A flue liner builds a barrier between the chimney walls and the flue, which insulates the chimney from heat and corrosion.
IT CREATES A SAFE PASSAGEWAY FOR HARMFUL GASES
Flue liners should be installed in homes with wood burning or multi-fuel stove facilities as a matter of course. The flue liner will serve as a safe passageway for harmful gases as they pass through the chimney.
IT CAN DECREASE THE CHANCES OF FIRE IGNITION
Heat in the chimney will be reduced, which in turn will decrease the chances of a chimney fire.
IT PREVENTS CONDENSATION AND MASONRY DAMAGE
Low-temperature gases can generate great amounts of condensation that can cause deterioration to the masonry within a chimney. If the chimney has a flue liner, this will be prevented.
IT CAN LESSEN THE BUILD-UP OF SOOT, TAR AND CREOSOTE
Condensation from wood burning can be attributed to the accumulation of soot, tar and creosote. Since these are combustible, this could potentially lead to a chimney fire if not removed. Installing a flue liner can lessen build-up because the condensation in the chimney is also reduced.
IT PREVENTS THE LEAKAGE OF CARBON MONOXIDE INTO THE HOUSE
The process of burning wood in a stove or an open fireplace mounted into the chimney produces harmful and toxic gases like carbon monoxide. Without a flue liner, carbon monoxide may leak into the house and become a health hazard.
IT CAN PROMOTE ENERGY EFFICIENCY, REDUCING THE CONSUMPTION OF WOOD OR FUEL
Flue liners improve the flow of air within the chimney. Wood or fuel consumption will be reduced since this causes better heat retention
Fitting an approved solid fuel stainless steel flexible twin-wall liner will improve the performance and 'controllability' of your stove by removing the typical chimney problems outlined above, and because this is a major 'quantifiable' component in the flue system, fitting a liner will always be the preferred option for professional stove installers.
A significant advantage of flue liners is that they provide a degree of insulation (especially when back-filled with vermiculite) which will help maintain the temperature of the flue gases to create a better 'pull', which is particularly advantageous where a chimney is on an outside wall and is subjected to a prevailing wind chill. A liner also offers the additional advantage of protecting paint finishes and wall coverings on the chimney breast from potential damage caused by condensates and tars.
If you plan to install a boiler stove then fitting a flue liner is essential. This is because boiler stoves operate at a much lower temperature than basic wood burners or multi fuel stoves. The boiler inside the stove effectively works like a car radiator drawing the heat from the fire chamber to warm the water as it passes through it, with the knock-on effect of the flue gases entering the chimney at a significantly reduced temperature than those from a stove without a boiler. Cooler flue gases reduce the effectiveness of the up-draught so that the gas rises even slower, thus cooling down further. Cooling flue gases cause a smoky chimney (possibly nuisance smoke) with potential creosote problems and most certainly additional soot deposits which would require the chimney to be swept more often to avoid blockages and chimney fires.
Further reasons for lining a chimney:
The existing chimney causes smoke and potentially dangerous fumes to enter the house.
The existing chimney void size is not compatible with the stove manufacturer's flue outlet specification (see Building Regulations Document J) and needs to be reduced by the installation of a flexi liner to maximise the stove's performance.
The existing clay or concrete liner has poor joint seals or has been installed upside down causing any condensates to run outside the liner rather than inside the liner, thus increasing the likelihood of staining.
Porous brickwork, weak mortar joints and condensates have caused unsightly staining on the interior or exterior of the chimney walls.
The chimney is 'cold', usually because it features a large void (see point 2), is on an outside wall or is subject to a strong prevailing wind, any of which will make it very slow to 'warm' and difficult to create the critical up-draught that a stove needs to burn properly.
With all of the above in mind it is obvious to see why, in The Stove Yard's opinion, a flue-lined chimney will always out perform a non-lined chimney in every category, including, most importantly of all – safety.