What you are describing sounds like a make up air interlock. Your house was probably built just after code required furnaces to bring fresh air in because houses are so air tight now. This helps with air pressure issues.
The easiest way to tell if you have a standard eff. furnace is the exhaust line is steel and exhausts out the roofline. You will have a chimney chase that may also be tied into the hot water heating system.
New construction Mid/high eff units exhaust through tubing out tbe side of the house typically. The exhaust is warm, but not hot. Much better heat exchange process.
Mid is no longer allowed to be sold in residential.
At 1998 you are mid most likely. (99% likleyhood.)
