I installed another Shelly 1 and got it connected with my heat recovery ventilation system (Vent-Axia Sentinel Kinetic).
The ventilation system also comes with a nr of terminal connections (see attachments). I have connected the Shelly 1 IO to SW4 and can use the Shelly app to switch from a normal (30%) to boost (50%) airflow (and back) modus. Initially I had to push the button twice before it switched back to normal but solved that with setting the Auto Off to 2 secs.
Now it works fine as explained above. One question... Is there a way to get a visual indication that the 50% modus is active? Now it's a just a gray button so if I am not in the proximity of the system, I can only guess how much air it is generating.