Ventbots are pretty reliable at using a temperature reading as a trigger. The temperature sensors I am using, BMP280 and BME280, and many other temperature sensors also provide a reading of atmospheric pressure. I think that's because in common use cases for these sensors, the interesting value is the pressure, and it is sometimes necessary to take temperature into account for pressure readings.

Anyhow, I got to wondering if it would have been feasible to use a change in the pressure reading as a trigger for Ventbots. The HVAC is blowing air, and it's at least conceivable that that would lead to a change in air pressure inside the vent cavity. If that worked out, I'd have to figure out a way to compensate for the additional effects of the Ventbot fans to correctly trigger turning the fans off.

Well, an interesting hypothesis, but basically a dead end. Here is a graph of 3 Ventbots during a couple of heating cycles.

The top graph is the Fan 1 tachometer reading, the middle graph is the temperature reading, and the bottom graph is the pressure reading. If you graphed more data and did some fancy signal processing, Fourier transforms, and a generous amount of squinting, I think you could convince yourself that there was some effect on the pressure readings. But it would be so tiny that it would be hard to reliably dope it out from the normal pressure variations, both in the physical world and in the tolerance of the sensor.

Now we know.