Flues, Draughts, and Liners

Any modifications to existing chimney systems, including installations of flue systems and flue liners, are subject to local Building Regulations, as well as any legal requirements. At Lee Davies Fireplace and Brickwork Specialists, we are fully qualified and registered to undertake all the necessary work involved with these installations and modifications. We are fully HETAS Registered, and work according to all the necessary guidelines stipulated by Building Regulations. We expertly carry out installations and modifications to chimney and flue systems throughout Buckinghamshire, Oxfordshire, Hertfordshire, Berkshire, Bedfordshire, and London.

One of the most important elements in ensuring the efficient functioning of your stove is the ability of the flue draught draw air into the stove’s combustion chamber and disperse the harmful air generated in the chamber into the atmosphere. A standard wood burning stove requires approximately 15m³ to 25 m³ of combustion air per hour, while an average home in the UK has 40m³ to 50m³ of air available. In order to ensure adequate ventilation, as well as the optimal functioning of the updraught system, when we install your flue system, we will adhere to the strictest Building Regulations guidelines in our services.

The updraught results from the difference in temperatures, densities, and pressures between the flue gases produced by your stove and the outside air. The difference in pressure results in the lighter, warmer air from your stove to be drawn out into the open air. A taller flue will result in a better draught, however the longer the gas stays at a high temperature, the better the effect of the draught will be. As a result, a taller flue may not always be the optimal choice, as the flue gas will lose heat as it rises through the flume, diminishing the effect of the updraught.

Chimneys installed prior to 1965 will have a rougher inner surface, and may likely have bent or warped at places, both of which may be a factor in the disrupted natural flow and draught of the flue gases, causing them to slow and cool as they travel up and exit the chimney. Post-1969 chimneys usually consist of clay or concrete liners with a internal diameter of between 200mm and 225mm, which is not ideal in the efficient functioning of a wood burning or multi-fuel stove. This is due to the fact that the joints between the liner sectors may be incomplete, or to the entire system being installed upside down.

In order to ensure the performance of your stove is fully optimised, any potential leak or cooling of flue gases in your chimney system should be identified and repaired. These leaks could be the result of the continuous heating and cooling cycles, chemical reactions, condensation of flue gases, and weather damage to the external stack.

It is important to determine whether your previously installed open fire chimney will be compatible with a stove system, as flue gases pass through the stove at a higher temperature than the gases from open fires. These higher temperatures could potentially compromise the performance and safety of the chimney system by, for example, expanding the old brick and mortar seals and leading to the development of a potential leak. It is important to remember that chimney and flue systems are subjected to prolonged, and intense heating and cooling cycles. With these cycles, excessive amounts of condensation as well as harmful chemical reactions, can occur.

Solid stainless steel flue liners have the ability to drastically improve the performance of your wood burning stove or multi-fuel stove, as it will remove any problems typically associated with chimney systems. For this reason, fitting a flue liner is a preferred option at Lee Davies Fireplace and Brickwork Specialists.
Flue liners also provide a large amount of insulation. This added insulation creates a better updraught by maintaining the high temperature of the flue gas, particularly where the chimney is located in an area with a chill wind. A flue liner can also aid in protecting the wall coverings and paint finishers from any damage caused by condensates.

Flue liners are also fundamental in the installations of boiler stoves, as they require a much lower temperature to function properly when compared to wood burning stoves, or multi-fuel stoves. Boiler stoves warm water by drawing it through the fire chamber, while the gases from the flue system enter the chimney at a cooler temperature than the gases from a stove with no boiler.

The cooler flue gases results in a less effective updraught, as the already cooler gas cools down further as it travels up the flume. These cool gases result in smoke which may cause creosote problems with an increased number of soot deposits, both of which require more frequent chimney sweeping to prevent blockages and chimney fires.

  • The current chimney is allowing potentially harmful fumes or smoke to enter the home.
  • The void size of the chimney is not in line with the stove manufacturer’s flue outlet specifications and needs to be reduced by installing a flexi liner.
  • The current installed concrete or clay liner has joint seals of an inferior quality, or has been installed upside down. This would result in any condensates running outside the liner instead of inside, resulting in staining.
  • The chimney is operating at a colder temperature, resulting from a large void or being located where it would be subjected to strong prevailing winds. This results in difficulty creating the necessary updraught to allow the stove to burn properly

When taking the above into consideration, it becomes easy to see why, at Lee Davies Fireplace and Brickwork Specialists, we believe that a flue lined chimney is superior to a non-lined chimney. Our team of specialists are able to install a chimney lining that will leave your stove or fireplace functioning efficiently and safely.

