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I moved your thread to the "AOP Residential HVAC and Refrigeration (Beenthere Zone)" forum as this is the only forum where homeowners can ask non-technical HVAC/Refrigeration questions, and seek advice.

Please take note of this new location.

 

I've read this several times and still have no idea what you're asking.

Why move the return grill?

 

Remodel?

 

Sorry I have drawn another photo. I want to move the grill to the underside of the bulkhead (arrow in blue). Then I want to reduce the height of bulkhead by 200mm (arrow in yellow). The bulkhead is the plenum I’m referring to. It’s just a box plastered up. The return air duct is connected to the side of that bulkhead.

I need to make the bulkhead smaller because I need to put a door under it. The door won’t fit at the moment. It’s too tall.

 

Return air surface is measured in cubic inches. The capacity of the system will give you an idea of how much return surface is needed. Too much surface is not a bad thing, too little is.

Google AI says :

To calculate return air size, determine your system's tonnage, then multiply it by approximately 400 CFM/ton to get the total airflow demand, and finally, use the rule of thumb of 2 CFM per square inch of filter grille area to size your return. Alternatively, use a Duct Sizing Chart or calculator

Return Air Duct Size Calculator - Calculator Academy

Enter the size of the air conditioner (tons) into the calculator to determine the size of the return air duct.

calculator.academy

 

I don’t see where it’s going to fit with all the structure that needed.