When I first went to this site, I had no intention of turning my input into a personal vindeta. However, you seem dead set on showing your ignorance, or lack of tollerance. For your information, I was still installing Stack Switches into furnaces into the 1980's because people did not want to spend the money to update their furnace to a newer model. Yes, soot can be a culpret, but I only provided a suggested input. I based my input on the fact that William said his furnace was a 1974 model. Since I do not have direct access to his furnace, I did not say that my suggestion was the only solution. No different than my suggestion about the limit switch. These were only suggestions,as I thought that this is what this site is for. If you are a HVAC mechanic and have a degree, then you especially should know that you cannot make a guarenteed judgement based on a single input. This holds true for any troubleshooting issue. In your last response to me, you asked if I need a HUG. It is not a matter of whether I need a hug, but more about whether your ego was hurt, because someone said something that you did not agree with. In closing, as I do not intend on keeping this matter an open issue, yes a gas control valve is a Primary Control. This was taught for many years in the industry, long before you were out of your diapers. In fact, the instructions that came with a thermostat stated that the heat anticipator is adjusted to match the current draw of the primary contorl, which could be a gas control valve, or whatever else is controlling the heat source.
No, I do not know what controls are being utilized today as they were updating them so fast when I got out of the business that we finally gave up trying to keep up with them. This also holds true in the automobile industry as well. Just ask any mechanic how frustrating it is to keep up with all the advances in technology. Oh, and one last thing, my parents still have the original oil fired furnace that was installed in the mid 70's which does have the original STACK SWITCH!!!