In some cases, it may not be possible to fit the general 150mm liner because of tight bends or other restrictions, and a smaller, 125mm liner will need to be fitted instead. It is, however, important to know and understand the legal requirements of the Building Regulations, as well as the requirements of the stove, before installing a flue liner.

The correct sizes of connecting flue pipes are carefully determined by the stove manufacturers to ensure that the stove and flue system works safely and efficiently. Because of this, care should be taken to ensure that the flue liner is no smaller than the flue pipe aperture. For example, if the size of the flue outlet is 125mm on a 5kW output stove, the liner should be 150mm in order to allow for the burning of fuels like coal, or for the potential replacement of the current stove with a larger one in the future. Some manufacturers recommend the installation of a flue line that is the same size as the aperture, rather than a larger one. It is important to note, however, that the liner should never be smaller than the diameter of the stove’s flue aperture, or of the diameter specified by the manufacturer.

In the UK, Building Regulations Document J states that flues with a diameter of 125mm should only be used along with smokeless burning appliances that have been DEFRA Approved under the Clean Air Act of 1993. Flues of 150mm should be used with appliances that burn other kinds of fuel, such as wood, peat, and bituminous coal. The document also states that for multi-fuel appliances, such as multi-fuel stoves, the flue should be of an adequate size to accommodate the fuel that, when burnt, requires the largest flue system. This is due to the fact that volatile fuels, such as bituminous coal, result in quick forming deposits attaching to the inside of the flue during burning, which could reduce the available diameter of the flue over time.

Ultimately, this means that if you want to burn wood while complying with Building Regulations, only a DEFRA Approved stove should be used. Additionally, when you are looking to burn multi-fuels in these stoves, only clean burning smokeless fuels should be used. When using a non-DEFRA Approved stove, no wood should be burnt, and only approved, clean burning smokeless fuels should be used.

If you require a 125mm liner, and would like to use a stove with an output of greater than 5kW to burn wood, you will only be allowed to use a stove with a higher output that features a 125mm, as a larger stove usually means a 150mm flue outlet. Unfortunately, this will restrict the choices available to you, since there are limited DEFRA Approved stoves with 125mm flue outlets that also have a larger heat output. This is also true if you are looking to use a higher output stove to burn multiple types of fuel, only approved smokeless fuels should be burnt in these stoves, and in a stove with a 125mm flue outlet.

Despite this difficulty, Lee Davies Fireplace and Brickwork Specialists will be able to source stoves, such as the Broseley Serrano 7, that meet these requirements. We will also be able to expertly install these stoves, flue systems, and flue liners.

Liner Diameter

Proposed Fuel

Appliance / Fuel Requirement

125mmWoodDefra Approved 5kW Stove with 125mm flue outlet burning wood with less than 20% moisture content (or approved clean burning smokeless fuels)
125mmMulti FuelMulti Fuel 5kW Stove with 125mm flue outlet only burning Approved Smokeless Fuels
150mmWood / Multi FuelAny non-Defra or Defra wood burner or multi fuel stove with a flue outlet size of 125mm to 150mm Subject UK Smoke Control Area regulations


When any type of fuel is burnt, condensation and water vapour is created. In order to ensure that the water vapour remains in its gaseous state until it exits the flue system and prevent long term damage, the optimal functioning of the flue system and the maintenance of the gases from the stove is highly important. However, if the vapour is allowed to cool excessively, moisture will form on the interior of the flue system which may cause more damage by allowing the gases to cool down further.

A combination of sulphur compounds, originating from the flue gases and sulphates in the brickwork and the burning of wet and unseasoned wood, may unfortunately produce an acidic moisture which can, over time, erode the brickwork and cement joints. In the worst cases, this moisture can even seep through the brickwork. An approved and vermiculite backfill insulated acid resistant fuel liner will effectively reduce the heat loss that causes condensates and moisture to form, limiting any damage to your chimney and flue.

This kind of damage can be avoided, however, by taking certain steps to ensure you get the most out of your systems. Keeping the flue gases are as hot as possible by ensuring a decent updraught, burning well seasoned wood, and having a taller, well insulated flue, will minimise heat loss and prevent problematic condensates.

Pipe, clay, concrete, and other liner joints are specifically designed to prevent condensate leakages and let them run down into the flue. However, due to the ability of the joints of these liners to work in reverse to other piping systems, some other installers and builders may install the liners upside down. In order for your flue lining to be operating efficiently, it is important to ensure that all your components, including wall flexible liners, are installed the correct way.