Important: Modifications to existing chimneys and installations of flue liners and flue systems are subject to strict local Building Regulations and legal requirements and, in the interests of safety, only appropriately qualified persons should undertake such work. The notes below should cover most of what you need to know, however Hi-Flame will always recommend seaking the advice of of a qualified, experienced fitter.

The benefits of a good flue system

Providing the correct amount of flue draught or 'pull' is critical to the successful performance of your stove. It helps draw vital air into the stove combustion chamber, removes the harmful products of that combustion through the flue and then disperses them to the outside air and into the atmosphere. A typical wood burning stove requires something like 15 to 25 cubic metres of combustion air every hour and an average UK and Irish living room has a volume of air around 40 to 50 cubic metres – less the volume taken up by any furniture and of course, the room's occupants who will also need the air. Draught-proofed and well-insulated rooms can limit the supply of air to the stove effecting the up-draught and therefore it is vital that Building Regulations' guidance on air supply and ventilation is strictly adhered to.

The hotter expanding flue gases produced by your stove will consequently be lighter than the equivalent column of cooler outside air and this will produce a slight difference in pressure between the two to create the important up-draught. The taller the flue (normally not less than 4.5m (15') from the top of the stove) then the better the draught will be (and the less likelihood of condensation problems, discussed later). Keeping the gases as hot as possible, for as long as possible, as they move through the flue will increase the effect of the up-draught. However, as the flue gases rise up through the flue they will naturally lose heat and the effects of the up-draught will diminish, so unfortunately it does not always follow that really tall chimneys work better than short ones.

Pre-1965 chimneys will certainly have rough inner surfaces, and possibly voids or bends, which could cause resistance to the natural flow of the flue gases, slowing and cooling them as they exit the chimney. Post-1965 chimneys generally feature clay or concrete liners, usually with a 200 – 225mm (8 – 9") internal diameter, an improvement, but unfortunately still not ideal for stoves. This is because the cement joints between the liner sections cannot be guaranteed to be complete (since they were never intended to be seen by the bricklayer) and it is not unusual for some liner systems to have been installed upside down.

Over the years the intense heating and cooling cycles as well as the chemical reactions from condensation and flue gases can take their toll inside a chimney, with the worst of the weather also effecting the exposed stack. Anything that could cause a potential leak (possibly of dangerous Carbon Monoxide), or slow down and therefore cool the flue gases, should be eliminated as this will affect the performance of your stove.

A chimney which appears to work perfectly well with an open fire may not necessarily work so well for a new stove. This is because flue gases leave the stove at a much greater temperature than those from an open fire and the higher temperature inside the chimney created by flue gases from a stove could affect the safety and performance of the chimney. For example the old seals made by the bricks and mortar on any mid-feathers (the brickwork separating the flues in a chimney stack, eg. between downstairs and upstairs fireplaces) could expand and open up from the additional heat leading to a potential leak that wasn't there with your old open fire.

It must be remembered that chimneys and flues are subjected to intense heating and cooling cycles over many years which can create condensation and harmful chemical reactions caused by the flue gases. A Victorian chimney will have been subjected to over a century of heat and the brickwork will have no doubt become porous and the lyme mortar joints loose. In some very early houses it is not unknown for the ends of the wooden floor joists to protrude into the chimney void.

Advantages of fitting a flue liner

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:

  1. The existing chimney causes smoke and potentially dangerous fumes to enter the house.
  2. 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.
  3. 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.
  4. Porous brickwork, weak mortar joints and condensates have caused unsightly staining on the interior or exterior of the chimney walls.
  5. 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.

Minimum Flexible Flue Liner Sizes

Sometimes it is impossible to fit the correct size of flexible liner (generally 150mm / 6") due to some restriction or tight bend within the existing chimney system and there is no alternative but to fit a liner with a smaller diameter (125mm / 5"). For some people fitting a smaller diameter liner can also be seen as a way to save money. However, it is important to understand the requirements of the stove manufacturer along with your legal requirement to comply with Building Regulations before the liner is specified.

