I am looking for a virtual device which sends constant or periodic update to manually activate the “constant fan” mode of my furnace to facilitate air circulation around the house.
I am currently using my motion sensor as a trigger, basically whenever it senses a motion, it turns on the furnace fan. (but when the furnace switches to heat mode, it automatically change the fan to auto mode). This works great during day time, but fails during night time.
Also I understand there is a Run Circulate in HVAC smart app which I am using to drive the other furnace right now. (unfortunately if I run this app on both furnaces, neither of them would work properly.
If your HVAC fan can be triggered in an automation, you could use the vEDGE switch driver. Since its seen as a switch, you can even set On/Off for a duration, 3 times within the device itself as a single routine (screenshots below).
Momentary Virtual Switch from @TAustin can do that.
It has delay settings, for how long it takes to change back to open or closed status. And you just add automation to trigger momentary on every x minutes.
Another solution requires zigbee temperature sensors, that can use @Mariano_Colmenarejo driver, and adds thermostat capabilities to temperature sensor. It has a lot advanced features including just fan schedule that you need. I am using it exactly for what you described.
I am using Aqara sensors currently $16.99 on Amazon.