Does my blower motor need replacement?


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Old 08-01-06, 12:47 PM
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Question Does my blower motor need replacement?

I have a 13 year-old Carrier gas furnace which needed service after a power surge. The circuit board and starter capacitor needed replacement.

While looking it over, the technician told me that the fan motor seems to be running slowly, and recommended replacing it for $400-500. He couldn't offer any way to check the airflow or the speed of the motor, but did show that the speed of the blower changed when he took out the air filter from the return plenum.

The blower runs quietly, and there are no funny sounds or odors. I would expect any motor to have an easier time if resistance to airflow (i.e. the filter) were removed, so this doesn't seem like much of an indicator. I would also expect an old motor to make noises rather than to slow down, since the electric coils and magnets don't age but the bearings do.

Should I be suspicious here, or is this just how old motors die?
 
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Old 08-01-06, 02:17 PM
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I'd be suspicious, too.

How did he determine that the blower speed changed when the filter was removed? Sound? It could have been the difference in the airflow that made a different sound. Were all access doors and panels, including any filter access, in place both before and after the filter removal test?

Was the filter extremely dirty? A clogged filter could seriously impede airflow and the removal of the filter (allowing free airflow) could have a distinctive change of the sound. Since this unit is 13 years old I assume that it is not a variable speed motor under the control of the furnace logic board.

It is possible that the motor is a three or four speed unit and it was connected to the lowest speed. Most furnaces of that era had their blower motors connected to run at the next to highest speed for heating and highest speed for cooling. A smart and competent installer (original installation) would have taken the temperature differential across the heat exchanger while running (input air temperature vs. output air temperature) and adjusted the blower speed to the lowest speed that did not allow for a differential (also known as temperature rise) across the heat exchanger to exceed the manufacturer's specifications. This would have resulted in a slower-turning fan (and quieter operation) than your current serviceman was used to seeing and hearing.
 
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Old 08-01-06, 02:25 PM
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Thanks for your reply.

This is a 4 speed motor that is controlled by the circuit board. He determined the change in speed by listening--and it did sound like the speed changed, not just the volume.

My main question is: Do these motors fail by quietly slowing down, or do they usually make noticeable sounds?
 
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Old 08-01-06, 03:10 PM
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It is my experience that a motor does not usually fail slowly. It could be that the bearings are shot or in dire need of lubrication and if they are sleeve bearings then there would most likely not be any noise to alert you to this fact.

Although you may have a heard a change in frequency (pitch) and not volume (loudness) between the filter in and the filter out this is not enough (not for me, anyway) to determine that the motor is failing. The added resistance of the filter should not be such that it would have an effect upon the motor speed, certainly not an effect that could be diagnosed by the sound.

Did the tech use an ammeter (probably a clamp-on model) and determine the motor's current draw during the filter in/filter out test? Is yes, how does the reading(s) compare to the label on the motor? If he did not do an ammeter test then the "sound" test is subjective and could be wrong.

As for the four speeds being "controlled" via the furnace logic...are you saying that the speed changes during the heating cycle? Unless you have a two-stage furnace (unlikely for that age) it is doubtful that you have more than one fixed speed for the heating cycle and one fixed (higher) speed for cooling.
 
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Old 08-02-06, 07:32 AM
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Thanks again.

The speed of the motor is fixed as you said, higher for cooling and lower for heating (or vice versa). It doesn't change, at least during cooling.

He didn't use an ammeter, and gave his advice based upon only the sound of the motor. He said that checking current draw would be unreliable, since it may start out fine but then decline after a few hours.
 
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Old 08-02-06, 05:02 PM
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I would just feel my hand over a register, up a certain distance, and then have someone pull the filter, and see if I could feel the difference...without them telling me exactly when they pulled the filter.
 
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Old 08-02-06, 10:40 PM
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Personally, I'd get a different repairman.
 
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Old 08-03-06, 10:23 AM
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Well, I said no to the motor change. Once the new capacitor and a new filter were installed, the motor seemed to run much better. It was probably the filter.

By the way, the repairman would have charged me $165 to replace that capacitor. True to the spirit of this forum, I bought the same thing and installed it myself for $10. All in all, I don't think I'll be using that company again!

Thanks for your help.
 
 

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