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A few weeks ago my furnace developed a vibration that can be heard throughout the house. If I remove the side panel of the furnace or if I remove the filter, the vibration goes away. My theory is that the fan works fine when it doesn't have to work hard but when pulling air through the filter (which is new) it has to work harder and vibrates.

My questions are: does this make sense? And if so, how can I troubleshoot it further?

-Edit- the old piece of junk ended up dying yesterday. We are replacing the unit.

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    No guarantees, but a video with good sound of you removing the filter and such would go a long way towards solving this.
    – Mazura
    Dec 25, 2014 at 6:49
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    When was the last time you lubricated the blower motor bearings? Some of them require it yearly. The info will be in your furnace manual.
    – Wayfaring Stranger
    Dec 25, 2014 at 13:51
  • @Mazura thanks for the idea. I'll try to get a recording of it.
    – kinar
    Dec 26, 2014 at 12:08
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    @kinar Have a look around the furnace area for the manual. Directions might even be printed on one of the removable panels. It's usually one of those manuals that someone stashes rather than tosses. Internet might even have a copy for your specific unit. Then again there might be labeled oil injection lines on the blower unit itself. I use 10w-30.
    – Wayfaring Stranger
    Dec 26, 2014 at 13:57
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    Are you sure the vibration goes away, or just the sound of it does?
    – rogerdpack
    Nov 9, 2020 at 6:57

5 Answers 5

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I read this post last night when one of the blowers in my system started vibrating, shaking the entire house. The comments above encouraged me to look into it myself, and I wanted to share what I've found in my case for the benefit of anyone in a similar situation.

In my case, it seems the Installation Instructions were left inside the unit and pages of it got jammed on the blades, causing the imbalance. After I removed these,enter image description here everything went back to normal. The AC is 6 years old, so these instructions must have been there since the installation, probably taped to a side panel internally.

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Try putting pressure on random things to isolate the location. Take the top panel off and start there. I'm not sure how you can check the rest if it doesn't do it while you have access open to deaden random things.

Try tightening the motor mounts and the blower cage bolts. Then do every screw you can find. A last resort might be shoving toothpicks in between where two pieces of sheet metal meet. Newer blower cages only have bolts on the front, the rear rests in a slide; another possibility.

If it's the bearings in the cage or the motor there's not much you can do except replace them, assuming you can verify either of these as the culprit.

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Your initial assumption about load on blower due to new filter is valid. Called starving the suction of the blower. If the MERV rating of the filter is too high, this will starve the blower suction and cause any minor vibration in the blower motor/wheel assembly to amplify. In my case the imbalance was due to a failing motor bearing.

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Just as with car wheels, vibrations happen at certain speeds. Putting the filter on, no matter how clean it is, slows the motor down a bit, which just happens to be an rpm where any out-of-balance vibrations show up (in your case).

It has been my experience that when there are excess blower vibrations, the cage blower itself has accumulated dirt. They can be in perfect balance, even though coated with dust, till a chunk of dust falls off from one spot, thus throwing the whole thing out of balance.

Clean the blower with compressed air (after removing it) and see if that helps. Note: do not disturb any balancing weights that might be attached to the blowercage.

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  • This makes perfect sense. I just bought the house in august. The unit is very old and judging by the condition of everything else, probably hasn't been properly cleaned in years and the house seems to produce (for lack of a better word) more dust than any other house I know.
    – kinar
    Dec 26, 2014 at 12:14
  • Unfortunately this wasn't the cause. As a side project, We had the ducts cleaned a dew days ago which included cleaning everything in the blower assembly as well. At first I thought it fixed the vibration too but it ended up comming back a few hours later.
    – kinar
    Jan 3, 2015 at 4:00
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Some things you can do to calm annoying vibrations:

In my case I took out the flywheel and discovered the connection between motor and flywheel had cracked. I connected it back together with some hose clamps "combined together into a big one" and it works better now, stronger air, way less vibration. Worth looking for a mechanical failure.

You could try just cleaning the flywheel. Of dirt. Might help.

Install a duct "skirt" "canvas duct connector" so that vibrations don't shake the structure as much.

enter image description here

You can try to "rebalance" your flywheel. Or replace it. Or replace it and the motor. Decreases vibrations.

You can add some padding underneath the furnace. A lining around the underside perimeter (if it's downdraft), or similar.

Could also add padding between the ducts and the wall.

If you want to be really aggressive, replace the furnace with a "two stage blower" then as long as it's close to temperature, it won't blow as hard. Or even variable speed even more so. But more expensive.
Some "normal" furnaces have an option for how fast they blow either "high high" or "high medium" so youc an vary the speed of the blower down a bit.

Or install radiant floor heating instead of forced air.

If your furnace hangs from the ceiling there are little rubber pads they can use to hang it with "compression fittings" that reduce vibrations.

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