I recently ran into a issue with a tripping vent stack switch on a Nordyne 90% 1 pipe upflow furnace. I found the blower wheel dirty along with a dirty secondary heat exchange and evap coil. I have blown out all listed componenets with CO2 as best as I can.. I also have checked the exhaust vent going outside. Everything appears to be clear. But i am still tripping out on a vent limit. Is there a better way to clean out the secondary heat exchanger. It still feel like the top of the furnace is getting real hot. I am going back to do a temp rise and combustion analysis. I ran out of time when I looked at it the other day. Any suggestions would be helpful. If needed, I can provide the model and serial.