How does RPM figure into this?

 

Well I thought feet per minute was abbreviated fpm.

 

It is. But FPM is not in your post.
Your question is “rpm to area served”.
RPM is revolutions per minute.

 

Alright- now that I saw the typo and corrected, any thoughts?

 

I would be more concerned with CFM. Are the returns balanced with the supply for that area?
Are any areas isolated by doors which could alter air flow?
Also consider length of the runs. A short, small return will have more air flow and probably more noise than a long, big duct because of what is called equivalent length.
Large ducts contribute to lower static pressure and lower noise. Return ducts are often more deficient than supply ducts. And the ducts and fittings close to the equipment are more important than those far away.

 

The supply Can NOT exceed what the return allows!

 

Kdean1, the contractor “designed” the ducts at 1500 cfm. That’s could/should be max possible-at least on the next lower line of equipment that I can get a good spec sheet from. ECM blower, inverter variable system. Going with that it has 800 cfm in return area 1-700 cfm in area 2. I requested the master suite at 200 cfm supply have a separate return deducted from area 1.

A 16” return at approximately 145’ total equivalent length for area 1
Master suite with an 8” return approximately 140’
The remaining area 2 in question would return 500 cfm. Total equivalent will depend upon fittings. Contractor proposed a 12” flex to panned joist section. I’d rather have duct even though the pan is between two conditioned spaces.

If the final proposed return was 12” round duct, the velocity would be higher than the other 2. My question is would that be objectionable enough to specify rectangular duct to more closely match velocities? Thank you for your time.

 

It is difficult to determine without seeing the whole Manual D. The panned return needs to be well sealed, especially if it includes an exterior surface such as a rim joist.
How have you determined the velocity will be higher in one duct? I’m inclined to think it won’t have higher velocity, but it will have less air flow than expected. What is the design static pressure? That’s going to be the critical matter.