Lining Your Flue
Building regulations state that all homes built with chimneys after 1964 should have a concrete or clay inner liner inside the chimney. As long as such a lining exists then you should not need to install a new liner when installing a wood burning stove or multi-fuel burning stove. Most homes built before 1964 were not built with lined chimneys. Whilst it is not a legal requirement to have a flue lining fitted when installing a wood burning stove or multi-fuel burning stove, it is definitely recommended. Find out more about why you should line your chimney and what it entails below.
Stoves are designed to work with a liner of a specific diameter, most commonly 6 inch diameter but sometimes 5 or 7 inch depending on the stove. They are not designed to work with a standard brick flue which is usually a 9 x 9 inch square or 12 x 12 inch square – both much bigger than a 6 inch diameter flue. Therefore lining the flue is essential to make sure the stove functions properly. Not lining a flue and therefore using a standard brick flue can result in a much more sluggish burn, lower heat output and more dangerous sooty and tar build up inside the flue.
Flue Design & Efficiency
The ‘flue’ is the cavity inside the chimney that runs from just above the fireplace right through the chimney up to it’s own dedicated chimney pot. Flues are usually square, sometimes rectangular and unlined, sizes ranges from 9 x 9 inches wide to 12 x 12 inches wide. These are not random sizes, flues are designed to provide an appropriate escape route for the emissions from the fireplace they service. The flue walls can be bare brick or they can be rendered with a thin layer of lime mortar.
Most wood burning stoves or multi-fuel burning stoves are designed to work with a 5 inch or 6 inch diameter flue. DEFRA exempt stoves, for use in smoke control zones, usually require a 5 inch liner. There are some stoves that require a 7 inch diameter flue (usually the much bigger stoves). The manufacturer will specify the flue size required for a stove and they will also specify the expected heat output and efficiency of the stove. These expectations are based on the stove being fitted to an appropriately sized flue.
This means that most standard brick flues are much bigger than stoves are designed for and a flue-liner should be installed inside the flue. If the flue is not the right size, i.e. the stove is attached to a 9 x 9 or 12 x 12 inch brick flue with no liner, the stove will not work as efficiently as it would do when attached to an appropriately sized flue. Smoke will move through the flue much slower as the flue is so much wider and the flue will take much more time to warm up. All of this reduces the draw and can cause the fire to burn sluggishly. You may need to burn a lot more fuel to achieve the same amount of heat as you would get with a with a lined flue. This can also lead to a greater risk of sooty deposits and tar forming inside the flue which can cause chimney fires.
If the fireplace is an open fire and it requires lining, perhaps due to damage in the flue that results in smoke leaking into other roods, the flue will require an 8 inch diameter flue liner.
Chimney Safety - Chimney Fires
Over time the render on the flue wall and mortar between bricks inside a brick flue can deteriorate and fall away. They are corroded by acids in smoke and washed away by drafts blowing through the flue when the fire is not in use. This can sometimes result in cracks in the brickwork through which smoke and other emissions from the fire like carbon monoxide can escape. These cracks can be in rooms above the room with the fireplace and even in adjoining houses.
An open fire is very inefficient, most of the heat produced (70 to 80% of it) travels straight up the flue and out of the top. This upward movement of heat carries smoke and other emissions safely away from the property. If there are cracks in the flue wall, the smoke is usually moving too fast up the flue to escape through them, it just passes the cracks by on it’s way up. But cool air can be pulled into the flue through the cracks, cooling the smoke and emissions down inside the flue before they are expelled out of the top. If smoke and emissions cool, they can condense on to the brick walls of the flue. If burning wood, these emissions include creosote which is highly combustible and can cause chimney fires. Read more about this on our Chimney Fires page.
A stove is far more efficient than an open fire, only 20 to 30% of heat produced is lost up the flue and if the flue is not lined, this smoke and heat will move up through the brick flue much slower than with an open fire. This increases the risks that some smoke can escape into other rooms within the property through cracks in the flue wall. It also increases the likelihood that more of the emissions will condense onto the flue walls as creosote, as the flue wall is cooler, so increasing the risk of chimney fires.
If the movement of smoke through an unlined chimney is very slow, common in very wide flues, the smoke can slow down so much that it can build up inside the flue preventing the stove from expelling more smoke properly. The resulting slow draw causes a sluggish burn, the stove will not burn fuel efficiently so there will be less heat produced and inefficient burning creates even more creosote and soot. In this situation where the draw is very slow, it is possible for smoke to be blown back into the stove if there is strong wind outside and in extreme cases it is also possible for hot gases to collect inside the stove and flue and ignite. This mini explosion can blow smoke back through the stove and into the room. Stoves need help to keep smoke and emissions hot enough to rise through the flue which is why an appropriate lining can make a real difference. Sometimes flue liners are even insulated, particularly if the chimney is on an outside wall, to keep them warm so smoke and emissions flow through properly.
It is not a legal requirement to have a flue lined but it is a requirement to have an installation signed off as safe, either by the installer if they are HETAS registered or by your local councils Building Control team. They will want to inspect the install has been completed competently and this will include demonstrating that an appropriate ‘smoke pressure test’ was undertaken in an un-lined flue, to ensure there are no leaks.
A smoke pressure test involves closing off the fireplace and blocking the chimney pot effectively sealing the flue, whilst burning a smoke pellet inside the flue. Every room the flue passes through must be checked for smoke escaping including in adjoining buildings.
A smoke test is not a 100% guarantee there are no cracks or that cracks might not develop in the future. There are plenty of horror stories circulating in the industry where people in other rooms and even other buildings have been overcome and killed by smoke and carbon monoxide entering a room through tiny cracks in the wall. One such incident involved a bedroom being redecorated, removing wallpaper uncovered a tiny crack that was missed when smoke testing, as the wallpaper had been covering the crack and stopping smoke escaping.
The only real way to mitigate against smoke and emissions leaking through cracks and damage inside flues is to have the flue lined with a good quality lining by a competent installer.
Many HETAS installers will refuse to install a stove without installing a lining due to the safety concerns. You can find out more about smoke test on our Smoke Tests page.
Types of Flue Lining & Cost
The most common liners used with wood burning stoves or multi-fuel burning stoves are stainless steel flexible flue liners. They are relatively easy to install as they are flexible, they are fed down through the flue from the top and connected to the top at the chimney pot and bottom at the stove so they stay in place.
There are two grades of stainless steel used to make flexible liners, 316 grade and 904 grade. The difference is in the thickness of the steel used. 316 grade stainless steel liners are suitable for burning seasoned hard wood. 904 grade stainless steel is for heavier use, e.g. burning smokeless coal or a combination of wood and coal, or if the intention is to keep the stove ‘slumbering’ for long periods, e.g. a slow over-night burn so the stove is still going in the morning.
Whatever you burn you produce smoke, soot and other emissions like creosote. These emissions are acidic and can corrode the lining, hence why there is a heavy duty version for fires that produce more corrosive emissions.
There are lots of cheap liners available and by cheap, we do not mean value for money. Good quality 316 grade liners cost around £25 to £30 per metre and usually come with a 10 to 15 year guarantee and good quality 904 grade liners cost around £35 to £40 per metre and usually come with a 25 to 30 year guarantee. Buying a cheap, poor quality liner really is a false economy as they are badly made and can unravel, i.e. the steel will be very thin and the steel coils that make the liner flexible can come apart either during installation or when being swept. If the coils do become loose when inside the flue then smoke and emissions will escape into the brick flue area where they will be trapped and could be forced through cracks in brick work into rooms in the property.