Chimney is a specific piece of a building both in terms of its function (transfer of fuel combustion gases and „used” air) and the fact of being exposed to weather conditions (especially water) all the way around. Therefore it is not surprising that durable and resistant clinker bricks are commonly used for chimneys.
Chimney brick – which one is the most suitable? 1. Technical parameters
Both chimneys laid in a traditional manner (although these are hardly ever used nowadays, replaced by prefabricated chimney systems whose installation is much quicker) and prefabricated chimney enclosures must be made of solid bricks.
This is required for technical reasons. Unlike many people think, it is not about the temperature (furnaces are becoming more and more efficient and the temperature of exhaust gases that enter the flue is relatively low), but about the humidity. At a certain height of the flue exhaust gases are cool enough that water vapour condensate adheres to the flue walls. Combustion of wet fuel (wet coal or wood) intensifies this effect. If the flue was made of perforated brick, the water together with the dissolved soot would get inside the perforated part through microscopic imperfections, at then, it would flow down gravitationally causing stains and damp areas on the walls.
In case of prefabricated chimneys where the bricks are not in direct contact with steam, the requirement to use solid bricks is aimed at keeping the structure tight against precipitation. The situation here is similar to what we have already mentioned in relation to traditional brick flues. If the body of a prefabricated chimney was made of perforated brick, water that flows down the external brick walls and the internal surface of the flue (if the flue is not protected with a cap) would eventually come across some microscopic imperfections and penetrate the perforated part. These problems can be solved by the use of solid bricks, preferably solid, smooth-faced bricks (not grooved) of as low water absorption as possible.
Chimney clinker brick – which one is the most suitable? 2. Visual effect
It is common practice to match the colour of a chimney brick either to the colour of the roofing or to the colour of the façade or a part of it (e.g. base) which is natural when the façade is made of bricks or clinker tiles. To avoid reference to colours of building components you can refer to the colour of other nearby pieces of architecture such as fencing.