I have an old customer that has an AV2 and a Tstat-EX. He wants to add an ST-PC power controller which will turn his HEPA filter on and off based on the fan being on or off on the furnace. The problem is that the only feedback that I can see for the TSTAT-EX is "Fan Call Status - Fan is Running If High, off otherwise". How do I write the conditional to turn off when low? Or is the status based on a fan call and the signal only goes high momentarily? This system was programmed initially in SB. Any help is appreciated.