Flue and Chimney Glossary

Are you baffled by all the flue and chimney terminology? Maybe you are unsure what items are used for or indeed what flue supplies you may need for your installation? Then look no further than our 'Flue Bible' - a Glossary of terms for flue and chimney products with lots of specification, information and helpful advice. Hopefully, this will tell you all that you need to know. However, if you need any further help or advice and are interested in our products for your wood burner or stove please feel free to get in touch. We offer a free flue design service and can assist you in your product selection.


Chimney - This is the structure enabling the ventilation of the combustion products / gases / smoke created by a fuel burning appliance. Chimneys are commonly constructed from brickwork rising from the fireplace up through the house and above the roof. A chimney may be internal or external (on the outside of the house). Houses that do not have an existing brick chimney can be fitted with a factory-made metal chimney system which fit together to form a route for ventilation to occur (see Twinwall Flue below). Note: The terms Chimney and Flue are often used to mean the same thing, however, technically they do have a different meaning (see Flue below).

Flue - The flue is the actual passageway / duct that the smoke and gases travel through. The flue is contained within the Chimney (see Chimney above). Technically the pipe itself is not the flue, the flue or flueway is the hole inside the pipe. In practice many people (including professionals) often use the term flue to refer to the pipework. Twin wall Flue - This is a manufactured system of component metal pipes which are slotted together and used to form a continuous chimney run. This type of chimney flue system can be installed where there is no existing masonry chimney in the position required that can be used. The twinwall flue pipes are made from two layers (twin walled) of stainless steel with an insulating material (such as Rockwool) between the inner and outer layers. It is designed to keep the heat inside the flue, so the outside of the pipe can pass safely through surfaces like walls or floors near (with a minimum gap e.g 60mm) to combustible materials e.g. wood. Different brands of twin wall flue pipes are not compatible with each other due to differences in the exact diameters and profiles at the ends of the pipes. Twin-wall flue pipes can simply push together and a locking band is used to tighten and hold around the joint.

Damper - a movable flap in the flue that can be opened/closed to allow more or less air up the chimney thereby allowing control e.g. to slow the rate of burning down, to keep the stove going over-night etc. Some old stoves had the damper built in on the top of the stove. Modern stoves being much more efficient (i.e. allowing less heat up the flue) do not normally need a damper built in. If you know you have a strong chimney draw, or particularly want to be able to slow the fire down a lot, then you can buy a damper unit to form the first section of flue pipe.

DEFRA - The Department for Environment, Food & Rural Affairs is responsible for, among other things, environmental protection and control, this is a term often mentioned in relation to stoves/Woodburners. Stoves are often described as DEFRA Approved (or DEFRA

Exempt - meaning the same thing) if they have been tested and proved to have low smoke

emissions when burning wood. Most large towns and cities will be Smoke Control Areas - if you live in these areas and want to burn wood your stove must be a DEFRA Approved / Exempted model. When looking for a stove either in a showroom or on the internet they will always clearly state whether or not they are DEFRA Approved. If it doesn't say DEFRA then it is most likely not. Note: a non-DEFRA stove can legally be used in a smoke control area but only if burning smokeless fuel.

Firebox/Flue boxes - The chamber inside a stove where the actual combustion (burning of fuel) takes place.

Fire Cement - A putty-like substance used to seal the joints between vitreous enamel flue pipes and also where the flue pipe connects to the stove. It can be applied by hand and will harden when the stove is first used. Take care to wipe off any excess before it has dried.

Fire Rope - A heat-resistant rope-like material sometimes called ceramic rope. It is used in virtually all stoves to make a seal at various edges/joins in the stove. E.g. most stoves will have a fire rope around the edge of the door to make a seal when it is closed. Most will also use a rope to seal between the glass and the door. Cast iron stoves will have a rope to seal between each individual part of the stove body e.g. sides, base, top etc.

Flue Diameter - Simply the internal diameter of the flue, many of the DEFRA Approved stoves can use a 5" liner right to the top as they have been tested to show they produce very little smoke. Note: many small stoves have a 5" outlet on the stove (so will use 5" vitreous flue pipe) but you must then step up to a 6" flue inside the chimney e.g. if connected to a flexible flue liner or twinwall flue system. Some older stove models (or new but very large models) may still need a 7" flue although these are quite rare now. Note: an open-fires usually needs a minimum of an 8" diameter flue. Larger fires/stoves need larger flues as they simply have a greater volume of flue gases that need to exit the fire. Flue diameters are still typically described in imperial (inches) in the UK even though the other measurements such as height are usually described in metric (m or mm).

Flue Liner / Flexible Liner - A pipe usually made from 2 layers of stainless steel that is used to line or re-line an existing chimney. It comes as a coil cut to your specified length e.g. 8m or 9m typical for a 2-storey house or 5m or 6m for a bungalow. Lining an older chimney will generally improve the performance of the stove and ensure a safe system. It is not a legal requirement to re-line the chimney if the existing chimney is in good safe working order and not leaking, however, we would always recommend it especially on an older property.

