Tell me what furnaces used a stack switch in 1974? They were phased out before 1970. If you know so much about stack switches, why didnt you tell him to clean the soot off of it, they dont go bad that often. I guess just go out and spend $200 for a new one. Yes if you didnt know SOOT is an insulator. That is why you are unqualified to give anyone advice except yourself. Call a gas valve a primary control? I have been in the hvac field for the last 23 years and have a degree. You taught this trade to people? I noticed you said when you used to work on those units, so you must have not worked on any for 30 years. If the furnace goes out on limit the fan motor is bad? I must inform you that there are a few other things that will cause a limit to trip and most of the time it is not the fan motor. One thing I will give you is this, you know pretty much know how those vintage 1950's era furnaces work. I bet you are a refrigeration expert too. Sorry I was so hard on you Bob, do you need a HUG?