return and feed size duct?

[acnoob1]

I have a quick question for you guys. In the process of building a new home. 2 story home rougly 2500 square feet. I have 2 2 ton lennox xp 16 Hp's going in and 2 g71 furnaces as well. Here is my question.

The furnace located in the basement went in fine lots of room. I don't have the exact measurements and the proper vocab so please bare with me. coming from the long duct runs that run the length of the basement floor. There are to spots were the furnace gets its return air and then the feed size.

output 20x16

return 20x10


Now on the second floor there is a truss that was interfeering with the install the HVAC guy pointed this out to me and because of this were the long runs in the atic hook into the furnace the

output opening is 15x 15
return is 11.5x14.

Im not sure if this is going to be a problem it seems like the amount of CFM is going to be greatly reduced. LIke I said I would have never known but the HVAC guy noticed and told me that they are a few inches smaller than normal. What do you guys think in your opinion. Sorry for the lack of proper terms.

[dash]

A rectangular duct duct is more restrictive then a square,when the cross sectional area is the same,due to the perimeter being greater.More perimeter,more restistance.


For air flow;
16X15= 20X11

14X15= 20X10

So they are closer in abilty to carry X amount of air then it would appear.

[deejoe]

A rectangular duct duct is more restrictive then a square,when the cross sectional area is the same,due to the perimeter being greater.More perimeter,more restistance.


For air flow;
16X5= 20X11

14X15= 20X10

So they are closer in abilty to carry X amount of air then it would appear.
************************************************** ********
16 x 5 = 20 x 11....huh!!! i don't think so.

[larobj63]

************************************************** ********
16 x 5 = 20 x 11....huh!!! i don't think so.

I think that was supposed to be 16 x 15

[dash]

I think that was supposed to be 16 x 15

Correct,and corrected.

[acnoob1]

I actually had messed up all the measurements. I just got back and re measured.

Return 20x10
output 20x16 First floor


second floor

return 11.5x14
output 15x15

With the dimensions being smaller is this a problem. I am paying a fair amount of coin for this system and don't need it comprimised because of a stud placement interfearing with duct work. Your advice is appreciated

[dash]

We typically see larger return ducts then supply,or the same size.

15X15=20X11

11.5X14=20X8,this one may be an issue,or the basement ducts are oversized.

Plus elbows and T's in the basement ducts could require them to be larger,where the attic ducts run strighter,less elbows ,no t'S,so they can be smaller.It's all about the total resistance of the ducts to air flow,Static Pressure.

Correct duct size could be transitioned to a smaller size to get past he truss,then back to full size,if that's the case.

If both were designed by using Manual D ,from ACCA ,then you should be fine.

With your concens ,I have a chat with your contractor.

[red73]

Is the duct lined inside, if so your return air will be too small. If it is not lined it will be adequate.