TLDR/BLUF - Does the [b]depth[/b] of the return plate/plenum that the return filter sits in matter? Ive received conflicting information from different techs. One says the deeper it is, the less noisy the system will be and doesnt want to go any larger than a 20x30 grille/filter plenum combo, while the other is stating that the size of the box behind the filter has no effect on the sound the HVAC system will create through the grille but also wants to up the grille size to 20x36 (which is a pretty big return to have on my wall). The long version - I have a single air return filter/grille in my condo that Ill be replacing in order to get the proper size return. (I currently have a 3 ton unit with a 20x25 return which is causing lots of "turbulence" at the grille/return). When I started to get into the math of HVAC systems, I found that the filter/return combo is undersized for the unit and would technically need a 20x30 which could also be the reason why Im experiencing the additional noise at the return... my initial thought was just to replace the grille with a more updated one as the current one is bent, painted over a dozen times, somewhat twisted... etc. (hasn't been replaced since the 70's?) but I figured if Im going to take the time to replace the grille, I should do it right while Im at it. Ive had a few HVAC techs come out to give estimates since I had never tackled HVAC stuff before... Ive re-wired my entire electrical panel, added recessed lighting, outlets, switches, fans... but never hvac. They all gave me conflicting information as to the correct way to go about rectifying my issue while also includes adding a couple supply's and perhaps another register in a loft area which usually remains very warm on warmer summer days (I currently only 1 supply to serve that area). I had one tech tell me I need a 20x35 grille/filter/air return plenum because the fins on the return grille reduce the overall performance by 10% and that should be added to the overall sq/inches of the filter, but the depth of the air plenum/pan(?) behind the grille does not matter because the depth of the return plenum plays no role in reducing the amount of sound that could emit from the grille. They also also suggested adding a 17x20 by 18" deep air return plenum on the actual air handler that would also provide additional noise reduction. Another tech explained to me that I don't need to go any larger than 20x30, but would build a larger box off the filter nearly 20 inches deep and that would provide the appropriate sound dampening I'd need and theres no requirement to build another plenum off the air handler itself (at least not until Id like to add an another return in my loft area) Does the depth of the return plate/plenum that the return filter sits in matter? One says the deeper it is, the less noisy the system will be, while the other is stating that has no effect on the sound the HVAC system will emit through the grille but also wants to up the grille size to 20x36. From the return its a roughly ~10 foot straight shot directly into the air handler (I have a heat pump), which I also know isn't ideal for sound. Any advice any of you may have would be greatly appreciated.

Hi, if I’m adding duct 10” to run to the attic in 3 ton down flow furnace when all ducts are in the crawl space that the opening will be exactly under the coil but very close to it at the base can ( I cannot / have no space to have 24” inch run/space and than to cut the opening) so the opening again will be very close 1) how much cfm will be in that 10” duct metal round will it be around 300 cfm 2) also do I need to insert like 45 elbow inside the can base to direct the flow to that duct or just open that base can and 90 elbow from the base can will be enough