i am working on a project and never delt with heating ducts. my project is finishing a garage. i am doing this for my brother-in-law. he wants all of the ducts framed in and hidden w/ drywall. he also wants to put a hole into the duct and have a vent to heat the room. sounds straight forward, but is it a good idea to seal it all off, and another question is will the vent affect the pressure of the duct in the rest of the house?

any help is apreciated. im going to post a picture soon for more clearity.