Cold weather is approaching so now is a good time to take a look at your furnace to be sure it is ready for winter. If you have a gas furnace check the inside of the flue pipe for any sign of soot. Soot indicates a potentially dangerous issue so if there is soot in the pipe be sure and have the furnace checked by a professional. Check the flue for any restrictions. If accessible birds will build nests in a flue pipe.
With the power off check the blower wheel for items that might have been sucked in and making the wheel out of balance. If belt driven check the belt for weathercracks and proper tension. Check the bearings in the motor and blower for any play. There should be be no play parallel to the shaft. In and out is okay. Up and down is not. Now is a very good time to change the air filters.
Next mix up some dish washing soap with water in a spray bottle and spray the gas pipe to be sure there are no gas leaks. Pay special attention to unions and gas cocks. Gas cocks are packed with grease and over time it hardens and cracks.
Next start the furnace and take a look at the flame. The furnace should have either end shot burners or ribbon burners. A ribbon burner is a long skinny burner that extends into the heat exchanger. The flame should be blue and very crisp with a well defined inner cone. The flame should not deviate when the blower is on and should not burn down around the burner.
The flame should also be blue on an end shot burner. An end shot burner is short and the flame burns at the end of the burner and extends through a flame retainer plate into a round tube. The flame should extend into the round tube heat exchanger and not roll out around the plate. It should not change when the blower comes on.
Pay attention to the draft inducer and make sure it is not noisy. Noise may indicate a potential bearing failure. If you have a 90% efficient furnace with PVC flue make sure the drain lines are clear and open.