Chimneys Explained

NB: The diagram below dissects a masonry chimney with one flue. A chimney may in fact contain more than one flue, and its type is dictated by the heat-producing appliance required. To open up an old chimney, the flue must be cleaned and inspected by a professional chimneysweep, and possibly relined to meet regulations.

1. Flue lining: An approved fire-resistant material which lines the inside of the FLUE, usually made of refractory concrete or impervious clay, but sometimes metal. All chimneys have to be built with a flue lining to protect the masonry from combustion gases, of which the lining also improves the flow. It was not uncommon for old flues to be pargeted (lined) with lime mortar as the chimney was erected, but many were not lined at all. There are two types of liner: Class 1 (solid fuel) and Class 2 (gas).

2. Flue: A vertical pipe or duct that provides a safe pathway for heat, smoke and other combustion byproducts away from the fireplace. Lies within the interior of the chimney. Flues must be high enough to ensure sufficient draught — around 4.5m in most cases.

3. Flue connector: Connects the fireplace to the FLUE. Bends shouldn’t exceed 45°, to enable them to be swept clean.

4. Smoke chamber: The space directly above the DAMPER, where the smoke ‘gathers’ before passing into the FLUE.

5. Combustion air inlet: The fire must be supplied with air from outside the home in order to safely burn fuel; this inlet controls the quantity of the air supplied for combustion.

6. Hearth: A base that isolates a heat-producing appliance from people and combustible items. The hearth’s thickness is dependent upon the appliance.

7. Firebrick: Laid masonry of refractory brick forming the rear and side walls of a fireplace. Refractory bricks are made of a ceramic material built primarily to withstand high temperatures.

8. Gather: A narrow opening between the outlet of the fireplace and FLUE, over which a DAMPER is usually situated, to improve draught and reduce pressure in the SMOKE CHAMBER.

9. Smoke shelf: A horizontal surface directly behind the DAMPER of a fireplace to prevent downdraughts. It also helps the chimney draw the smoke up into the FLUE.

10. Damper: Also called a ‘throat’. A pivoted or sliding metal flap in the FLUE that regulates the amount of draught, preventing excessive variations. It can also close off the fireplace from the outside of the house, preventing air loss when the fireplace is not in use. Sometimes it is located on the appliance.

Further reading:

• Open Fireplaces Explained
• Rainwater Goods Explained
• Showers Explained