Sorry in advance for the length of this post, but I wanted to try to be as specific as possible on this problem.
I have two 16+ year old Armstrong natural gas furnaces (model number in subject line above) for my residence. Both identical spark ignition,
125,000 BTU 5-burner units.I had a soot build up problem with the lower level unit (the one that is used most frequently) that caused the flame on the left-most burner to slightly roll out from the heat exchanger chamber, so I called a reputable local service company to come out and clean both of the units.
They removed the flue and front panel and then thoroughly cleaned the sooted up heat exchanger and flue vent. (They found the heat exchange cavity by the leftmost burner completely blocked with soot). They also removed and cleaned all the burners too and then re-assembled everything. In this process, they also discovered that the air mixture controls for the 5 burners were shut down completely and opened them up (all the way to max settings) to let the burners "breathe" properly. The flame tip color turned a much deeper blue and the dark blue "inner" flame mantles appeared to go down to about 3/4" long. The good news here was that this stopped the orange tipped flame from rolling out of the combustion chamber and resulted in a noticeably hotter flame. (I've also been monitoring CO in the area around the furnaces and it has gone from 20+ PPM when the furnace was running to 0 PPM now.)
Unfortunately, changing the air mixture seems to have introduced a new problem. Now during extended runs of this furnace (for example when coming up to temperature in the AM after a setback period the previous night), the flue roll out switch trips shutting down the system. When this happens it requires a manual reset of the roll out switch button. If the furnace is simply "cycling" to maintain temperature, then it does not seem to trip and everything is fine. The drafting on the furnace seems to be good so I suspected that it was just burning too hot for the switch.
Thinking this was possibly a weak/old/failing roll out switch, I swapped the switch with the one in my upstairs furnace, but this did not fix the problem and it still tripped just the same (and the upstairs unit which now has the switch from the downstairs unit did not trip). At this point, I have tried backing off the air mixture to about 50% from being wide open on all 5 burners, but this too seems to have had no affect.
The only thing that I've found that does seem to prevent this switch from tripping is to manually feather the gas shut off valve to the point where I can observe a noticeable reduction in the flame height on the 5 burners.
I'd appreciate thoughts on what is happening here. I do not believe it is an issue with a blocked flue since both furnaces were thoroughly cleaned and share the same flue stack/chimney and the upstairs unit does not have this same problem. I suspect it may be that the air/fuel mixture is still not correct or possibly that the gas pressure from the gas valve to the burner orifices on the lower level furnace may be set too high. (I am not sure how these are set.) My service company is stumped and after being charged for two trips, I am not anxious to have them come back till I understand what is happening here.
Thoughts and ideas on what I can look at or try are very much appreciated. I know these units are only about 65% efficient and getting old, but replacement at this time is not an option since they are otherwise still running well!
Bob