Wiring in a Aprilaire 500m to an older (11-86) Trane furnace w/o A/C
I would use a Honeywell sail switch like the S688A-1007 from SupplyHouse.com for about $50.00 bucks and wire it in series with the power source for the humidifier. You could also use an air pressure switch from the same supplier. Those sail switches work very well.