We had a similar situation in our master bedroom where there were a total of three vents serving the room, but with the carpet under the door, there was less than an inch of return underneath the door. I alleviated the problem by installing an additional vertical offset air return through the wall.
If return venting is what you need, there are essentially 4 different methods that I know of to do it, but I'm not an HVAC tech and your mileage may vary:
1) Ceiling return in this case, providing a vent in the ceiling ducted back into the main airspace or directly into the HVAC return ducting.
2) Vertical offset wall return. This involves using the wall cavity as a duct, placing one vent high on one side, and low on the other. This provides an extra sound and light barrier between rooms.
3) Straight thru wall or door return. No ducting, just vents on opposite sides of the wall. And probably the most half-assed way you can do it.
4) Under the door venting. An air gap of just an inch or so is adequate for most smaller rooms and would probably be enough for a single vent fed room.
It's sad, but I tried asking about this stuff on a professional HVAC tech forum, and they took the smug attitude of not helping at all and suggesting that only a professional could possibly fix my problem. Apparently they are afraid people will figure out how air flows or something. Anyway, that's my little ***** about that. :P
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