I have forced air at my home and air ducts run pretty much straight. However I want to add bathroom in the basement and because of low ceiling hight, the main air conditioning duct has to be re-routed around the bathroom (9 * 7 ft) as shown in the diagram below.

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As can be seen, it introduces 4 right angles. Will this significantly effect efficiency of the air conditioning and air flow?

Should I avoid re-routing this main duct?

Alternatively my other option is to make bathroom in another place in basement where location is somewhat less ideal and sewer lines are quite far away but it will not interfere with the air ducts.


The reason that 90 degree turns (or any tight radius bends) in duct systems are discouraged are because they reduce air-flow. The friction that is encountered by the moving air as it hits the wall of the turn slows it down decreasing the distance it can travel.

There are equations that can be used to calculate the number of bends before air performance is affected. Duct size and shape, motor wattage, register opening, etc. will need to be considered. If you can orient the duct system and configure the shape with "gentler" turns such as using (2) 45 degree bends in place of (1) 90 degree bend. This would restrict air-flow less. Installing a motorized fan duct in-line with the new duct branch would allow for longer runs if needed and account for sharp turns.

Is it feasible, when viewing the drawing you posted, to branch off the main duct sooner with a 45 degree duct and circumvent the restroom with 45's? Or consider using flexible insulated duct which is much more bendable than stamped sheet metal ducting. I'm not positive that code will accept this type of configuration ( rigid to flex back to rigid), but someone should comment about it either way).

  • Thanks for the input and suggestion! I am thinking it might be better to relocate the bathroom. – zar Jan 28 '16 at 16:40
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    Flex duct has a thousand times more friction than sheet metal. Do not use flex duct unless you oversize it by a significant amount. – Zach Mierzejewski Jan 28 '16 at 18:35
  • @ZachMierzejewski thanks but why do you think Flex duct has more friction? It does seem to have smoother turns!? – zar Jan 28 '16 at 22:41
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    @zar The constant bumps create eddies and the air tumbles for the entire length of the duct. Compare that to a sheet metal duct where the air can get flowing in-line with little turbulence for 10's of feet. In flex duct, there's not just turbulence around the edges and the middle flows along happy; the turbulence around the edges distorts the entire air stream. – Zach Mierzejewski Jan 29 '16 at 20:18

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