How to size flex duct for returns

[bmsemple1]

I have a 5 ton system. I am not exactly sure how to size flex ducts for returns. I installed 4 12x24 filter grille, 3 with 12" flex duct, and 1 with 14" flex duct. It is not enough for the 2000cfm required. I have since decided to install a 4/5" media filter box and install it in the unit. Then I want to remove one of the 12x24 and replace it with a larger grill. Either 20x20 or 20x24 or 24x24 and replace the 12" flex duct with a 18" or 20" flex. This grille is located closest to the unit about 10' or less.

What do you recommend I replace it with and will this be enough for the 2000cfm required. I prefer to size it a little larger.

Any help would be appreciated.

 

[rodder98]

A larger size is good because it will be quieter. I would replace the 3 12" with 14" returns and leave the other one at 14" each. I would use either 20x20 or 20x25 filter back grilles. Those filter sizes are very common, so they are easy to find. I am a retired A/C contractor.

 

[zmaxmotorsports]

Just say no to flex.

 

[gregtwojeeps]

Before we bought this house someone forgot to tell that to the HVAC guys that put the system in my crawlspace Zman. ...

It works o.k. I guess on the main returns and with the physical locations of the furnace, main trunk line and two returns grills in the floor...I guess flex it had to be. And, hey...they even supported it from the 2 in. PVC sump pump discharge line. What more could a man want ?

 

[Falcon67]

There's nothing but flex here, never see anything else. Works well enough, better if the joints are sealed to prevent leaks.

 

[Jamie V]

Just say no to flex.

X1000

Flex is made for ceiling diffusers in drop ceilings not for houses.

Flex restricts airflow, 12" flex will not flow the same amount of air as 12" metal duct.

 

[zmaxmotorsports]

Before we bought this house someone forgot to tell that to the HVAC guys that put the system in my crawlspace Zman. ...

It works o.k. I guess on the main returns and with the physical locations of the furnace, main trunk line and two returns grills in the floor...I guess flex it had to be. And, hey...they even supported it from the 2 in. PVC sump pump discharge line. What more could a man want ?

send me a plane ,I'll ship the tools.
We'll have that fiasco fixed in no time!

 

[LS6 Tommy]

Flex is horrible for returns. There are much large sizes available than you have now if you have to stick with it.

Tommy

 

[zmaxmotorsports]

Flex is horrible period ,unless its,inside of a,drop ceiling in a commercial building where it's constantly being torn out or moved.
It's right up there with Trac pipe/green field/flex water heater install lines in my book any way.