Fan relay has a NC and NO switch, low speed is the
heat speed and is run through the fan relays NC switch and is controlled by the temp delay.
When the fan relay is energized the NC opens and NO closes bringing on high speed. The fan relay is only energized in fan or cool modes, the temp delay controls heat/low speed fan
Originally Posted by HVACTECH96
The part on the left is high temp limit,on the right is fan control.Fan control has been replaced,with a more modern fan/limit control but only fan control side is being used.To me it sounds like the fan center that is on the outside of furnace isn't wired properly.Power should be coming out of fan control,then into normally closed contacts in fan center then to the fan motor.Sounds like this is where you might be wired wrong.Fan center is added when ac was installed.