Rectangular Vs Round AC ducts 1

[Golestan]

Hi- I would like to know in the HVAC mechanical system, when should we use rectangular ducts (I think it is called the Trunk line) and when we should use round ducts.
And what is the rule of thumb for transitioning from rectangular to round duct.
And when do you use flexible round duct.
And is the return line back to the units has the same rules.

 

[ChrisConley]

Round ducts are generally preferred. The reason we use rectangular is 1) cost, 2) ceiling space. Oval duct is nice alternative to rectangular. The flow is better and you can change the aspect ratio to 'flatten' the duct to fit in the ceiling.

Return ducts are often rectangular as the flow path is less critical, sealing is less critical and the runs are usually shorter.

Flex duct should be used only to connect a branch duct to a diffuser, and even then try to keep the length of the flex to under 4'.

 

[stanlsimon]

In vernacular language, round and oval duct can be made to look more attractive when exposed to view. Rectangular duct is uglier and is generally concealed.

 

[willard3]

The hydraulic radius of round and oval ducts is also less than rectangular ducts.

Lower hydraulic radius=less pressure drop.

 

[AbbyNormal]

In most cases rectangular is less expensive

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

Rectangular fits in tighter spaces, an elbow with vanes is a lot more compact than a round elbow with a proper radius and similar pressure drop

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[Tmoose]

Round ducts have much lower noise breakout than rectangular, so come in handy close to airhandlers until the various fan flow noises settle down, or when passing thru a sensitive area. Sloppy duct transitions and elbows can stir flow noise right back up again, especially when flow is fast.