If this is a standard efficiency furnace (approx. 80%) then the furnace uses the air in the basement for combustion. There may also be leaks in the return duct work. These leaks will draw air from the basement when ever the furnace is operating.

You build a furnace room with a vent to bring in fresh air from the outside. Inspect and seal all the exposed return duct work in the basement.

Brother says the intake for the furnace is outside, so I guess that means there must be leaks in the return duct work (?). I'm just glad to hear there's a potential remedy for the problem.

Thanks for the reply!

Does your HVAC System have a return located in the basement?