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Question DetailsAsked on 10/28/2017

my furnace blower sometimes starts very slowly then little by little it picks up speed. motor or capacitor?

I'm not sure if it's the capacitor or the motor. The unit is direct drive. Not sure how to check either.

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1 Answer


Check your performance info in the owner's manual - this could be a bad capacitor (which can be checked with a volt-ohm meter with capacitance test capability - could be a bad section of winding or dragging bearing or debris like lint in the motor - or could be your motor (especially if a DC infinitely variable speed one) is responding to the info from the sensors in the furnace, especially if a high-efficiency unit. If the blower motor is readily accessible and removeable (some are, some not nearly), if you dismount it and (with power to unit off of course) turn the fan by hand, if it drags or has significant or variable resistance to turning or makes any sort of dragging or scratching sound, then the problem is almost certainly motor bearings or debris in the motor, or debris between fan blades and housing or maybe the fan is loose or distorted and is dragging on the housing. (Some models with plastic fans have had problems with the plastic overheating and deforming, causing contact with the housing).

If this is not a new behavior, some of the most sophisticated furnace units continuously detect the exhaust input and output temperatures from the heat exchanger and regulate the blower motor speed to extract the maximum feasible amount of energy from the flue gas as it passes through heat exchanger - starting off with slow blower while the amount of heat in the heat exchanger is low, then cranking up to higher speeds as the heat exchanger comes up to heat. And there may be several different steady-state speeds during firing cycles because some units have variable gas/oil burn rates depending on the sonrsor readings of outdoor temperature - producing lower hwat rate (hence less heat, hence slower fan speed) in warmer conditions, cranking up to full power in very cold or cold start conditions. Then when the unit shuts the fuel off, as the heat is extracted from the heat exchanger, the fan would (for that type of higher-efficiency unit) crank down to lower speed to most efficiently extract the residual heat from the heat exchanger. So - long answer short - if yours has a continuously variable speed blower - usually meaing a DC motor but not always (would say if so in owner's manual), and it is also slowing down similarly (roughly doing the reverse) after burner shutdown as the unit cools down, it is probably doing what it is supposed to. BTW - same can happen with the eductor motor in some condensing units (the flue gas extraction fan in the exhaust piping) - soemtimes they are variable speed depending on the flue conditions - commonly running full speed at startup top ensure the flue gases begin flowing properly, then slowing down whenver the flue gases are cooler, faster when hotter. The "sometimes" nature of this does not indicate which it might be - because dragging fan or bearings tend to be intermittent at least initially - but when a starting capacitor fails commonly it is an intermittent "breaking-down" of the capacitor initially, then generally getting worse - ditto to an internal short in the motor windings - tend to start off as a sproradic thing, then eventually becoming more and more consistent as the electric breakdown becomes more pronounced. You could also contact the manufacturer about this at their technical support eMail or website eMail link - emphasizing that this is intermittent, not every time - which is the one factor which indicates to me it might be a faulty sensor or bad blower motor or capacitor or fan blade. But I suspect they will advise havging a Heating and A/C tech first check the resistance and starting amperage of the motor, and the capacitance of the capacitor.

Answered 2 years ago by LCD

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