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I have some of this duct tape Nashua 557 Premium Grade Flex Duct Tape. Is this the correct stuff to use to seal leaks in a forced air heating system, or is there something else I should be using?

Note:

It says it's rated for Class 1 ducts, I haven't been able to find out what exactly a Class 1 duct is yet though.

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That is the fanciest looking duct tape I've ever seen. –  Mike Powell Dec 11 '10 at 4:08
    
@Mike Powell: It's real duct tape, that can actually be used on ducts! just not sure if it can be used on my ducts. –  Tester101 Dec 11 '10 at 4:10

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Looking at the specifications on the page you linked to, it appears to be made of the same basic materials as other duct tapes: plastic coated fabric with a rubber adhesive. Premium Grade it may be, but I think you'll run into the same problems -- it'll get brittle and crack with age. I've used aluminum foil tape where I've needed spot repairs; Home Improvement 1-2-3 says to use duct mastic when installing ducts.

I found this article very useful: Can Duct-Tape Take The Heat?

Duct Tape Cannot Take The Heat

Although our testing has not been able to differentiate amongst other sealant products, the data shows that cloth duct tape is not a good sealant for use in ducts that operate at much above ambient temperature. We believe this is due to the rubber adhesive, but cannot state so definitively. For the most part, cloth backing and rubber adhesives go hand in hand. Thus it is not surprising that the other sealant products have not demonstrated any of the failure modes we have seen in the duct tapes. There are a few products that use rubber adhesives with non-cloth backing and we intend to test these in the future.

To address your side note about Class 1 ducts: from the scope for UL 181 Standard for Factory-Made Air Ducts and Air Connectors

Class 0 - Air ducts and air connectors having surface burning characteristics of zero.

Class 1 - Air ducts and air connectors having a flame-spread index of not over 25 without evidence of continued progressive combustion and a smoke-developed index of not over 50.

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Thanks Niall. I found the same information on the classes of ducts, but wasn't sure what that means? As for the tape, it's more like metal tape than "duct tape" so it might last better than "duct tape". I am more concerned about the flammability of it, if it can stand the heat. –  Tester101 Dec 11 '10 at 7:00
    
I agree with Niall. I only use foil tape for any kind of ductwork, even cold ventilation ducts. That Nashua tape may feel like metal, but it says right on that page that it's really made of cloth and rubber. Duct tape may have thousands of uses, but taping ducts simply isn't one of them. :) –  Chris Jaynes Dec 11 '10 at 23:31
    
@tester101: the specifications also list a flame spread index of 15, less than the 25 maximum cited on the Wikipedia page I linked to, so I don't think flammability would be a concern. The taps is probably just metalized, i.e. a few atoms thick, so I still think you'd be better off with foil tape. –  Niall C. Dec 12 '10 at 0:49

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