> It's not supposed to be sealed, it's just supposed to have draft.
It depends on the model. High (>90%) efficiency, condensing furnaces are completely sealed off from the house: they take in air from the outside (via PVC pipe), combust, extract the heat, and exhaust the results (via PVC pipes).
Older lower efficiency (<85-90%) furnaces suck in air from the house, exhaust into metal pipes (PVC would melt because not as much heat is extracted from the fumes), can be back-drafted and you have to worry about CO.
Basically: if the furnace has vents in which you can see a pilot light, it's the older style.
