HVAC question -- best way to plumb an additional 16x30x1 return to keep furnace noise down?

[surrealone]

Scenario:

• I am the third owner of the house
• House has a centrally located 16x30x1 return grille
• House originally had 2 ton HVAC; in 2009 the current owner upsized it to 3tons to add heat to the basement
• No return was added to the basement.
• I'm planning to add a 16x30x1 return grille to the basement in a reasonably central hallway (furnace is on the other side of one wall of said hallway).
• I want this size specifically for filter parity with the existing return grille on the main floor (so that I only have to stock one filter size) ... and, as I understand it, one can't really have too much return air.

When added, an efficient basement return grille would take air through the wall straight into the return box, and I already plan to insulate the furnace room with sound-absorbant Rockwool to reduce through-wall noise from said room. However, I have concerns about blower noise exiting the new return grille since it'll basically be a short, straight run to the return box.

Is it sane to use round duct and add a S-curve to the mix knowing that air flows well around curves while sound ... doesn't? Or, perhaps, is there a sound deadening technology I should be looking at for the new return, itself?
Finally, I have extra round duct on hand and it'd save me money to use it: is there a good way to come off the new return grill and go straight to round duct? I'm aware of square-to-round transitions, but I'm not sure that'd apply here....

Please advise and, as I'm a relative HVAC n00b, please feel free to provide course correction if you think I'm off course.

Thanks in advance to anyone who can assist!

 

[jjwestbrook]

Flex duct would be quieter , but if you already have metal I would use it. The larger the better for quietness

 

[Darkhorse]

@BlackGun

 

[LeeMajors]

Are there the same number of supply vents in the basement as the upstairs? If not, I would not add the same size return in the basement. IMHO that will throw the balance off. Especially is there is a door that is going to be closed between upstairs and the basement. But that's my opinion

 

[BlackGun]

Good suggestions above. You can add the same size return and yes bigger return is good for less noise and flow. But you need to have a duct size on that return consistent with the number of supply registers for the room. For example if you have two 6” supply registers your return duct size will be 8”. If you put in a 12” duct with them being in different air supply zones you will in effective not have a balanced system. The air you put in will be pulled out quickly.

Usually as stated by jjwestbrook, flex dampens noise much better than metal. If the downstairs return is directly behind the newly installed grill not only could you have air noise but you are going to get motor and fan noise with very short distances. Those noises are at levels that make it hard to hear TV and have conversations. If its two feet to the air handlers return box, go to the other side with a turn and make it 6 feet.

 

[surrealone]

Are there the same number of supply vents in the basement as the upstairs? If not, I would not add the same size return in the basement. IMHO that will throw the balance off. Especially is there is a door that is going to be closed between upstairs and the basement. But that's my opinion

There are NOT be the same number of supply vents in the basement. My plan to address this was to install dampers in the rigid duct from both returns (so that I can manually adjust balance) and, of course, test copiously with a manometer to balance things. (I already know the house needs jumper ducts in some places, as I can feel the pressure difference when opening doors to certain rooms. I'll eventually get to that....)

Good suggestions above. You can add the same size return and yes bigger return is good for less noise and flow. But you need to have a duct size on that return consistent with the number of supply registers for the room. For example if you have two 6” supply registers your return duct size will be 8”. If you put in a 12” duct with them being in different air supply zones you will in effective not have a balanced system. The air you put in will be pulled out quickly.

Usually as stated by jjwestbrook, flex dampens noise much better than metal. If the downstairs return is directly behind the newly installed grill not only could you have air noise but you are going to get motor and fan noise with very short distances. Those noises are at levels that make it hard to hear TV and have conversations. If its two feet to the air handlers return box, go to the other side with a turn and make it 6 feet.

Yup, we're on the same page in terms of duct size. The noise (and noise level) of which you wrote is precisely what I'm trying to avoid!