Proper way to connect flex to metal plenum?

Greg__RJuly 9, 2012

What is the correct method for connecting flex duct (R-8) to a metal plenum? Mine were taped in place (metal duct tape) and my air handler has blown them off the metal flange. Should these be screwed in + the tape? The flex duct doesn't seem to have much to screw into.

Also, is a big zip tie OK to hold the insulation in place?

We had our duct work replaced due to flooding last year and one of the connections has broken. I want to bring the company out to fix the problem but want to know the correct way to install so I can ensure a correct fix.


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start collar mastic sealed to plenum.
inner liner mastic sealed to start collar.
insulation jacket and insulation brought in
contact with plenum. mastic sealed insulation
jacket to plenum.
lock ties are often used to hole insulation
jacket and insulation in place.

either paint on mastic or mastic tape can be used.
paint on mastic takes longer, as mastic has
to dry.
mastic tape is more expensive, but good long
lasting seal. because it doesn't have to
dry, it goes quicker.

I use Hardcast brand 1402 mastic tape.
silver with sticky gray (mastic) backing.
costs about $30. per roll at hvac supply.

foil tapes are not strong enough. just like
duct tapes, they fail quickly in extreme attic

best of luck.

    Bookmark   July 9, 2012 at 4:34PM
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Looking @ it closer, I see a gray sticky material on the inside of the flex and the tape has a grey sticky backing (tape is metallic). It looks like they used the right materials but for some reason the duct still fell off (metal was dusty?).

    Bookmark   July 9, 2012 at 4:50PM
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inner liner connected to supply box,
same install for plenum install
don't know how to do multiple pics
so outer liner pic to follow

    Bookmark   July 9, 2012 at 5:34PM
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and last.

    Bookmark   July 9, 2012 at 5:40PM
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