blower fan coming on between cycles

I have an older Heil furnace (from around 1991-92), standing pilot, and I've noticed at seemingly random times, between normal cycles, a noise coming from it. It sounds like it's trying to start the fan, but when I get close to the furnace it's making a kind of buzzing noise, almost like something's shorting out. After doing this several times, the fan will come on for about 5 seconds then shut off. Then it seems fine. It doesn't always do this, only about once a day that I've noticed. What would cause it to call for the fan to come on, and why would the fan only run for a few seconds ? What would be causing the shorting type noise ? I had it checked last fall and all seemed well. Any help would be appreciated.

Oscar Hobson

 

Check the ability to spin the blower by hand(with furnace off!) so you can tell if the bearings are shot. Another cause for the motor to be locking up, causing the buzzing sound, is that the capacitor is shot. A motor shop could test yours for you. Those only cost $10. Lastly, it be the motor is shot, if not these other two causes. You could test to see if proper ohms and no short to ground. Or, motor shop can test. Motors often can be bought at motor shops for about $80- $125. Easy DIY job if you are at all handy. Maybe a 1 hour job.

 

Have the run capacitor checked out. Blower motors are usually what's called PSC Permanent Split Capicitor and require a run capacitor wired into the circuit (Usually to two brown wires). It will normally be mounted on blower casing, silver in color, oval shaped and rated in microfarads. If it is leaking or swollen then it is bad. Run capacitors are fairly inexpensive

 

Thanks for your replies. Had someone out today to check it, and of course it didn't do it when he was out. It's happening about once a day now. He checked the run capacitor and thought that was ok as to the microFarad reading, but did comment it looked like a little oil had leaked. He also adjusted the fan on and fan off settings slightly. He said it could be the blower motor going, but didn't want to put a new one in without being sure that was the problem. Also asked me to monitor it, maybe keep a little log of when it did it and the exact timing of the symptoms.

Any other thoughts ?

Thanks,

Oscar Hobson