I am looking to upgrade my HVAC and going through the usual research between single stage, 2-stage, and modulating furnaces. I have read some of the threads here on this already.
I have also looked at the thermostat connection schematics for the York 9C series furnaces and see that generally it just takes a single stage thermostat "w" wire connected to the Modulating Furnace control board W terminal.
So all the board ever sees is just a typical call for heat. How then does the furnace decide at what blower speed and flame % to run? Is it just a timed thing or are there other inputs and sensors in the furnace? I really like the idea of extended lower % running for comfort but wonder just how it works and if it is adjustable to any extent?