Stove manufacturers determine the appropriate size of the connecting flue pipe to ensure that their stove will work safely and efficiently and therefore the flue liner should be no smaller than the size of the flue pipe aperture. In the case of 125mm flue outlets, generally on stoves with 5kW output and below, ideally the liner should be one size larger (150mm) as this diameter will make it suitable for burning high volatile fuels, such as coal, or replacing the original stove with a larger output stove at a later date.

Some manufacturers recommend a flue of the same size as the flue aperture throughout the system. The flue pipe or flexible liner used should never have a smaller diameter than that of the stove's flue aperture and that specified by the manufacturer.

In England and Wales, Building Regulations Document J (table 2) states that 125mm flues should only be used with smokeless burning appliances or those appliances exempted by DEFRA under the Clean Air Act and which allow you to burn wood in a UK Smoke Control Area. 150mm flues should be specified for appliances burning other fuels such as wood, peat and bituminous coal. It also states in paragraph 2.6 that for multi-fuel appliances, the flue should be sized to accommodate burning the fuel that requires the largest flue. This is because high volatiles fuels such as bituminous coal quickly attach deposits to the inside of the flue during combustion and over time could potentially reduce the usable cross sectional area of the flue.

So to sum up, if you fit a 125mm flexible liner and you want to burn wood, in order to comply with Building Regulations, you should only specify a DEFRA Approved Smoke Exempt stove and when burning multi-fuels in this stove only use clean burning approved smokeless fuels. If you specify a non-DEFRA Approved stove then you should not burn wood at all and you should only burn clean burning approved smokeless fuels.

If you need to use a 125mm liner and you wish to burn wood in a stove with a greater output than 5kW (which usually means a 150mm flue outlet) then you can only use a stove with the higher output that also features a 125mm flue outlet. This will severely limit your choice as there are only one or two DEFRA approved stoves with larger outputs that also feature 125mm flue outlets, for example the Broseley Serrano 7, being the most well-known (available from The Stove Yard's Cheshire showroom). The same applies if you wish to use a higher output stove and burn multi-fuel – you should only burn approved smokeless fuels and specify a stove that unusually features a 125mm flue outlet.

Liner Dia  Proposed Fuel Appliance / Fuel Requirement
125mm Wood Defra Approved 5kW Stove with 125mm flue outlet burning wood with less than 20% moisture content (or approved clean burning smokeless fuels)
125mm Multi Fuel Multi Fuel 5kW Stove with 125mm fl ue outlet only burning Approved Smokeless Fuels – but NOT wood
150mm Wood / Multi Fuel
Any non-Defra* or Defra wood burner or multi fuel stove with a flue outlet size of 125mm to 150mm
*Subject UK Smoke Control Area regulations

Please note: Without regular cleaning all flue liners could become constricted or blocked.

Condensation and staining

Water vapour and condensation is created when virtually any kind of fuel is burnt with some fuels producing more water vapour than others. Providing you can keep the water in its vapour state, by maintaining the flue gases from your stove at the highest possible temperature until they exit the flue system, then usually they won't cause any problems. However, if they start to cool down too much they will form moisture on the interior surfaces of the flue system, potentially cooling the flue gases even further.

Unfortunately a combination of sulphur compounds from the flue gases and sulphates in the brickwork produce moisture which is effectively a mild acid which over time will attack the brickwork and cement joints. in the worst cases it can eventually seep through the brickwork. Burning wet or unseasoned wood will only exacerbate the problem.

Generally speaking, the taller the flue and the better insulated it is, then the better the up-draught will be, thus minimising the heat loss and reducing the chances of producing problem condensates. These blackish noxious liquids can ruin decor and stain exterior brickwork as well as create an unpleasant odour in your home. A combination of the hottest possible flue gas, a good up-draught and using dry, well-seasoned wood fuel will all help to minimise this potential nuisance. Flue pipe and clay and concrete liner joints, as well as other joints in a flue system are all designed to avoid condensate leakage and allow the condensates to run down and inside the flue. However, because these joints work in reverse to other piping systems (eg. the female link must point upward) some builders, odd-jobbers and DIYers simply do not understand this important issue and fit them upside down. For the same reason twin wall flexible liners must also be fitted in the correct direction.

Lining the chimney with an approved flue liner, which is also acid resistant, and then insulating around it with vermiculite backfill will help prevent any further condensation by reducing the heat loss which causes the condensation.