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Is this proper reasoning for replacing my furnace?
My Rheem, propane fired, Classic 90 Plus furnace is 14 years old and I have lived in this home in far north Michigan for 2 years. For two days this week the unit fired up and ran through a heating cycle only about every 6th attempt at starting. The remainder of the time only the induced exhaust blower energized, and then cut out after about 45 seconds. During the many failed attempts to fire the panel light continually flashed 2 times, signifying an "open pressure switch."
The technician I have used for 6 years evaluated and worked on the furnace about 2.5 hours. Pressure switches & hoses, condensate drain, pvc intake & exhaust, firing components were all operating properly. He did determine that the furnace fired up and operated properly ONLY when the upper access panel was not in place, which allowed more air into the combustion area than could be supplied by the 2 inch pvc intake. He also evaluated the pvc intake & exhaust on the exterior of the home: The 90 degree elbows of the pvc intake & exhaust on the exterior were both pointing to the ground and the exhaust gas was immediately being suctioned into the intake pipe. The interior of the exhaust pipe is coated with an oily, black film. He removed the elbow from the exhaust pipe so that it now exits horizontally so that the exhaust would not readily mix with intake air. He also removed a plastic plug from the side of the combustion area of the furnace in order to allow additional combustion air into the chamber, from the basement. The furnace has been operating normally since his visit.
Recommendations of the technician: Seriously consider replacing the furnace. In all liklihood the heat exchanger is at least partially plugged due to the furnace using the exhaust gas for combustion. The pressure switch was sensing the exchanger problem and not allowing the furnace to ignite.
My Questions: I'm not familiar with scorched air as my previous homes have had hydronic heat (certainly my favorite).
Does the technician's reasoning for replacing the furnace sound legitimate? Any suggestions or recommendations would sure be appreciated.