Let's see if this helps . . . A 90 degree bend is approxiamately equal to 10 feet of pipe, so installing 90's can reduce the air flow somewhat. However, some factors need to be considered; what size is he ductwork?, how long of a run is it now and after rerouting?(to determine drop of duct flow)Etc. Some installers(or homeowner installers) run the ducting at an angle simply to save on cost of material and labor during the installation, because it will be hidden anyway, so why not. The duct may be large enough to handle the decrease in flow, or increase in static pressure as it is. But one elbow may not affect it's flow enough for you to notice, so I have a suggestion for you ( not knowing your home layout, it's a rough idea); why not run the duct from the corner it's in, down one wall(at ceiling height), use one elbow, andthen connect again at the other corner. To hide it, you could box it on the two sides exposed and have it appear as a step in the ceiling. As for flex ducting, you would(if used) want the insulated ducting(bare metal 'will' sweat). You'll need to do a little math to make sure you have the same sq. in. of duct area. Assuming your duct is square now. I have made similar changes like you speak of, and had little or no noticeable decline in supply, usually because of the "installer thing" I mentioned above. If you want some free information, you can go to a Grainger's somewhere and get a catalog. In the back they have handy references for air flow. Duct and furnace supply companies also have some handy literature sometimes. If you give me sizes of the ducting as it exists, the approxiamate length of the current run, wether it's round or square, a North, East South, West wall dimension description of the existing room, and the relationship placement of the furnace, and what rooms are adjacent to each wall, I can probably help you. My e-mail is Rb249@aol.com, you should RE: ducting (my credentials - I am a licensed Stationary Engineer) Good luck with your project.