Q: I have sometimes noticed that the stones on the outside of the end wall of a house are blackened in a way which quite clearly marks the course of a chimney inside the wall. Surely this cannot be caused by smoke diffusing through stone, so what does cause it?
A: The blackening of the external wall of domestic chimneys is most commonly caused by water penetration due to condensation collecting on the inside flue.
This can happen when a gas fire is fitted to replace an original open coal fire. Because the gas fire is much more efficient than the coal fire there is, for a given heat output, a marked reduction in both the volume (and hence the velocity) of the combustion gases which pass up the flue and the amount of heat they carry with them.
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Therefore, the walls of the flue do not heat up as they were designed to do. Burning gas (most of it methane) produces more water vapour than coal does. Some of the water vapour in the slow-moving gases condenses in the cool flue and the acidic water soaks into the cement and brickwork.
The problem is heightened because gas fires are often used only intermittently and for short periods and the flue cannot heat up enough to evaporate the condensation.
Serious structural damage can result from this. The solution is to install an insulated chimney liner which is resistant to chemicals. This will produce a narrower flue which is better matched to the needs of the new heating appliance.
A: If the questioner scratches at the stained mortar, he will find that it is softer than the rest. This is a phenomenon seen in pre-1965 houses where the chimneys were lined with a cement skin, which is alkaline. The acidic constituents of smoke and soot deposited in the chimney transform the alkaline particles of cement into a softer, crumbly material, staining it in the process. This weakening process is demonstrated where there is a prevailing wind that is cooling one side of the chimney, and causing more condensation on that side. The acid attack expands the mortar as it reacts. So the chimney develops a marked curve with the windward side outwards.
This damage should not occur in modern chimneys if they are built correctly, thanks to the terracotta liners required by law since 1965.