My home is about 1500 sq ft with 3 bedrooms. The doors are undercut only about a 1/2". I am going to install some return air pathways above the doors to improve the circulation. I am looking at the sound dampening ones made by Tamarack. I am just debating between the 12"x4" or 12"x6" rough opening sizes.

Each room is supplied with a single vent about 5.25"x9.5" opening. So the 12"x4" is about the same area but I was thinking since it has the baffle it might be better to go up to the 12"x6". Per Tamarack, the airflow @ 2.5 pascals is 48 for the 4" and 69 for the 6". Thanks for the help.

 

Will have to do a heat load on the home to know what size u need

 

Without sounding like an ass, I'm not really trying to get technical here. My home has been without the return vents since I bought it about 5 years ago so it isn't anything critical. Just want to increase the circulation. Think I will probably just go with the 12x6 and be done with it. Don't think it can hurt to go a little larger.

 

A good rule of thumb is that return air ducts should be 1-1/4 to 1-1/2 times the area of the supply duct. This holds true from individual rooms to main supply and return ducts. It never hurts to go bigger on return ducts except maybe in the wallet.

Since you had nothing before any return air opening will help.

 

You probably only have a 6" pipe feeding this room which would not let more than 110 CFM on the average into the room, so your 12x 4 " would work fine.
You do have some under the door so your room can pressurize a little and let the right amount of air out of room. Paul