What a chimney liner actually does

A chimney liner is the tube inside your chimney that carries smoke, water vapour and combustion gases safely up and out of the house. In many older British properties that tube was simply the inner face of the brickwork, or a clay pipe built into the stack by the original builder. Both can serve well for decades, but neither lasts indefinitely, and neither was designed for the hotter, more efficient appliances we now install.

The liner has three jobs, and all three matter. It contains the products of combustion so they cannot seep into bedrooms, lofts or roof spaces. It protects the surrounding masonry, and any timber that happens to be near it, from heat and acidic condensates. And, less obviously, it shapes the draught. A flue that is correctly sized and smooth inside pulls air through the appliance steadily, so the fire lights easily, burns cleanly and responds to the air control rather than sulking.

Why an unlined or damaged flue is a genuine risk

Old flues fail quietly. Mortar softens, clay sections crack, and a chimney fire can spall the inner face badly enough that the flue is no longer gas-tight. Once that happens, hot gases and carbon monoxide can travel into the fabric of the house instead of up and out. This is why a liner is not an upgrade you fit for tidiness — it is the barrier between your fire and your home.

Worth investigating if you notice any of these:

• Soot or mortar debris collecting at the base of the flue or in the fireplace
• Smells of soot or smoke in upstairs rooms, particularly when the fire is first lit or when it is windy outside
• A fire that smokes back into the room, or proves stubborn to light
• Staining on chimney breasts, ceilings or roof timbers
• Visible cracks in the stack, or a flue that has been damaged by a previous chimney fire

Carbon monoxide is the serious one, because it gives no warning at all. A suitable liner, correctly fitted and properly ventilated, is the most reliable way to keep it out of the living space. Fit a CO alarm as well — it is a legal requirement with most solid fuel appliances and simply sensible with the rest.

The main liner types, and where each suits

There is no single best liner; there is only the right liner for your chimney, your fuel and your appliance.

• Flexible stainless steel liner — the workhorse for wood and multi-fuel stoves. It bends around offsets and is available in single-skin or twin-wall form. Grade 316 suits most domestic solid fuel use; grade 904 is the tougher option for corrosive fuels and long-term resilience.
• Twin-wall insulated flexible liner — the same idea with an insulation layer wrapped around it. Ideal where the chimney sits on an outside wall, because it keeps flue gases warm and reduces condensation and tar.
• Rigid sectional liner — stainless steel sections for straight flues. Smoother bore, easier sweeping, and often the preferred choice for larger open fires or commercial-style installations.
• Cast in-situ or pumped liner — a vermiculite and cement mix poured or pumped around a former. Excellent where the flue is irregular or the chimney is structurally sound but the void is uneven.
• Clay and pumice liners — traditionally built into new chimneys. Pumice has good insulating properties and is often used where a new stack is being constructed or an old one is being rebuilt.

Matching the liner to your appliance and fuel

Different fuels behave differently inside a flue. Wood produces moisture and tars, which are corrosive when they condense. Smokeless fuels burn hotter and drier. Gas flues run cooler and produce acidic condensate, so they need a liner suited to that chemistry rather than one designed for solid fuel.

As a general guide, most solid fuel stoves want a 150mm (6 inch) flue unless the manufacturer specifies otherwise; some smaller appliances are approved on 125mm. Never go below the appliance manufacturer's stated minimum, and never squeeze a large flue into a small liner to save money — an oversized flue runs cold and sluggish, while an undersized one strangles the fire.

Height matters too. A solid fuel appliance typically needs around 4.5 metres of effective flue, with the terminal clearing the ridge line and any nearby roof surface by the margins set out in Approved Document J. If you live in a smoke control area, the appliance itself must be exempt or you must burn an authorised fuel — a good liner will not change that.

Insulation, sizing and the draught you can feel

Draught is the engine of the whole system. Warm flue gases rise, fresh air is drawn in to replace them, and the fire gets the oxygen it needs. Insulation is what keeps that engine running. An uninsulated liner in a cold external stack loses heat quickly, the gases cool, draught weakens, and tar begins to deposit on the liner walls. That deposit is fuel waiting for a chimney fire.

You should also think about the air the room can supply. Modern homes are draught-proofed, and a stove needs a permanent air supply — sometimes a dedicated vent. Without it, the appliance competes with extractor fans and bathroom fans for air and can spill smoke back into the room.

Fitting, compliance and keeping it right

Liner work is not a DIY job. The installation must satisfy Building Regulations Approved Document J, and in practice you want an installer registered with a competent person scheme, who can self-certify the work and notify your local authority. Ask for the liner's specification and the manufacturer's approval for your appliance before anything goes down the chimney.

Once it is in, look after it. Sweep at least once a year — more often if you burn a lot of wood — and keep a record. Burn well-seasoned wood with a moisture content below 20 per cent, use a stove thermometer or flue gas thermometer to avoid over-firing, and have the liner inspected periodically. Looked after properly, a good liner will outlast the stove it serves and quietly do its job for decades.