Siren-like noise coming from blower exhaust fan?

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Old 02-05-13, 04:16 AM
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Siren-like noise coming from blower exhaust fan?

Sometime in the past couple weeks my furnace developed a weird siren-like sound when it kicks on. I took the side cover off, and shut it off, and noticed that the sound really seems to be coming from the exhaust blower fan (it's most noticeable as the fan is spinning down). Is that normal, and is it a sign that the exhaust blower fan is on its way out? I can't tell externally and may try to pull it off tonight and check for any play. The furnace has written on it that it was installed in '98.

I originally thought it was the blower motor, but it definitely seems like the sound is coming out more up top than below where the blower motor is. I honestly didn't even know furnaces had fans for the exhaust
 
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Old 02-05-13, 08:05 AM
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The top fan is called a draft inducer.
Those blowers run hot and the bearings can dry out. Most have no real way of lubricating it.

The main blower may have oil fittings on the motor.
 
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Old 02-05-13, 11:33 AM
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Often the inducer motor assembly, motor and fan together, can be unbolted from the furnace and removed for inspection or replacement pretty easily.

Pulling that off to inspect it is a good idea, since the siren like sound you describe might be something caught in the fan that could be removed easily.

You can inspect the fan for any debris or dirt and clean it as needed. Check it to see if the fan wheel turns freely or is stiff from lack of lubrication.

You should power up the inducer motor and observe how it starts and runs while you are holding and inspecting it, to see if you can identify where the noise is coming from.

With the additional information you have from that inspection you will probably be able to make a better decision as to whether it needs to be replaced.
 
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Old 02-05-13, 06:28 PM
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OK...I checked it again tonight and the noise is definitely coming from the draft inducer (the furnace's parts manual just calls it "blower exhaust," I dunno).

I pulled the fan out and looked it over. Can't find any play in the shaft (it's worth noting that the motor says it is a sealed ball bearing unit), both lateral and in-out. I spun the fan by hand and it seems to spin about as freely as I would expect a fan to, and I can see nothing caught in it. When you spin by hand you can hear a faint rubbing noise but that's it. The fan blade itself is completely enclosed in a turbine-style housing and the cover is glued on, and I decided not to risk breaking it to try pulling it off. There's a little dust inside the motor itself but nothing more than what I would expect to see inside a furnace.

So, I'm kind of at a loss as I found nothing unusual. I can't see anything that might cause that weird sound. I did not power it up while it was in my hand as the breaker box is several feet away and I'm at home by myself at the moment, I may try that later when my fiancee is home, though I'm not sure if I'd be able to see much else especially considering that the it's so enclosed.
 
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Old 02-05-13, 07:21 PM
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I'd power it up to see what that might tell you, especially since it's an expensive part.

I might cut the end off an extension cord and use that to power up the motor if that happens to be convenient.


Unless powering it up tells you something, sounds like you will need to replace it. Rather than just replacing the motor, I'd look for a replacement assembly that includes the fan and the part that bolts onto the furnace.

That simplified installation, and getting the fan off the motor shaft is often impossible since it's frequently corroded on.

Having the whole assembly makes replacement easy, usually, and that's usually to be preferred especially for do it yourselfers.
 
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