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Posts: 303
Reply with quote  #1 

I came up with an idea to replace the long (~ 20 ft.) flexible air duct that connects the input of my 2x16 ZP collector.  Referring to this:

Ducts, covered, 022617.jpg 

I would still need some short flex sections at the ends, but I'm thinking about making a long straight section. about 15 ft., with a square 'tube' of 1 inch polyiso, with 6 inch starting collars at the ends.  Here's a sketch, showing view from one end:

Polyiso duct, 022518.jpg 

The new 'duct' would be 8 x 8 inside (10 x 10 outside).  The ends would be 8 x 8 pieces of polyiso with  6 inch round hole in the center, surfaced by sheet aluminum with the 6 inch start collar attached.  The aluminum sheet would be glued to the polyiso square, then the whole assembly would be glued into the end of the tube before the last side is glued on.  May have to tape the corners for strength and to ensure good seal - don't know about that part yet.  Probably also have to glue / tape two sections in the center to get the required overall length, and maybe wrap the joint with a strip of flashing and more glue.  I could attach the whole thing to the Unistrut pieces on the wall with some 'soft' hangers.

Has anyone tried anything like this?  I am hoping to improve air flow over 6 inch flex duct.  I guess the other option is rigid duct with exterior insulation.  Also - I'm working on a plan to make a whole collector out of glued-up polyiso with flashing skin, so this would be a chance to work out the bugs of a new construction technique.

Now if it would just warm up around here...


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Posts: 2,461
Reply with quote  #2 

I like the idea...

I recall that Scott Davis used XPS to line a trench for the water tubes running out to the far end of his yard. After lining the trench and inserting the required pipes and sensor wires, he filled the remaining space with expanding foam, such as Great Stuff. I see no reason why you couldn't do something similar. But why put 6" ducts inside a 8" square interior? Could you skip the round ducts altogether and use square ducts made out of polyiso? Yes, I know round ducts have better flow, but you would have 64 square inches(8" square interior) versus 50 square inches(8" round), and you wouldn't have to purchase solid duct pieces except for the corners and transitions. You would still need something different for corners, but maybe that's where you have to use metal ducts inside flex duct insulation, which is what I have been using for years. 

With a single 4x8 sheet of polysio, you would only be able to make 8' of square duct(8" interior), so a long duct with metal ducting inside could be somewhat pricey, but would last for years. I know my flex ducting insulation is in need of replacement after several tough winters. The exterior skin is not designed for exterior use, so I wrap a cover of a heavier gauge plastic around it for added protection.

Greg in MN


Posts: 303
Reply with quote  #3 
Greg, I was not planning to install rigid duct inside the square polyiso tube, but rather to use the polyiso itself as the duct.  I will just be replacing a long, straight section of flex, at least at first, so I won't have to worry about any corners.  That's why I figured to stub out the ends of the tube, so I could connect short flex sections to the collector on one end, and the building on the other end.

Thinking down the road, it seems you could incorporate a long duct passage into the collector back, make it physically part of the collector instead of a separate piece.  If it was used for the intake, for example, (what I am planning), the air passage could be rectangular, say 3x20 or 4x20 inside, and the intake port into the collector could also be rectangular.  That's what you end up with inside a ZP anyway.

You could make corners too (may work better with a 'squarer' cross-section), and glue in a curved piece of flashing to help the air around the corner.  I'm even thinking of doing that on first experimental model.  The end of the tube could butt right up to the back of the collector with some sort of bellows joint to seal.  Gotta think about that part.  Too many ideas; I'm getting ahead of myself.
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