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House shaking when furnace/cenrtal AC comes on


umby63's Avatar
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03-20-13, 11:59 AM   #1  
House shaking when furnace/cenrtal AC comes on

I have a 1150 square foot ranch built in 1951 on a crawl. I had a new furnace and central air installed about 6 years ago. Late in the summer '12, I notice the kitchen floor vibrated (could feel but not hear) under my feet when the air came on. This location is about five feet from the blower for the furnace/ac. Around early Febuary this year I noticed the exterior walls and windows are now vibrating when the blower comes on. One of these windows is at the farthest point away in the house: 20 feet or so. I don't like change! I met some concrete contractors who worked on my house just before I bought it (in 6/2000), and they told me they had to jack a floor joist up, pretty much in the center of the house and place some patio bricks in under this joist to hold it up in place as it had sagged over the years. I just thought I'd mention that as I haven't been in the crawl in about two years. I can't imagine this issue 'resolving' itself. Please let me know what you think. Thanks.

 
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03-20-13, 12:11 PM   #2  
How much does the furnace shake?

 
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03-20-13, 12:21 PM   #3  
The furnace doesn't shake, or at least not that I can hear. And I would imagine with a shell of sheet metal I would hear something. However, I will go home and put my hands on it and make sure. I have also noticed that our dog's toy balls are rolling to a stop near the corner of our living room where, directly underneath, the contractors jacked and supported the joist years ago. Having not gone under the house in a while, I got a feeling that sagging problem has come back. Could this be causing the issue?

 
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03-20-13, 12:59 PM   #4  
What you need under the jack that your contractor's installed, is a 'footing.' A footing wide enough and deep enough that can support the jack.

What's happend is that the patio blocks have sunk into the soil, and the jack is no longer supporting the joist. Someone should crawl under and give the jack a couple of twists.

Then this spring/summer you should get a correct footing installed.

 
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03-20-13, 01:13 PM   #5  
So this (possible) sagging would cause the vibrations I am feeling and hearing above?

 
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03-21-13, 02:52 AM   #6  
Exactly what dgb049 said. I can't believe "concrete" contractors would put patio stones in to support a beam in a house without pouring a sufficient footing. Go under and check the pile of stones to see how tight they are to the beam. You will probably find them loose, or a sagging beam, which you can test with a string tied across your joists on either side of the beam, or along the beam itself.

 
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03-21-13, 05:21 AM   #7  
Check the bearings on the squirrel cage blower and the blower motor. I've seen bearing on squirrel cages vibrate enough to rattle the teeth out of your head....lol Sounds like the harmonics are just traveling through the house.

As for the sag, as was said, get a better footing. (who knows, maybe they never jacked it high enough to start with.) Me, doing too much, would put a jack post on each side of the exsisting post, like 3 feet from each side of what's there. You can dig a good footing for the new posts and jack it all up until level. As you jack, you shim up/screw the one that's there now. My luck is the post falls over , the jack kicks out and my upstairs becomes downstairs. Take your time.

 
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03-21-13, 05:35 AM   #8  
I think the floor sagging is the likely cause but I have also seen debris get sucked through the return duct. If a piece of paper, insulation or furnace filter got into the blower it can throw it out of balance causing it to vibrate or wobble.

 
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03-21-13, 01:05 PM   #9  
Chandler: As I recall being under the house a couple of years ago, they left no jack behind. These two booze-soaked 'contractors' didn't know they were talking to the new house owner and told me they used a car jack to raise the joist. They then slid two stacks of patio bricks in under the joist, on either side, removing the jack when they thought it was level. They said they were in a hurry. This conversation occurred post-sale and I somewhat blame my inspector for missing it. So yah, no footings. How dangerous is the type of jacking if I take my time?

To the rest of you: There is a slight vibration in the actual furnace blower unit that may be resonating out to the walls. And it is possible for the vacuum produced on the input side to have dragged some debris in. I'll check into it.
Thanks to all for your help.

 
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03-21-13, 02:29 PM   #10  
Make sure you disconnect power to the unit before removing panels. You don't want your arm in there when the thermostat tells it to turn on.

A proper footer obviously would be a more proper solution but there is probably no sudden thread from what was done. The blocks were set on the ground. The floor joists jacked up and blocks inserted. Over time the 5, 10 or 20 of house bearing down probably pushed the cement blocks down into the soft earth. A proper footer would be dug down to virgin soil that is of a strength to bear the weight and the footer would probably be larger in size to better spread the load over the ground.

As yet another short term fix the house could be jacked again and the blocks replaced with a steel lally column. Most home centers and building suppliers sell adjustable height ones with one or both ends having a threaded adjustment that allow them to be easily adjusted later. You might have to crawl under the house once or twice a year with a wrench to raise the floor back up but eventually you'll likely compress the soil to the point it can bear the load and further adjustments will be further apart or not needed at all.

 
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03-21-13, 04:49 PM   #11  
Jacking is not that dangerous on the beam if you take your time. You will need to crib up your jack position about 36" away from where you want your footing. Dig a square hole, 24"x24" x12" deep and make a pour of concrete. Once it has cured....a few days, you can re jack the house to level and install 24" blocks and shims.

Dane types faster than me.

 
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