The flue is the interior passageway inside the chimney that allows these byproducts to escape outside.
It is possible for a chimney to have multiple flues, which are separate passageways inside the chimney that can vent different heating systems or fireplaces. This is common in homes that have multiple fireplaces or heating appliances, such as a furnace and a water heater, that require their own flue to vent their byproducts.
Each flue has its own opening at the top of the chimney, which allows the byproducts to escape outside. The flues may be located side-by-side or one on top of the other within the same chimney structure, depending on the design and layout of the building.
It is important to note that each flue must be properly sized and installed to ensure that it is able to handle the volume of gasses produced by the appliance it serves. Additionally, regular maintenance, such as chimney cleaning and inspections, is crucial to ensure that each flue is functioning properly and safely.