Often times your furnace and or water heater could be venting the exhaust out through your chimney. Making sure that the liner is intact at all connections and in good condition, along with having the proper cap installed at the top of the liner, will prevent any possibility of a negative air flow to allow the dangerous exhaust fumes to filter back into your home.

The furnace flue liner is also a great home to many critters. Small rodents and birds love to make their nests in the clay flue liner that hosts your furnace or water heater liner because of the warmth and safety it provides in the winter. This can obstruct flow, allow the critter to damage your liner, or even allow the critter access into your home.