Chimneys which may look and seem perfectly sound on the outside can exhibit extensive damage on the inside. A chimney must be structurally sound both inside and out in order to meet code standards for operation. Chimneys are made of different kinds of materials, whether brick, stone, or other construction material, and are used to vent wood burning fireplaces and inserts, gas appliances, or even furnaces. Chimney flues generally are comprised of clay flue tiles, stacked one on top of the other, which are able to stand high temperature fluctuations and safely vent the corrosive byproducts of combustion.
Cracks in the flue tiles of a chimney liner can be caused by many occurrences. Chimney fires and the shifting and settling of the house into its foundation are two main causes. Only call out chimney professionals who run a video inspection of your flue system after sweeping. Use of a closed circuit camera system is often the only way to see damage in the flue system. If cracks exist in the flue tiles then the chimney must not be used. A chimney liner with cracks in it should be resurfaced, or in cases of very bad damage, a stainless steel chimney liner pipe should be installed.
Slip casting is one way professionals describe the resurfacing of a chimney liner system. If a chimney liner has cracked over time, applying a new surface onto the old flue tiles after they have been cleaned will extend the life of your existing liner. The product is applied in the flue system and a large, foam head is dragged up from the bottom of the chimney to the top, smoothing it over any cracks or missing mortar joints and once again making the flue system safe to contain products of combustion after the application of a “tie coat” or a primer. The product I recommend using is HeatShield, which is a cerfractory material: a mixture of ceramic and high-temperature refractory cement, which combines strength with heat resistance.
HeatShield carries a 20 year warranty when installed by a professional and has been UL tested to withstand temperature up to 2900 degrees Fahrenheit. This material is pushed into open voids and smoothed over completely so that no soot will be prone to get caught on ridge in the system. This resurfacing of the flue is a cheaper alternative to a stainless steel liner and should be discussed with a professional as not all cases should be repaired using HeatShield technology.