The flue liner
Where possible, we will always try to line a chimney when we install a stove, because it is the best way to ensure the safe enjoyment and easy operation of the stove for years to come. For more information please see our FAQs page We will use one of two grades of top quality stainless steel liner – 316 or 904 – depending upon which types of fuels you intend to burn on your stove. The liner is then normally insulated, either with loose-fill vermiculite, or – in large chimneys – using high temperature wrapping.
Lining not only ensures the integrity of the flue that is evacuating the smoke and more importantly, Carbon Monoxide from your home, but increases the flue draught, and helps to prevent the potentially dangerous condensation of flue gases inside your chimney. An insulated stainless steel liner warms up quickly, enabling you to light your stove quickly and easily.
With fixed joints between the liner and any rigid flue components, this ensures a strong, integrated flue system from stove to chimney terminal, allowing you to safely and responsibly enjoy your stove to the full.
The chimney will be sealed using a horizontal register plate at the top of the fireplace opening. This prevents any heat loss from the room up the chimney, as well as retaining the insulating materials within and helping to keep your fireplace safe and clean by eliminating falling dust and debris.
Chimneys and chimney lining
A wood burning stove is an extremely efficient means of converting firewood into useful heat. And a highly efficient stove should really only be connected to a well-insulated and lined chimney. The problem however is that most older properties built prior to 1966 have only unlined brick flues (usually one brick square inside, ie. 9” x 9”, although sometimes larger as with large inglenook fireplaces). Such old chimneys are often only one brick thick, have already been exposed to the weather and had substantial use over a period of perhaps 60 years or more, and were in any case built using soft lime mortar rather than modern cement mortar. All of this poses a problem. Some poeple may understandably say, “We have been lighting an open fire in this fireplace every winter’s day for the past 50 years, and there’s never been any problem yet: so why should I consider spending a lot of money just because someone claims its a desirable thing to do?” A reasonable question, and certainly one which deserves an explanation. And the explanation is as follows.
Two things are basically very different when comparing a modern stove with an open fire. The first major difference is one of efficiency.
Typically, only about 5%-15% of the total heat produced by a fire in an open fireplace benefits the house: the other 85%-95% goes straight up the chimney. This is of course very wasteful and expensive in fuel, but at least the flue gases (the “smoke”) remain hot, and relatively little condensation of steam or unburnt volatiles takes place. The loss of heat up the chimney means that the flue gases remain above the “dew point” (you may remember the term from chemistry at school), and are thus mostly emitted safely into the atmosphere.
With a wood burning stove however, between 60%-75% or more of the heat released by the burning wood will be radiated and convected into the room as useful heat. This means that, depending on the size of the original open fireplace and its flue cross-section (9″ x 9″ in most houses), a stove is typically 6 to 10 times more efficient in turning fuel into useful heat.
When so much heat is retained within the building with a stove, the “downside” is that the flue gases are consequently much cooler and are more likely to condense onto the sides of the flue/chimney and will result in a shiny, and often sticky black layer of tar and creosote forming on the inside surfaces of the flue/chimney. This condensate can cause certain issues: Tar is highly flammable, so the risk of having a potentially dangerous chimney fire at some time in the future is considerable.
Tar is also highly corrosive, attacking and eating into old brickwork and the soft lime mortar. Old bricks are porous and tend to absorb moisture, so one result is that brown stains penetrate through the chimney breast and begin to appear through the plaster or wallpaper in an upstairs bedroom. An examination of the chimney stack if it passes through the loft will often reveal the tell-tale evidence of brown staining before it reaches that stage lower down.
Another major difference between a stove and an open fire is its ability to be operated in “slow burning mode”.
It is really not possible to burn an open fire slowly in the same way as a modern airtight stove, ie. By closing off its combustion air supply. To slow an open fire down, two methods have traditionally been used. The first approach has been to place one or two “green” (ie. unseasoned) logs on the fire: because of the high moisture content, it would take some time before sufficient sap had been driven off as steam to allow the logs to burst into flame. As you will probably appreciate better by now, this approach is not really recommended.
A second way to obtain a longer burn with an open fire is simply to use very large logs which have a minimum surface area to mass ratio. As mentioned on our solid fuel page, when dealing with stoves, the larger the surface area, the quicker the burning time for a given mass of timber. Exactly the same principle applies with an open fire, and perhaps even more so due to the lack of a stove’s forced draught. With a closed airtight stove, it is physically possible to close down the air controls to such a degree that almost no combustion air can get to the fire. By thus starving the fuel of oxygen (air), the wood gases released by the heating of the unburnt wood are unable to ignite, and the fuel then smoulders at a much lower temperature. It is possible to burn a stove so slowly by this means that it can be made to “stay in” for perhaps 6 to 10 hours at a stretch without refuelling.
Since the wood gases (or “volatiles”) then escape up the chimney unburnt, and since almost half the available energy in wood is obtained by properly burning those gases, it therefore follows that the efficiency of a stove can drop markedly during any slow burning period. The only exception to this is where all the volatiles have already been burnt off, and all that remains in the stove is charcoal: since there are then no wood gases remaining unburnt, under these conditions slow burning can be carried out safely and efficiently.
Whilst a woodstove can on the one hand be a highly efficient and safe means of converting firewood into useful heat, and on the other hand can offer the means of burning (or more accurately, smouldering) wood, using it in slow burning mode is relatively inefficient. When the air supply is shut right down before the wood has been reduced to charcoal, the efficiency can drop from over 70% to perhaps only 30%. More importantly, slow burning for long periods can be unsafe, especially when the stove is fitted in an unlined chimney. As a rule, in order to be able to burn a stove slowly and safely on a regular basis, it must be connected to a lined and insulated flue of a high standard.
By using a double skin insulated flue system, a stove can be installed practically anywhere. This can be a real option if there is no existing chimney. A scaffold tower is needed for this particular external installation.
The system is constructed of two tubular sections with a difference of between 25mm and 30mm diameter. The centre is constructed with insulating materials and binds them together. They come in lengths, general up to 1150mm, and lock in to place with a locking band or wall bracket. It is a very strong and durable system. Optional colours of stainless steel and powder coated black. You can expect to pay around 40% more for the black.