The amount of combustion air today is very limited due our attempt to reduce our fuel consumption. We are replacing our windows, doors, weather stripping, caulking and adding insulation. When I started in this trade there were no double pane windows in the basements much less steel insulated doors in the basement leading to the outside. There were old single pane glass windows and old wooden doors which, a lot of times you could see daylight around the door frame.
Today we have added thermopane windows in the basement, new steel doors complete with magnetic seals. We even insulate the exterior perimeter between the floor joists. All these ideas are good, but they steal much needed combustion air required for a safe combustion process. If your heating appliance is using indoor air for combustion you must be aware of the amount of combustion air that is required.