A house chimney of an old building is one with brick-built chimneys without inner tube. Their task was essentially to dissipate flue gases into the atmosphere. Since modern fireplaces have a very low exhaust gas temperature, masonry chimneys are not suitable for their connection. The operation of modern heating systems on old domestic chimneys would have risk of sooting, because the chimney cross section for the low exhaust gas temperatures is usually too large and the acidic condensates in the exhaust would attack the chimney tiles.
In order to avoid such a destruction of this kind is advised to refurbish the chimney by drawing in an inner tube.