Fireplace flues are intended to provide the inflow of oxygen to aid combustion and the outflow of fumes and gases. Both of these objectives are best accomplished by a flue that rises straight up from the fireplace. When the realities of your home's architecture require building a bend into your flue, attempt to avoid any slope, or offset, of 45 degrees or greater.
Efficient Exhaust of Byproducts
The hotter the temperature inside the flue, the more efficiently gases will be conducted up and out of the house. Therefore, fireplace flues need to be constructed to maximize internal temperatures. Stainless steel, an excellent heat conductor, is a common choice for flues and flue liners. Clay and concrete are alternative options. Flue design also has an impact on flue efficiency. Bends, or offsets, in the flue can retard the steady outflow of gases. They also interfere with the flow of oxygen through the chimney to the fire.
Flue Bend Disadvantages
Bends in the fireplace flue not only impede the inflow of oxygen and outflow of byproducts, they also can make fireplaces more difficult to clean, as soot and creosote can collect at the offset points. It is also easier for a maintenance person, or chimney sweep, to see into the flue and down to the fireplace to identify problems, if the flue boasts no bends. Further, it is more costly to install a flue that includes slopes, as building the offsets into the design takes time and expertise.
Offsets built into a flue should be as shallow -- and as few -- as possible. Angles of up to 30 degrees are easier to accommodate than angles of 45 degrees. If you can meet your offset requirements by building two slopes of 15 degrees each, so much the better.
Chimney manufacturers typically publish offset charts to give installers complete information on how much pipe is needed to construct an offset of 15 or 30 degrees. Variables include the diameter of the flue and the distance between the slopes you intend to create. You can compensate for creating offsets by increasing the diameter of the flue. The downside to this technique is that a wider flue leads to a slower flow of gases from the fire.