Heat rises, and so does the hot air, so you need to make sure there is a source to resupply the room with air or the negative pressure will occure in the room when the heat (and air) go up the stack, The room then will actually be at a lower pressure then atmospheric, so it will want to pull the air into the room from the “easiest source”, so if you don’t have a door or window cracked, it pulls air from outside down the stack, and you get the downdraft. Opening a window will keep the room pressure equalized and hot gases going up the stack. Comming from a guy who burns lots of stuff for a living .