I have a Payne 'Plus 80' central heating/airconditioning forced air unit. The blower motor just hums instead of turning when either the air or heat is turned on. I notice that there's a box that appears to be a capacitor on the side of the squirrel cage blower and am not sure if the motor or the capacitor is likely to be the problem. Any suggestions on how to diagnose this as well as good sources for replacements would be welcomed.

PS. Further checking with a cheap analog VOM shows the capacitor (labeled as 7.5 uF) to be an open circuit whereas I would have expected the meter to initially show low resistance and then drop back to infinity as the capacitor charged up. Would an open capacitor account for the hum rather than turning of the blower motor? Or is there likely a problem with the motor as well?

PPS. Replaced the capacitor and the blower motor now runs properly. Unfortunately the furnace doesn't light. It's a natural gas, induced draft ignition model and the igniter glows red periodically but the burners never light. The control box has a self-test feature and that runs normally - sequentially turning on the induction draft motor, then the igniter, and then the blower motor.

Thanks,
Peter

You most likely have an open limit switch-the unit overheated when the blower failed. Look around the burner compartment for little "gizmos" (safeties) with 2 wires and a little button between the wires. Push the button. Once you've found and pushed all buttons (may be as many as 3 depending on model) one of them should give you a "click" when you reset it. Once you've heard/felt the "Click", restore power and cycle it again-you should be in business. If it continues to trip, call in a licensed tech to troubleshoot thoroughly.

And change your filter as well... a clogged filter will cause the same problem.