Flue systems and chimney systems are designed to evacuate combustion exhaust from furnaces, boilers, and other fuel-burning devices. They are also used with gas stove vents, pellet stove vents, and other heating systems. Flues are ducts or pipes that use buoyancy or the stack effect, sometimes in conjunction with a blower, to exhaust noxious gases or fumes. Flues for furnaces are called vents, while flues for boilers are called breeching. Chimneys and chimney systems contain one or more flues. These vertical structures are usually made of brick or other refractory materials, and are designed to vent hot flue gases to the outside atmosphere. Specialty flue systems and chimney systems for specific applications are also available. There are many types of flue systems and chimney systems. Products include chimney liners and chimney caps; stove pipes, black stove pipes, double-wall pipes, and triple-wall pipes; and pellet vents and direct vents. Chimney liners are made of clay tiles or stainless steel. In addition to metal flue liners, cast-in-place liners that are made of poured concrete are available. Chimney caps are made of metal or masonry, and differ in terms of shape, size, and style. Metal chimney caps are made from steel, stainless steel, copper or aluminum. They usually attach to the flue with screws. Draft-increasing caps, chimney caps with integral damper mechanisms, and custom caps for flue systems and chimney systems are also available. Stove pipes for flue systems and chimney systems may be equipped with adaptors, collars, connectors, dampers, elbows, flue stoppers, pipe increasers, plugs, and reducers. They are available in black or blue, and are sometimes galvanized. Corn stove pipes, direct vent pipes, gas vent pipes, insulated stove pipes, and pellet stove pipes are also available. Corn stoves use whole-kernel shelled corn or other types of biomass as fuel. Pellet stoves burn wood pellets to provide a consistent heat while producing much less ash than traditional fireplaces. Heating systems that use natural gas also require flue systems and chimney systems. Often, stainless steel flue systems are equipped with condensation traps, cowls, draught stabilizers, join clips, increasers, adjustable pipe, and other accessories.