Capacitors?

DifferentDan

Does this make sense? I went on a no-heat call recently and by the time I got there the homeowner had the wood furnace cooking and blowing into the plenum of oil furnace. The fan/limit control was against the high limit which kept the oil burner from running. When I checked the fan compartment, I found the fan running backwards!! I won't amuse you with my "whoever installed this" tirade, only to find out it was wired OK.

When I turned the swich off and stopped the fan, the wood furnace fan would pressurize the plenum and force the fan backwards. When I turned it on, the fan powered backwards too, but when I turned the switch off and manually stopped the fan completely and then turned it back on, it just hummed. SO I replaced the capacitor and then it seemed to work properly.

Does this sound crazy? In my mind (don't go there) it fits with the no-heat call since the fan probably didn't start at all until the wood furnace fan started it by pushing it backwards. Will this happen again?

Thanks!
dan

dandyme

You`ve fixed the problem.
you`re not out of your mind.

Big Ed

Dual Fuel

The cap is being used as a starter for the motor . But with the cap shot.......the wood burners blower took over the job. But too bad it the other way


Was the main fan running backwards slowly ? Steady or alternating RPM's ?


~##~Being feeling like i'am living on a ice burg ~##~

Frank_17

You didn't fix the problem. It will happen again, You need one way dampers installed .

steve_26

PSC

It may happen again,If and when the cap fails.
The motor is most likely a PSC type motor and the cap is there to assist the run windings and help dissapate heat. They are a low tourq motor.
I see this happen alot in commercial VAV systems,the RTU comes on and the primary air damper on a vav may be open before the vav motor can start and when that happens the motor gets spinning backwards under power.
steve