Possible heating duct problem

I belong to a little brick bungalow in the midwest US built in 1954. Original forced-air (galvanized ductwork) heating, 2-yr-old Heil 80% furnace.
Lying in bed, very quiet, furnace begins heat cycle. About 15 seconds after the burners fire, I hear a big POP. Very consistently. Every night.
I know that the furnace heat makes the sheet metal expand, and a certain amount of noise (creaks, squeaks, etc) is to be expected, but this one sounds too loud.
Walked the length of the ductwork, tapped each joint with a mallet, inspected everything. No apparent problem.
Still I get the big POP.
Anything with 50-year-old galvanized ductwork that I should be looking for?
Thx, Peetie
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wrote:

Not sure if it is the sound you are talking about but some forced air furnaces will cause the sheet metal to suddenly shift out when the fan starts. Similar to what a balloon does when you blow air into it but just enough to make a "Whoomp" sound when the sudden air pressure is turned on.
In that case, it is usually the larger pieces of sheet metal near the furnace that connects the furnace to the cold air return that make that sound.
David
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Just from the force of the air ...

I don't think that's it.
Mea Culpa: I did a poor job of describing it yesterday.
I measured again last nite. The burners fire, the blower kicks in, and about 40 seconds later, I get the big POP!
I 'm pretty sure it has to do with increasing temp in the supply (not return) ducts.
Thx, Peetie
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wrote:

my furnace does the opposite. i get the loud POP when the blower shuts off.
--
C.D


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Immediatly? Before it's had any time to cool off?
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wrote:

yep.
--
C.D


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Even more curious than my little problem.
Hope it doesn't keep you awake at nite.
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wrote:

nah...but now i'm worried about the CO2.
--
C.D


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CO2 is carbon dioxide (in which we swim, all our lives).
You are worried about CO (carbon monoxide).
Buy a detector. Not too expensive.
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I had a furnace that made a poping noise, similar to what you describe. It was on a fairly new unit and was found to be a broken heat exchanger. It was leaking CO2 into the house.
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On Sat, 17 Jan 2009 05:47:31 GMT, snipped-for-privacy@yahoo.com (John Normile) wrote:

...

Fairly new and broken HE? I assume 'twere under warranty.
I gotta CO detector: no apparent problem.
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wrote:

If it's happening "Very consistently. Every night" then grab a sleeping bag and sleep next to the furnace one night, halfway to the bedroom the next, etc.
Seems like within a few days, you know exactly where the noise originates and then be able determine the actual cause.
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You *could* have a damper in the duct that opens in closed. Common in commercial buildings, I've never seen one in residential but that doesn't mean there isn't one.
But if I had to guess, I'd say the most likely cause is something called snap-through, pop-through, a couple of other terms. A slightly convex surface can be stable in two configurations, and under air pressure (more likely) or temperature (less likely) changes can pop through to the other. You can probably play with a beverage can and reproduce it.
If that's what it is, it is harmless.
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I think it's also called "oil canning" and is not anything to worry about, but can be annoying. Mark
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On Jan 16, 10:19�pm, snipped-for-privacy@yahoo.com wrote:

I had this problem a LONG time ago:( Added a couple drywall screws to elminate it.
That must be over 20 years ago. Time flies.
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go down there and search for the pop during operation.
---------------------------------------------------------------- http://www.minibite.com/america/malone.htm
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On Jan 16, 9:49pm, snipped-for-privacy@webtv.net wrote:

Or set a trap.
Spray the duct with something that will crack when it moves. This is commonly done to check for vibrations in machinery. I can't remember what is used anymore, maybe some kind of shellac?
It occurs to me that if you can see a galvanized metal duct, it is not insulated. Bad idea.
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I did have to chase every duct-line and vent.
The kitchen vent was partially closed. Looks like that duct was bent funny back in 1954 and the plasterer plastered up into the dent. With the vent panel partly open, evidently the expanding <something> was binding the panel causing it to pop.
That vent won't close 100%, but with it wide open, no more pop.
Not the first time I've tracked a problem that had it's roots in the original construction.
Thx, Peetie
On Wed, 14 Jan 2009 19:28:52 -0600, Peetie Wheatstraw

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