How are they built, what are they comprised of and just how can you make sure your chimney works to its full potential? Natasha Brinsmead explains

The History

Although holes in the roof and walls of ancient homes is evidence that our ancestors knew it was important to get smoke out of their homes, chimneys were not really widely adopted until the Tudor times, and even then, only by the upper classes, with more common folk having to put up with smoke-filled rooms.

Even when chimneys were used, they were highly inefficient and often dangerous too, being made of wattle and daub and susceptible to fire. By 1710, all clay-built chimneys in England were ordered to be rebuilt in brick.

However, a poor understanding of heat and smoke meant that chimneys were still not drawing in the way they should, and it was not really until the mid-18th century when the likes of Benjamin Franklin (once called ‘The Universal Smoke Doctor’), Charles Willson Peale and Benjamin Thompson – later known as Count Rumford – got involved that things improved. Count Rumford invented a fireplace and chimney designed to reduce the smoke pollution in London. The new Rumford fireplace reflected heat back into the roof, with the chimney being incorporated into the wall.

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Today, powerful chimney stacks are back in vogue, as on this new Hall + Bednarczyk Architects barn-style home

The Flue

The terms ‘flue’ and ‘chimney’ are often confused. The flue is the working section of the chimney which takes the products of combustion up and out into the atmosphere, while the chimney is a structure built around a core of clay or concrete flue liners, terminating with a pot.

Chimneys are categorised into ‘classes’.
A Class 1 chimney is common in houses built up until the 1960s. They consist of a brick-built stack, situated on either an internal or external wall and containing multiple flues for multiple fires (although the fires cannot share a flue). This type of chimney can be used with all types of solid fuel fires and stoves, and gas fires too. They will need lining only if they fail a smoke test.

Class 2 (5” diameter) gas flue systems are often found in houses built from the 1960s onwards. They consist of an interlocking metal pipe running through the house, but can be used with certain types of gas fire only. Class 2 (pre-cast) systems consist of a rectangular hollow cavity made from concrete or clay blocks, travelling up the wall cavity to a ridge vent or metal flue terminal on the roof. They can be used with slimline gas fires.

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Most chimneys are built from blockwork and can be clad in a variety of materials — usually stone or brickwork

The Materials

Both chimneys and flues are available in a variety of materials, including stainless steel, concrete, pumice, clay or ceramic, and plastic. Concrete, pumice and clay or ceramic are collectively known as ‘masonry chimneys’. Plastic flues can only be used with low-temperature condensing appliances, and some stainless steel chimney systems and liners are designed only for use with gas-fired appliances. Clay and pumice chimney systems however are suitable for use with wood, multi-fuel, oil and gas appliances.

Factory-produced pumice, clay and ceramic chimney systems can be retrofitted, but they tend to be reserved for new builds as they require foundations and their construction is really best left to a skilled bricklayer.

The Build

Chimneys require foundations while steel or ceramic flues don’t. If you want a traditional pot then you need a chimney, as flues have terminals. While flues work with any construction type, chimneys work best with blockwork.

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The Regulations

The construction and use of chimneys and flues is covered by Building Regulations — and Building Control approval is necessary for building new chimneys and, in some cases, relining existing chimneys, particularly if you are planning on changing the heat appliance.

For a chimney to work well, it requires a good flow of air and for the flue to maintain as high a temperature as possible. It is important (although not something you have to do) for chimneys to be insulated, as this keeps the smoke warm and lessens the chances of it condensing as tar deposits — this is particularly important with woodburning appliances, as they burn wood at a cooler temperature than coal.

It is also essential to have a flue diameter to match the output from whichever type of fire you have along with an adequate air supply.

Chimney Terms Explained

Flue: The void through which the products of combustion are removed into the atmosphere

Flue liner: The material used to form the flue within the chimney

Flue pipe: A metal pipe used to connect an appliance to the flue

Chimney: The structure surrounding one or more flues

Chimney terminal: Another word for pot, cowl or other method of finishing off the top of the chimney

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