This is a long post. I believe giving all of the relevant information will better enable good responses.
My 2800 ft^2 house in Massachusetts built in 97 uses a Bryant 383KAV 80%, 154KBTU/hr input, 2 zone (pressure side only dampered)gas furnace, model #383KAV060155.
I have come to realize that when I run only on one zone and the system has to run for a while to reach thermostat setpoint (like on a cold morning, when I wake up and turn on/crank up the 2nd floor zone only) then the limit switch in the heat exchanger will trip prior to setpoint being reached. This creates a 3-3 code. The burners shut down, the blower will keep runnning as it should to cool the heat exchanger (so theres non heated air blowing out of the registers). The temp of the heat exchanger drops and then the burners relight and eventually the setpoint is reached or the process repeats until the setpoint is reached. Because the process will repeat until the setpoint is reached, it is not readily apparent there's a problem.
I've done some investigating to arrive at this and I believe the root cause is an inadequate duct system for running this furnace on only one zone. When only the 1st floor damper is open the system will run for about 7 minutes, 2nd floor damper only open then it will only run for about 4 minutes (this is in Jan/Feb), both dampers open it will run for at least a half hour, possibly indefinitely. The limit switch is a 160F. I've replaced it. I've also measured the temp in the duct just before the dampers. If both dampers open then it will go up to 154F, only one damper open then it will go to 160F and the limit trips. The temperature rise per the raying plate on furnace is supposed to be 50 to 85. 70room + 85 = 155 Max< 160. Further leads me to believe inadequate airflow.
I also checked out 2 neighbors houses. neighbor A has same furnace, different larger house, different duct work. He has no problem and air at damper is only ~130F. I checked out neighbor B's house. I told neighbor B of my problem and he said he hadn't experienced it. Anyway neighbor B has identical furnace and house. I measured temp at closed damper and I saw it reach 160F just like on mine, and then whaddya know, the burners shutdown, blower kept blowing and same 3-3 code. he has same issue and didn't even know it.
If I run the furnace temporarily with the door removed to the blower compartment (unrestricted air supply, door safety bypassed) then it runs fine and temps are fine.
I replaced the motor run capacitor (not a start capacitor, this is a PSC motor) to eliminate the cap as a possibility. New motor is like $400+. Won't replace that unless I'm 98% sure and I'm inclined to think it's running the same as when it was new.
I should mention that I've replaced the filter and its very non restrictive and none of the outlets are blocked. The blower speed for heat is the 2nd highest of 4 available. 1st reserved for AC use. Switching to the A/c highest speed doesn't stop the limit from tripping. I also "clocked" the meter to confirm that the unit wasn't being supplied excess gas.
At this point I'm pretty convinced that the root cause is that the ductwork doesn't allow the stock blower to push enough CFMs to adequately dissipate the heat produced, at least not for when only one damper is open (the majority of time).
MY QUESTION IS: what are my options for preventing the tripping without replacing the furnace with a smaller unit? Are their higher speed blower units available (if so from where)? Any ideas?
Thanks for any help.