Trion HE Series Air Flow Sensor
The HE Series has a built-in air flow sensor. When the blower is running, the vacuum in the duct will cause air to be drawn in thru the hole shown below.
A thermistor on the back side of this hole will sense the air flow. The thermistor heats up to 130° and when cooled by air flow will power up the HE2000. Blowing in to this hole with a straw will also power up the unit.