What Your KNX Blinds Do When the Wind Sensor Goes Quiet
Everyone asks what the blinds do when the wind picks up. Almost nobody asks what they do when the wind sensor stops talking.
That second question matters more. A working sensor will retract your awning. A sensor that has died, been unplugged or lost its cable does nothing at all. If the actuator is not set up for that case, it keeps waiting for an alarm that never arrives, and the first Highveld storm of the season finds a fully extended awning.
The actuator can watch the sensor
A KNX blind actuator does not have to sit and wait for commands. Most decent ones can monitor the alarm object itself. The weather station sends a "no wind" telegram at regular intervals. If the actuator does not see one inside a set window, it assumes the sensor is defective or the bus line is broken, and it drives the blind to a parameterised alarm position.
That is the right instinct. Silence from a sensor is a fault, not a calm day.
The setting goes by different names depending on the actuator, and the options differ. Some let you enter a window in seconds, some in hours, minutes and seconds, and others give you a short menu such as 10, 20 or 60 minutes. Some have a separate monitoring setting for each alarm type, and zero or "off" disables it. Check the parameter page in ETS for the device you are installing.
The sensor has to do its half
This is where installs go wrong. Monitoring on the actuator is useless if the weather station only sends when something changes.
Both ends need to agree on a heartbeat. The usual rule: the sensor's sending interval should be no more than half the actuator's monitoring window. Set 20-minute monitoring and the sensor must send at least every 10 minutes, or the actuator trips on a perfectly healthy system. In practice I leave plenty of margin on top of that.
It is also worth knowing what the sensor does when it loses its own power. Better rain sensors fall back to a "raining" state and repeat the telegram. That is the safe way round, and your actuator settings should lean the same way.
Priority: who wins when everything is alarming
Wind, rain and frost can all fire together. Most actuators let you choose which one ranks highest, and the highest-priority alarm decides where the blind goes. Weather alarms, blocking and forced operation also sit above manual operation, so the wall button stops working while an alarm is active.
That last point is a design decision, not a footnote. For a family home, it is what you want: nobody lowers the awning in a gale because the button worked. For a restaurant terrace where the owner insists on control, have that conversation before commissioning, not after the first complaint.
Power failure and power return
Two more parameters decide how the system behaves after a bad day.
Most actuators let you choose what happens when bus voltage fails (stay put, go up, go down or stop) and what happens when it returns, often after a short delay so the bus is not flooded as everything powers up. With monitoring turned off, some actuators reset their alarm state to "no alarm" after a mains failure. The system can then wake up believing the wind is calm until the sensor says otherwise.
My own view, and this is practice, not a standard: for exposed awnings and external venetians, I set the bus-failure reaction to up, turn cyclic monitoring on, and keep the sensor interval well inside half the monitoring window. I would rather retract a blind that did not need it than lose a three-metre awning to a bad cable. A blind in the up position is a minor irritation. A bent arm is a quotation.
What I check before handover
This is my commissioning habit, not an industry checklist:
Confirm the weather station sends cyclically and note the interval.
Set the actuator monitoring to at least double that interval, on every alarm object.
Pull the sensor's bus connection and watch the blinds. They should reach the alarm position within the monitoring time. If they do not, it is not finished.
Check the priority order matches what the client cares about. Wind first on exposed elevations is the usual answer.
Switch bus power off and on, and see where each blind ends up.
Step three takes ten minutes and is the only test that proves the fail-safe works. I would not hand over without it.
Why this belongs in the design
In my experience, Highveld storms arrive fast and often hit when nobody is home. A system that only works when every part is healthy makes a nice demo. A system that does something sensible when a part fails is the one worth paying for. That is a big reason I like a wired, standards-based bus: the behaviour is written down, parameterised and testable on site.
If you are planning external blinds or awnings and want to talk through what the system should do when things go wrong, I am happy to have that conversation. Just a conversation about what the project needs.
KNX Logic
wayne@knxlogic.co.za | 082 564 3982 | www.knxlogic.co.za
Based on a review of KNX blind actuator and weather station manuals from several manufacturers. Parameter names, ranges and defaults vary by device and application version, so check the ETS application description for the device you are actually installing.
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