Flue Pipe - This usually refers to the pipe that connects the stove to the chimney so also known as Connecting stove Pipe. Usually made from steel and coated with vitreous enamel so can be called Enamel Pipe, Vitreous Pipe or simply Vit. Some connecting pipes are made from stainless steel, although this is not very common in the UK. The Flue Pipe most commonly joins to the top of the stove (via the Collar), and is sealed with Fire Cement, although many stoves also have rear flue outlets, to which a 90° bend or Tee piece is fitted before the flue rises vertically.

HETAS - Heating Appliances, Fuels and Services, HETAS is the official body recognised by Government to approve biomass and solid fuel, including the registration of competent installers and servicing businesses. Primarily they provide training for installers - if your installer is HETAS registered they are allowed to self-certify the installation, so you do not have to go through the local Building Control process. You can find a local HETAS installer here: http://www.hetas.co.uk/find-installer/ .

Insulated Flue - Usually referring to the Twinwall. Insulated Flue Pipes used to form a chimney in a modern house where there is no brick chimney, or in an older property where the original chimney has been removed. Can also refer to the practice of insulating (either with a special blanket wrapping or loose fill material) around a flexible chimney liner. Spigot/Collar - a steel or cast iron ring that bolts to the top or rear outlet of a stove, where the first section of flue pipe sits. The collar is usually a bit larger than the flue pipe so the small gap is sealed with fire cement (and fire rope if required). Note: in the UK the flue pipe should fit INSIDE the collar, however, on some European stoves the collar is the same size or smaller than the flue pipe - in these cases you can fit a Continental/European Adaptor so you can use standard UK size flue pipe. On stoves with both top and rear outlets the outlet not in use is sealed with a Blanking Plate.

Vitreous Enamel pipe - Refers to the most common type of pipe used to connect the stove to the chimney. It is most commonly black so that the vitreous flue pipe matches to the colour of the stove. Other colours are also available. Vitreous enamel is a material made by fusing powdered glass to an underlying substance by firing at high temperatures. The powder melts, flows, and then hardens to a smooth, durable vitreous coating on metal, so in technical terms fired enamelware is an integrated layered composite of glass and metal. This type of flue pipe is supplied in various diameter sizes to suit the size of the appliance it is fitting to.

Register Plate/Debris plate - This is an essential component when installing a stove into a fireplace with traditional masonry chimney above. The register plate is usually made from steel (but can in theory be made from a suitable non-combustible board material). The Register plate will have a hole cut in it through which the flue pipe passes. The Register Plate seals off the chimney from the room so that air can only be drawn directly through the stove and into the flue / chimney. This is important because the draw on the stove is created by the chimney - if there was no register plate air would be "pulled" from the room around the outside of the stove and into the chimney, little air would actually enter the stove so starting and maintaining a fire would be very difficult. The Register plate also serves some secondary purposes too - it helps prevent excess heat loss from the room into the chimney void and also protects the stove and room from any dirt or debris that may fall down the chimney. If you are using a flexible flue liner to re-line the chimney then the primary function of the register plate (to seal off the chimney) is not really significant as all of the draw will be through the flue liner which is then connected directly to the stove flue pipe. However, it is still normal practice to fit a Register plate in this case as it will still serve the secondary purposes of heat retention etc. Also, if you want to insulate the liner e.g. with Vermiculite, the Register plate will be needed to stop the insulation falling straight down into the room. Technically in the case of an installation with a liner the Register plate can be called a

Closure Plate - to most people it means the same thing though and is actually the same item, although used for a different purpose.

Wood burner - A closed heating appliance designed to burn wood logs. Generally described as a stove. Unless stated or described as multi-fuel then it is for wood only, not coal. Wood burning stoves have a flat base (without a grate) on which the logs are burned.

Multifuel - If a stove is described as multi-fuel it means it can safely burn fuels other than wood logs. Typically the other fuels will include natural smokeless coal (e.g. Anthracite) or manufactured coal products, but may also include fuels like peat. Some solid fuels e.g. petroleum based Coke (Pet Coke) or coal designed only for use on open fires should not be used in any stove - it will potentially damage the grate and inner components of the stove. Multifuel stoves always have a raised grate to allow air from below and for the ash to fall through.

Gather Hoods - These are used at the base of a chimney stack and allow the gases from an open fire to gather with minimum resistance to flow.

Cowl/ Terminal - A cowl is a usually hood-shaped covering used to increase the draft of a chimney and prevent backflow. The cowl, usually made of galvanized iron, is fitted to the chimney pot to prevent wind blowing the smoke back down into the room below.

In strong winds, the pressure of the wind may overwhelm the updraft and push the airflow in reverse back down the flue, in these situations you can install a weathering cap or anti- downdraught chimney cowl.

Other cowls may come with bird guards, fans and rota tops.

Flashings - The flashing is sheet metal installed for the purpose of ensuring that the connection between the chimney and roof is watertight to prevent leaks through the roof.

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