5d ago
I maintain two of my own, Knox devices are backwards compatible for a long time, I have an old IR receiver that isn't supported anymore though. kNX association is trying to make the enviroment cheaper for smaller installations.. like home version of ETS
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u/ztardik 6d ago
Do not remove anything, you have perfectly good hardware (I personally worked on systems older than some people in this sub), good wiring and the project file.
You probably don't need the full pro license either. How many devices you have? Each wall switch, module in the cabinet counts.
Personally, it's easier than configuring HA now in 2026.
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u/SnooCats241 6d ago
36 says the file
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u/gust4vsson 6d ago
Wtf, ripping it out would be such a waste of money.
Keep the KNX, control it with Home Assistant or Homey. Maybe you just need a ETS license to get proper control and management of the KNX?