can i get more air flow out of 2 separate pipes 8" and 6" or 1 10" pipe on furn

THE 10 INCH.

The formula is pi times the radius squared. There's a little more to consider than that, but that will get you close enough when working with round duct.

Greetings tra,

Please include CFM of blower fan.

Is not the size of tube which is restricting air flow. You are adding to a system, meaning a group of components configured so to be efficient together.

The elbows effectively add about three feet to the runs.

Without CFM info no calculation can be made, no solution discerned.

The surface area of a 6" and a 8" is exactly the same as a 10". (working on the nominal diameters) If the actual inside diameters are used the result will be different.

The flow may further be influenced by the perimeter length. Where the 10" having less than the combined 6 + 8 (5/7 ratio)

If air perform anything like water I would choose the 10" and it should be cheaper and less problem to install as well.

As you probably know the 6" and 8" pipe are equal in area to the 10" pipe, but looking at it a bit further the surface area or circumferences are larger for the two pipes so if you take friction into consideration, the 10" would have less surface friction? and more flow?

Regards JD.

jd has the right answer, assuming there are no impedance matching problems. 'Tis the old boundary layer that gets you.

The comparison between the two options indicates :

1. The same cross sectional areas are nearly the same (6" & 8* : 50903 mm² and 10" 50829 mm²).

2. The circumferential length adjacent to the inner wall pipe for 6" & 8" is 1217 mm, where for 10" 875 mm (1.5 times).

Since the velocity of any fluid inside a pipe is a parabolic where it is a max. at pipe center line, where it reaches zero adjacent to interior wall of pipe. For this reason, the friction loss (pressure drop) in case of 6" & 8" shall be more than in case of 10".

So, I prefer using 10", also economically its initial cost and running & maintenance cost is considered the best solution.

The plenum is usually reduce after a line is ran off it to produce an even positive pressure through out the system. All so the return registers insure that there is a negative pressure in the room being fed. With out ether the flow from these new runs will be low. The problem can be solve by installing in line blower. They make them just to fit the 6" and 8" lines. Will have to add controls to the furnace to switch them on and off with the blower there. All so I would put some type of damper on the lines so that you can regulate the amount of air drawn from the system to the new rooms.

Okay if everybody's going to get technical. Rated at .1 static the actual numbers are.

10 inch= 433 CFM 8 inch=248 CFM 6 inch= 121 CFM

My ductulator is at the shop. So I went online and found this free trial download.

http://www.members.optushome.com.au/aerial1/

PS. It's amazing somebody has designed a computer program to do the same thing I've been doing with a cardboard dreamwheel for 30 years. I don't know what the program costs, but I'll bet it's cardboard counterpart is cheaper.

The cardboard deamwheel is a wiz. You can get it from Bacharach Instruments at $5.00 to $8.00 as well as Dtraft gauge air flow indicators plus he can go to engineering toolbox for air flow data on both square & round ducts.

i would go with a single pipe /duct with 2 ducts you will lose more heat and flow at the end/ outlet and also add a return. the flex pipe for a long run you will lose flow. insulate any ducts in cold areas

nick

qc. ca.

If coming off the side of the plenum would eliminate use of elbows that would be beneficial otherwise leave them be.

Try opening the plenum and attaching a 22" x 4' piece of foam insulation material to the inside and cut out for piping as necessary. Then check if flow to the addition has improved, then adjust size of foam piece as necessary.