Hi Frank,

Calculate the area of the supply duct then cut the same area from the
bottom of the door. for instance if you have a six inch internal
diameter duct feeding your office the supply area would be 28.26
square inches so if your 30" door has an inch gap above finish floor
you should have enough return area. an eight inch diameter duct would
require about 1-3/4" above finished floor level. If you cannot get
enough by doing that put a return grill in your door. a lot simpler
than putting in a return duct.

I'm not sure if it is 6" or 8", but under my door between carpet and door
bottom is about 1/2". I would say that is probably not enough of a return
duct. This room is my office and it does have some equipment running in it
to keep it warmer (computer, monitor, printer, etc.), but I have to keep
noise out so I can't put a vent in the door or cut it shorter. The temp in
the rest of the house is about 77 deg and my office is running 83 deg now.

I could have the company who installed my system (house is about 3 years
old) put in another vent or an air return and/or I would still consider
doing it myself if it was hard work. The area in question is pretty easily
accessible.

Do you think I need a return vent, or an additional vent, or both?

Thanks,

Alan