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I have a 30 year old Plus 90 furnace. Recently I noticed a…

I have a 30 year...
I have a 30 year old Bryant Plus 90 furnace. Recently I noticed a humming noise when the blower starts up, and is gone when at full speed. In other forums (and this one), the capacitor has been identified as the likely culprit so I installed a new one. The replacement part for my furnace model was listed as 12.5uF, 370v. However, when I replaced it the hum seemed louder, and though it got quieter at full speed, it was still noticeable. I put back in the old capacitor and hum level returned to the original lower audible levels at startup and quiet at full speed. I noticed the old part was a lower 7.5uF, 370v part. Any idea what is going on?
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Answered in 2 minutes by:
1/17/2018
BillyHvac
BillyHvac, Journeyman HVAC Tech
Category: HVAC
Satisfied Customers: 23,037
Experience: Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
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Hello,

My name is Billy. The first thing to do is look at the motor and see what capacitor it was rated for. Many times motors get replaced and the capacitors do not so they end up mismatched.

Do you know what the motor in the furnace has on label for cap size?

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Customer reply replied 4 months ago
The furnace has not had either the motor or the capacitor replaced (until I just replaced the cap). I am the original home owner.

then you simply were given the wrong capacitor as a replacement.

If 7.5 uf was previous part you must match that. So at this point you need to get a 7.5 and try that. Plus....using the wrong cap can cause a early failure for a motor so I hope it is still ok...

Billy

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Customer reply replied 4 months ago
Understand, but all the on-line parts stores recommended the same 12.5uF capacitor for my furnace model number (which fits many other carrier furnace models as well). I think Carrier must now recommends this size for these models. Why would a higher capacitance make the hum worse?

Did anyone ask for serial number? The motors change from series to series in the same model number.

If you had a 7.5 then it was OEM. A 12 will burn the motor out. What is your model number and serial and I will confirm

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Customer reply replied 4 months ago
Model # 398AAW060120D
Product # 398AAW060120AEBA
Serial # 2487A05308

I show that as an OEM oval 7.5uf 370 volt

See here

If you like you can enter your model directly in the model field and find your parts.

I went to Carrier/Bryant parts and found the original part was HC91CA007...this crosses to what I sent link of.

Billy

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Customer reply replied 4 months ago
OK, I will test the old part before I get a new one and try to return old one. Not sure the old one is bad and that a new one will quiet the startup hum. Is there a chance the hum can be a result of the motor going bad? Thank you.

It could be start windings getting weak...weak capacitor....

Many of these "groaned" at start up and in lower speeds. Sometimes changing the speed tap on the board to a higher speed changes it or shortens the duration.

Honestly...a hum is generally the "get to ready to replace me" sound.

Billy

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Customer reply replied 4 months ago
Thanks for the advice. The speed is set to lowest (for heating). I can try a higher speed with the negative tradeoff of a louder overall fan sound and higher air flow from vents. But if it helps with motor longevity it is worth it (I know, 30 years is pretty good already). Did have to replace the inducer motor 25 years ago, but that turned out to be a known issue.

the good thing about this board is the blower wires set up...you can easily swap speeds by looking at diagram on back of the blower door.

I hope all goes well for you and if you need further info let me know....I cut my teeth installing those 30 yr old units:)

Just rate service to close ( 5 star system)

Billy

BillyHvac
BillyHvac, Journeyman HVAC Tech
Category: HVAC
Satisfied Customers: 23,037
Experience: Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
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Category: HVAC
Satisfied Customers: 23,037
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