Duct Question

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rockncountry101
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Joined: Sun Mar 11, 2012 3:56 pm

Fri Feb 15, 2013 3:19 pm

I have a mobile home that has a central unit. The unit itself is quiet. However, in one spot of the home (right under the master bedroom) there is a continuous popping sound after the heat and a/c kick off. I took all the registers off to make sure that was not what I was hearing. Even then I still here a popping noise. It sounds like someone is hitting underneath with a hammer. Someone suggested that it could be expansion and contraction but it goes on for a good hour before it ever stops. If it is the ductwork, what should I do to correct it? Could it be anything to do with the main unit. The sound is just a few feet away from it. A couple of mobile home people suggested to put a brick inside the ducts to weight down the problem area in hopes that it may weigh it down. I'm beside myself. No a/c man will attempt to correct it because they don't work on mobile homes. (Too much trouble for them to cut and repair the underbelly.) Thanks for any help.


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Greg
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Fri Feb 15, 2013 5:15 pm

If it lasts for an hour AFTER the furnace kicks off I would question the ducts expansion/contraction. Is it possible that the fan kicks on & off to cool the furnace essentially "inflating" the ducts making the noise?

Greg
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Spydrweb1977
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Fri Feb 15, 2013 8:11 pm

If this is a gas furnace, it could be delayed ignition from clogged or overly dusty burners. You should be able to hear you igniter click and then the whoosh of the burners lighting up right away if your near the unit when it fires up.

Just make sure that's not potentially the cause first, then beyond that it probably is some type of expansion/contraction in the ducts.

If it is a gas unit, call your gas co and ask to have your furnace looked at. I believe most utility companies have service branches. I know mine does...

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Greg S
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Sat Feb 16, 2013 11:58 am

My money is on expansion/contraction. Very little else causes duct noises. My guess is that something is fit/attached too tight restricting noise free movement.
The solution is to open the under belly and determine what is restricting the metal movement.
An individual must enforce his own meaning in life and rise above the perceived conformity of the masses. (Anton LaVey)

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Greg
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Sat Feb 16, 2013 5:51 pm

If it is lasting an hour after the furnace kicks off I seriously doubt it is an expansion issue. There is NO way that the tin foil ducts would retain heat for that long. I can heat a piece of 1/2" steel red hot and have it cool in an hour. That was why I was leaning toward the fan "inflating" the ducts if it was still coming on.
"If I can't fix it, I can screw it up so bad no one else can either."


DaleM
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Sat Feb 16, 2013 11:48 pm

I would say duct expansion and contraction. The reason it's goes on for an hour or so is because the strapping holding the ducts up against the floor joists is really tight. The duct heats up at a much faster rate than it cools down. It will expand faster when heated but will contract slower when it cools down. The duct may be completely cooled down but the strap holding it tight to the joists restricts the contractive forces for some time until there is enough pressure to overpower the straps hold. That may take 10-20 minutes, longer even. Then you get your BOOM. I would say crawl under your place when the noise is happening, slice it open and cut the starpping holding up the ductwork.

rockncountry101
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Mon Feb 18, 2013 11:31 am

I know that as soon as the heat kicks off the ducts are warm to the touch. In about 5 minutes they are back to room temperature. The "popping" will start about three minutes after the heat kicks off and continues on for a good while. It never did this until about 9 months after I moved in. It is an electric heater. I'm not sure which strap to cut, because the sound goes from wall to wall. It even transfers to the wall. It sounds like someone is in the wall and under the floor. It's a bizarre issue. I can also cause the sound by standing and walking across the floor. No sounds whatsoever will come from that area on days when nothing runs. I'm not a handy man, so I couldn't get under there. And I can't find an hvac person that will work on mobile homes. I did get someone to go under there and they made sure everything was tight. But since the floor joists are 16 inches apart and the run is about 20 foot, it would just be a guessing game as to which brace to cut.

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Greg
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Mon Feb 18, 2013 5:21 pm

If it's a strap issue, you really don't want to cut the strap. I would try loosing it, you may have to unfasten it and reattach it. I think it will take 2 people to find the location, on on top and one under.

Greg
"If I can't fix it, I can screw it up so bad no one else can either."

ponch37300
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Mon Feb 18, 2013 9:47 pm

Please do not just start cutting straps. Think about it, these ducts are flimsy enough as is. If you go cutting straps you are only going to make things worse when the duct is sagging down.

IF it is something to do with the straps then undo them and add some kind of foam between the duct and joists and also between the duct and strapping and then put the strapping back up.

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