It needs to get combustion air from someplace. Most of the newer homes are built to be tight, so the combustion air source needs to come from outside the house. Some units will have a type of heat exchange unit that tempers the outside air with the exhaust, but that comes at a price; the exhaust gets cooled too much, so you get condensate / acid / sulphar build up in the flue. The type of fuel burnt also plays into this. Gas and/or propain exhaust is generally much cooler, with a much lower sulpher content.
Bummer you need to replace your furnace TAM. When it rains, it pours… A couple weeks ago, I decided to change out an instant hot water dispenser I installed about ten years ago. Had to remove the garbage disposal unit to get to it. After I installed the new dispenser ($350), and then the disposal unit, the friggin disposal unit developed an internal leak. $200 later, the new disposal unit was installed, but then the new hot water dispenser stopped working. Called the warranty / service line… They have to have one of their authorized installers check it out…. He finally shows up, “yep, the unit is bad and needs to be replaced”…. Four weeks later, no progress. And, three weeks ago, the dishwasher crappped out on me…. $900 later, a new Bosch was running. Now I’m just wondering what the hell else is going to crap out on me…..
