A flue is a pipe, duct or opening in a chimney which removes exhaust gases from a heating component such as a fireplace, furnace, boiler, water heater or generator to the outside. Also known as vents, they operate when the combustion products induce their use with a blower. These products contain carbon monoxide, so proper draft or removal of these dangerous gases is important. Building codes regulate the materials use to build one, how it is designed and how to install it.
Oftentimes flues can be adjustable and are specifically designed to release dangerous gases into the atmosphere. They can however release household heat to the atmosphere if they are not properly set. Fireplaces are notorious for wasting energy when the flue isn’t being used properly. This happens when the flue itself is left open too wide after a fire is started in the home. A flue should be open all the way after a fire is first started and adjusted as the fire burns. In some cases, a flue will use a second combustion chamber where these combustibles are burnt a second time, which reduces the amount of emissions and soot and makes it overall more efficient.
Furnace and water heater flues are most often double wall metal pipes with spark arrestor flue caps and terminate above the roof. Some high efficiency furnaces recycle heat to a point that their flue gas is cooler and they can be equipped with CPVC flues. Tankless water heaters, depending on efficiency have stainless steel, CPVC or galvanized flues.