Acid will corrode and damage lime and cement based mortars as well as the brick work
When any fuel is burnt there are flue gasses produced. These gasses are hottest neat the flame source and rapidly cool down as they enter the chimney. If the gasses cool down too much and the temperature falls below the dew point the constituents of the gas will condense on the chimney or liner wall.
The problem with this is that all fuel gases are acidic due to the Nitrogen Oxide and Sulphur Dioxide produced during combustion. Domestic house coal and particularly smokeless coal is high in sulphates and when the flue gasses condense sulphuric acid is formed. Leading to a compromise of structural integrity and a very expensive repair bill.
With a stainless steel liner the acid causes rapid corrosion with a reduction of life expectancy to as low as 3 years in some cases. Replacing a stainless steel liner is a costly business. “Ah, but I’ve got a super efficient purpose designed stove so I’ll be OK” I hear you say.
Well whilst a high efficiency stove is great for putting heat into the room, it’s true to say the more heat going into the room the less heat is going up the chimney and this increases the risk of acid build up within the chimney as the flue gas is cooler and more likely to condense out.
In terms of prevention the chimney can be insulated and this is best done at the installation stage if possible. Double skin insulated flue pipes, pumice liners, insulated flexible flue liners, using vermiculite all help prevent problems. Contact your local stove stockists for advice. For an existing fireplace avoid burning any wet fuel as this increases the likelihood of acid condensate.
Fuels low in sulphur have reduced risk but you must follow any regulations regarding the burning of smokeless fuels. However having your chimney swept regularly removes the buildup of acidic soot and this helps look after your investment. A really good time to have a chimney swept is just after the last time you use it at the end of spring. This leaves the chimney clean and free of acid during the summer period and extends its life.