help me get positive pressure in a room....

i have done a bunch of research on positive and negative pressures and am hoping someone can point me in a direction to research the topic...

basically, i have a 2700 sq ft house above ground and a big basement and a 5 ton trane heat pump and furnace

the unit works great except in my office... its a large room (maybe 28x25ft) with 3 exterior walls and lots of windows but none of the windows open.... basically, the room also has a fireplace and my house pulls air into the house from around the chimney (not from the chimney itself but from around the insert and the chase box itself)

i am going to have the chimney box insulated in a few spots to help but i also want to try and get rid of some of the negative pressure in the room

i have blocked the return vents in the room

what else can i do? i am looking for permanent solutions, not opening windows or anything in another part of the house

any thoughts? i know it is difficult to diagnose with the little info i have given but if you need more, let me know... would putting more air into the room form my vents work? taking air away from main floor or upstairs?