I’m working on a single room recording space with an already installed minisplit. Because the room is sealed very tight, ventilation is required. I already have an ERV installed and some silencers in place. My target is 50-100CFM at about 100FPM to minimize noise. This in the NW USA.

The attached drawing shows the basic system layout. The main ducts in the room are built within joist cavities lined with duct liner. Dimensions about 23″ x 6″. On the supply side, the duct would run about 8′ with registers dumping air right over the minisplit intake.  The drawing shows two supply registers but I will just use one here.

On the return side, it’s a similar situation. I am still working on these ducts and the register placement. Here are my questions.

I came late to this concept of the “2 foot rule” which says the last register should be placed about 12-24in before the end of the duct. My intention was to place registers at the very end, especially on the supply side where the minisplit cannot be moved. I could pull it back 15in or so but am wondering if this would really be worth doing. Lengthening the duct would also be problematic.  

On the return side, it’s a similar situation although I have more leeway. I had considered placing two registers there to help distribute return flows of stale air. Do you think it’s worth using multiple registers vs one larger one? And the same question applies to leaving space at the end of the duct or just going as far into the room as possible.

I think the registers at the end would mainly impact the flow rate and add additional pressure, but am not totally sure.  As you can see in the drawing, the path is fairly short but the four silencers definitely add a fair amount of static pressure to the system.

Thanks.