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Class 1 Flue
Older houses usually have a Class 1 solid-fuel flue (identifiable by the ‘chairbrick’ which has a minimum 7″ diameter and generally terminates with a brick or stone built chimney.
Class 2 Flue
Newer homes may also have a Class 2 flue with a 5″ minimum diameter that terminates with a cowl. The most common is a gas cowl as illustrated.
A more modern home may have a pre-cast flue (usually identifiable from the starter block or letter-box opening) that terminates with a ridge tile, as illustrated.
A fan flue alternative is available for new homes built without a chimney or older properties where the flue is no longer of sound integrity. All that is required is a suitable outside wall.