I have a 100,000 btu/hr Trane XL-80 natural gas furnace. It seems like it may not be running at peak efficiency. The metal exhaust stack from the furnace gets quite hot to the touch, even with new filters and all the duct vents open. Does anyone know what the exhaust flue gas temperature for this furnace should be? The flames look ok.
Is there a tag on or inside the appliance showing measurements taken either at startup or after a service call? For that efficiency, I believe you should see ~375°F, however attempting to evaluate a burner or boiler with only one value is improper. Checking combustion O[sub]2[/sub]%, CO[sub]2[/sub]%, CO ppm, stack draft and temperature will enable a technician to properly diagnose and correct a problem.
Possible causes of elevated stack temp include dirty filters, blower wheel, or evaporator coil, low blower speed, ducting restriction, dirt or scale on the heat exchanger, or a chimney restriction.