Why does a chimney need a liner?

Chimney liners are generally needed when a traditional brick or stone built chimney (a Class 1 Chimney) is to be used to flue a wood burning or multi fuel stove that is more than likely replacing an open fire. There are a number of important reasons you should line a chimney when installing a stove but perhaps only one reason why you don't always have to.

1. Modern stoves produce much higher combustion gas temperatures than open fires. When compared to a stove, open fires are incredibly inefficient because they allow the chimney to suck lots of room air up the chimney. All this additional air cools the hot combustion gases to a point where an old chimney is under much less stress from the reduced heat expanding and damaging the brickwork and mortar bonds than it would be when used by a stove. Therefore what worked in old chimney for an open fire may not necessarily still work for a new stove with its much hotter combustion gases. A chimney liner will remove this potential issue.

2. Many older chimneys have been subjected to decades of acidic condensates produced by inefficient combustion from open fires which will have attacked the brickwork and mortar joints so that there is the potential for smoke, fumes and dangerous invisible carbon monoxide (CO) leaking into other parts of the building or, if the chimney or chimney stack is shared, into a neighbouring building. A chimney liner will help prevent this from happening.

3. The odd bends and kinks found in many older chimney designs, combined with the rough inner surfaces of old brickwork and mortar joints, can slow down combustion gases possibly to the point where the updraught is adversely affected and condensation is produced. Poor updraught may affect the efficient performance and safe operation of the stove and condensation can create odours and possibly eventually damage interior decorative surfaces. The smooth inner surface of a chimney liner will remove this problem.

4. Many houses built since the mid sixties have chimneys which are lined with clay or concrete sections. There is plenty of evidence that indicates that a significant proportion of these liners have been poorly installed with missing or incomplete mortar joints between the sections and some chimneys having been built with liners fitted the wrong way up so that condensates leak causing unsightly staining on interior and exterior walls. A chimney liner will prevent this from happening.

5.The cross-sectional area of the passage through a brick or stone built chimney, whether or not it is clay or concrete lined, will rarely match that of the stove's flue outlet which is usually much smaller (generally 125 mm or 150mm diameter). Large differences between the two will reduce the velocity of the combustion gases and cool them to produce inefficient updraught possibly affecting the stove's safe operation particularly at the end of the burn cycle. Cold combustion gases also create more smoke (which your neighbours will love) and therefore more soot and potentially problematic acidic condensates. In addition any cold void will have to be warmed first before the updraught improves and this can be very difficult if the chimney is large, is on an exterior wall and / or is also subject to a prevailing wind. A chimney liner can be used to provide a consistent flue diameter in line with the stove's flue outlet. If necessary additional insulation can be provided by also back-filling the chimney with Leca or vermiculite granules, or for larger voids sleeving the liner in a specialist chimney wrap.

6. The flue is a major factor in the efficient and safe performance of a stove and it's hard to quantify exactly how an old unlined chimney will work when it's connected to the stove. For installers a flexible liner reduces doubt exactly because it is quantifiable. It provides a consistent flue diameter in line with that of the stove's flue outlet and it has a regular smooth inner surface to minimise interference with the speedy passage of the combustion gases – both of which will improve the updraught and provide greater control of the stove and its fuel efficiency. It's also much safer because when it's properly installed it won't leak. For cold chimneys and chimneys with large voids, backfilling with insulation such as Leca or vermiculite, will keep the gases as hot as possible, again improving the stove's performance.

7. Stoves are CE tested with a flue pressure of 12pa and so most stove manufacturers will recommend that you use a flexible chimney liner. This is because the liner counteracts all of the issues above and will also contribute to achieving the manufacturer's desired minimum flue pressure which in turn will have a positive impact on the how well their stove performs for you.

The only reason you wouldn't fit a chimney liner...

If the existing chimney is same diameter as the flue outlet on the appliance.