Replacing furnace ducts with flexible?

[chilipepper]

I have a '71 Overlander I'd like to replace the long piece of ductwork aft of the furnace withe 4" flexible ducts. There are only two vents to the cabin from this piece as well as a couple of diverted pieces to the belly. Would I need to run more than one 4" duct to compensate for removing the 8" x 2" duct that is there now? Would it be 'better' to run two lines for the vents? I have some of this ducting material from a previous home improvement project (Dundas Jafine BPC425R6 Insulated Flexible Duct with Black Jacket, 4-Inches by 25-Feet - Amazon.com) or is that just for return venting?

My reasoning for wanting to do this is to accommodate the flow from the drain lines from the back to under the galley where I want to put a grey tank.

[dznf0g]

You do not want to reduce the size of the duct cross section. Your metal duct is 16 sq.in. A 4" round duct has a cross section is only 12.57 sq. in. Add to that, the flexible duct is not smooth, introducing more flow restriction. I'd definitely use two 4" ducts.

[Jammer]

Originally Posted by chilipepper

I have a '71 Overlander I'd like to replace the long piece of ductwork aft of the furnace withe 4" flexible ducts. There are only two vents to the cabin from this piece as well as a couple of diverted pieces to the belly. Would I need to run more than one 4" duct to compensate for removing the 8" x 2" duct that is there now?

Two 4" round flex ducts will move less air than what you have now but will probably work out OK.

I have some of this ducting material from a previous home improvement project (Dundas Jafine BPC425R6 Insulated Flexible Duct with Black Jacket, 4-Inches by 25-Feet - Amazon.com) or is that just for return venting?

I could be mistaken but I believe that the RV stuff has a higher temperature rating. You can get the RV stuff from the place that makes it:

Flexible Ducting / Uninsulated Flex / 4 in dia. X 25 foot long, Premium Flexible Heat Duct (4 X 25 SLP-10) - DW Inc.

My reasoning for wanting to do this is to accommodate the flow from the drain lines from the back to under the galley where I want to put a grey tank.

Might want to try a shower sump and pump since it doesn't look like you have enough fall anyway

[chilipepper]

Perfect that is what I was leaning for but just wanted to make sure. Seems to be a good amount of space there and I might just add a collar to the duct under the tub to keep from having to flatten out the flex and fish it through there. Thanks again for the reply!