Ducted heat pump static pressure, and noise

a) I am looking to measure the static pressure loss across my slim ducted heat pump system. It has one central return with a bend and two supply trunks with takeoffs (see attached sketches)
b) I’m also trying to reduce the noise through the central return (it has a 12” by 24” sized filter, 2” thick), it’s a bit loud.
a) At which points do I take the static pressure readings to see the drop across the system if I have a central return?
b) What strategies could I pursue to reduce the noise at the central return?
Thanks!
-Luke
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Replies
Insert the probes right before and after the air handler. If the measurements tell you to drop the static pressure, that might reduce the noise a bit. If not, there's some duct insulation you could try inside the wall cavity of the return. Otherwise, this type of setup is more noisy.
Thanks. Do you think making the filter opening larger—from, say, 12 by 24 to 12 by 36''—would make things quieter?
You measure between B & C. Make sure you use the right type of guage, the common internet special units have a 3PSI full range and simply don't have enough resolution to measure properly. One of the Dwyer Magnehelic ones rated for about 1" WG works.
As for noise, you can put a length of duct liner (heavy fiberglass with a facer) or duct board in the riser facing the filter on the bottom and another piece on the side facing away from the blower on the top.
Thanks, is this sketch describing what you're saying re: duct board? (You're saying the duct board should be on the inside walls of the riser, right? Based on what you're saying, they sound to me like both pieces are on the same inner face of the riser: the part opposite the filter opening, right? And just small pieces, like 2 ft max, not the entire length of the riser? Or the entire length)?
Also, do you think making the filter opening larger—from, say, 12 by 24 to 12 by 36''—would make things quieter?
Would love your thoughts, thanks!
Yes. It could likely reduce the noise if you make the return air opening and filter larger.
If you remove your filter is it still noisy?
The bottom is fine. The top one should be on the other side of the duct, Essentially you want the sound from the blowers to hit the insulation instead of hard pipe.
If your riser is large enough, you can have the duct board run full length on both sides. The full length is the quietest. This two piece approach is good when you are airflow limited as it doesn't block as much cross section.
The filter size is mostly about losses. If one you have there has low enough pressure drop, I would not make it bigger. Won't change sound enough to make a difference.