Wood Burning Stoves: Why Chimneys Need 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, i.e. 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.
But some may understandably enquire, “People 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 it’s 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, already alluded to above, 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, a stove is typically 4 to 10 times more efficient in turning fuel into useful heat. Large inglenooks are the worst culprits, having huge draught requirements and providing relatively little benefit except in the immediate vicinity of the fire: but then it should be remembered that their original purpose was as much to preserve meat and fish by smoking as it was to heat the room.
But when so much heat is retained within the building with a wood burning stove, the “downside” is that the flue gases are consequently much cooler and are more likely to condense onto the sides of the flue. Now, for those who can recall their chemistry lessons, one simple experiment involved heating scraps of wood in a glass retort over a Bunsen burner. After a short while, the surface of the wood begins to char, and if a lighted taper is held close to the neck of the retort, the wood gases which are being driven off by the heat of the Bunsen burner ignite and burn with a yellow flame. This in fact mimics what is happening when you burn wood in a fire.
The second part of this simple experiment illustrates the problem with unlined chimneys. Using a rubber bung, a glass “condensation tube” is fitted onto the neck of the retort -- a length of glass tube surrounded by a glass coil through which cold water can be circulated. This apparatus now presents a cooled surface to the passing wood gases, and the result is an almost immediate film of sticky black condensate. (If you were never shown this experiment, now you know what you missed!)
The same thing happening up a chimney will result in a shiny, and often sticky black layer of tar and creosote (general terms used in the stove industry to describe what is in fact a complex mix of distillates) forming on the inside surfaces of the flue. This condensate can then do several unwelcome things. For a start, 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 (and often the first symptom to be noticed) 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.) Stains are not only unsightly, but the semi-liquid tar is also highly pungent and almost impossible to get rid of without stripping the plaster, inserting an impermeable barrier of some sort, and then re-plastering and redecorating. Arguably this is rather an expensive remedy for what is only the avoidable result of poor stove management.
The second 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 wood burning stove, i.e. 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” (i.e. 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 above 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 wood burning stove, however, 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, but 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 slowburning 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 slowburning can be carried out safely and efficiently.
Thus, whilst a wood burning stove 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 slowburning 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, slowburning 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 wood burning stove slowly and safely on a regular basis, it must be connected to a lined and insulated flue of a high standard.