Sealing Round Metal Duct Joints On Forced Hot Air Furnace (other than duct tape)?

Hello:
Regarding all the round sheet metal duct work exiting our typical gas fired, hot air heating furnace:
What's the best way to seal these duct joints, at least where one can actually see them ?
Duct tape doesn't really sound like the best way to go.
Would appreciate any suggestions as to brands, etc.
I guess I should also ask:
Is it necessary to do so for the hot air ducts ?
How about for the duct leading to roof flue exhaust (no fireplace in house) ?
Thanks, Bob
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You would be correct to not use duct tape on the joints.
Find metal tape. Here is an example: http://store.yahoo.com/poseysupply/2x60yarfoilt.html
A good brand name is Nashua.
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I bought a mastic , a thick plastic cement like product you brush on from my hvac suplier, im sure HD may have a product for it. It is for supplys and returns not hot chimneys.
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Robert11 wrote:

"Duct" tape may work on ducks, but not ducts. You want tape made for ducts (Duct tape is not, but that is another story).
Look around in the area where ducts are sold. You will find some slightly more expensive tape made for the job. It will be shinny metal looking and it will have a liner over the adhesive that you will need to remove when you use it. Get it straight the first time, it's not coming off. That liner makes putting it on easier.

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On Wed, 20 Apr 2005 18:47:57 -0400, "Robert11"

the BEST product to use. you want a brush on duct sealer/mastic. Not roofing mastic, nor any petroleum based product. the best duct mastics are water soluble & have zero or very little flame spread. The better ones have reinforcing fibers mixed in. These products are not typically sold at home depots or DIY home centers.
do not apply duct tape or duct sealer to the exhaust venting. check the building codes for your geographical area.
try these sources:
http://www.hardcast.com/products/pr_detail.asp?pid=1
http://designpoly.com/sealants.htm
http://www.wardind.com/Products/Sealants.htm#swb-n
http://www.mcgillairseal.com/textDocs/newsReleases/uniFlex_Mastic.htm
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If you use tape for sealing ducts, pressure sensitive tape should be rated UL-181-AP. That is in the new International Residential Building Code. ShurTape AF-100 is the one I normally use. It has an acrylic adhesive instead of the rubber based adhesive that regular duct tape does. It should be rubbed down with a squegee like you use for applying body putty. It is called a rub down tool. That helps the adhesive to cure and gives better adhesion. UL-181-AP tapes have the brand name ant the tape model number written on them. It MUST be at least 2-1/2" wide It has UL-181-AP rwitten on it in block letters. It also has the last test date on it as well. It will normall have a white adhesive protected by a white release paper. If it does not have all of those features, it is not the correct tape. There are 5 brands with 5 different tape model numbers that are acceptable to code and the UL.. Fasson also makes one as does Ideal. I'm not sure of the other two brands.
Stretch
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We use silicone
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Duct tape is exactly what you want... It's metal and shiny silver and has a wax paper strip on the sticky side. You do NOT want "DUCK" tape, which is grey cloth tape and really sticky/gooey.
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