Proper way to connect heating air ducts?

Some of the air duct from the furnace to the registers are several shorter ducts joint together. A while ago I found some of those joints failed and separated in the crawlspace. The ducts look to be 8 to 12 inches in diameter, and they want to pull away from each other at the joints, perhaps the ducts were taut when installed. I duct taped them back. Although they hold, they seem to be leaking air.
What is the best way to seal the joints?
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lots of options, but depends on the type of "hole" Are you sure there isn't crimped pipe joints that came apart ?
Duct tape is easiest, and you can get different grades, some is garbage, some will last years. it is also available in either the cloth or metal types.
You can also buy what we call ductseal, a brick of caulking type compound that you press in like play dough. (electrical and refrigeration wholesalers always have it)
Or even pop rivet another piece of metal over the hole.
Even just regular silicone caulking may work.
AMUN
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I went into the crawlspace to investigate. Turns out the leaks are coming out of the metal ducts' Y joints.
Here are some photos to illustrate: http://www.smugmug.com/gallery/849974
I did not take photos of all the suspected leaks.
Although it may seem that the gap is small, but a lot of air is coming out. What is the proper way to seal those joints? Some sort of sealant? I don't want to use duct tape because they become dry and brittle after a few years and then lose its effectiveness.
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peter wrote:

    Have you tried the "Metalized duct tape?" There is one not made of cloth and it lasts a LONG time. Home Depot and other such stores have it.
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I would add hex drive, sharp tipped sheet metal screws - three per joint. Gray cloth duck tape (original name) was never meant for HVAC install. You need real metal duct tape. Example: <http://www.pplmotorhomes.com/parts/caulks-sealants/388521.htm
After screwing and taping you can also apply duct sealant. Here is one we use: <http://www.fosterproducts.com/Images/childers_tds/cp-10-11.pdf It is quite easy to apply with a throw away paint brush. If the holes or gaps are huge, add fiberglass reinforcing mesh.
(top posted for your convenience) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Keep the whole world singing . . . . DanG (remove the sevens) snipped-for-privacy@7cox.net

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peter wrote:

that is doing the job puts three screws in each joint, then brushes on a permanent duct seal over the joint. They say it should last forever.
Of course I don't expect it will last a hundred years, but then I don't expect to be worrying about it in a hundred years.
Bill Gill
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