Chimney flue lining: requirements & installation (2026)

When is chimney flue lining required in 2026?

Existing chimney: when the flue must be lined when replacing an appliance

When an open fireplace is replaced with an insert, a wood stove or a pellet stove, the existing masonry flue must be adapted. If the flue is not airtight, if it is cracked, oversized or degraded, continuous lining (often stainless steel) becomes necessary to limit backdraught, deposits and the risk of chimney fires.

New installation or major renovation: lining required depending on use (wood, pellets, gas) and the flue's condition

For a new installation, either a metal flue or a masonry duct with lining is fitted. For pellets and gas, the presence of condensate requires a compatible, resistant, well-sealed flue, sometimes designed to operate under pressure. For wood, lining becomes necessary as soon as the flue does not meet sizing, insulation or safety distance requirements.

Insurance and liability: what safety requires to be covered in the event of a claim

After the work, keep proof of compliance (instructions, invoice, any certificate) and have the chimney swept according to the local frequency. In the event of a claim, the insurer may request them. Without a compliant installation, coverage and liability can be disputed.

Choosing the right lining for your chimney flue

Single-wall or double-wall stainless steel: how to decide based on the route and temperature

In an existing masonry flue, single-wall stainless steel is often the logical choice for lining a chimney. As soon as the route becomes exposed, passes through an unheated space, or runs near combustible materials, insulated double-wall provides better safety. With hotter flue gases (wood, pellets), insulation also helps maintain draught and limit condensation.

Rigid or flexible lining: advantages on site and limits in renovation

Rigid liners suit straight flues. The joints are stable, and sweeping is easier. Flexible liners are a good ally when the flue is offset or irregular, without breaking into the masonry. They require careful installation, in the right direction, with controlled bends to avoid snagging points.

Diameter, airtightness, corrosion resistance: checkpoints for a reliable chimney

Follow the diameter prescribed by the appliance and keep a constant cross-section up to the chimney outlet. Check the airtightness of the joints, the safety clamp, and a suitable cowl. On durability, choose stainless steel designed for acidic condensate, especially for intermittent use.

Installation: the key steps for lining a chimney without any nasty surprises

Preparing the flue: sweeping, scale removal and checking it's clear

Before lining a chimney, do a complete sweep, then scale removal if the flue is fouled. The goal is simple: a clean flue with no narrowing. Finish with a check that the full height is clear, ideally with a visual inspection or camera, to spot cracks, offsets and obstructions.

Fitting the lining: fixing, chimney cap, connection to the appliance and managing bends

Fit a suitable liner (rigid or flexible) while keeping a constant diameter compliant with the appliance's instructions. At the top, plan for a sealing plate, a retaining clamp and a cowl. At the bottom, take care with the connection using an adapter and an inspection element for sweeping. Limit bends, avoid sharp angles and keep access clear for cleaning.

Site safety: distances to combustible materials and floor penetrations

Follow the safety distance to combustible materials, per the requirements of DTU 24.1 and the manufacturer. At floor penetrations and in lofts, plan for non-combustible cladding, insulation designed for flue pipes, and ventilation if required. Also check the airtightness of the joints before commissioning.

Safety and compliance: checks to plan after chimney lining

Tests and checks: draught, airtightness, hot spots and condensation

After lining, validate correct operation before the first fire. Check the actual draught, run a smoke test for airtightness, then check for hot spots (hatch, penetrations, masonry ducts) and check condensate drainage and the absence of drips at the base of the flue.

Documents to hand to the client: certificates, photos, traceability of the lining and flue

Hand over clear proof of what was installed. Certificate of compliance or installation, technical data sheet and marking of the lining system, before/during/after photos, a diagram of the route and sweeping access points. Keep complete traceability of the references and diameters.

Maintenance: sweeping frequency and monitoring the flue to avoid chimney fires

Explain the maintenance schedule based on the fuel and local rules. In practice, plan for two sweeps a year for wood, one of them during the heating season, and at least one annual check for gas appliances. Emphasise regular monitoring of the joints, the cowl and deposits.

Special cases in the field: chimney renovation and common constraints

Offset flue, irregular cross-section, cracked masonry ducts: suitable lining solutions

When the flue is offset or the cross-section changes, a visual check, sometimes supplemented by a camera inspection, avoids nasty surprises. A flexible stainless steel liner adapts to bends and irregular masonry ducts. If masonry ducts are cracked, the top and penetrations are also secured, then airtightness is checked before putting the system back into service.

Chimney in a co-owned building or an old house: access, penetrations, and coordinating the works

In a co-owned building, the chimney quickly touches on common areas. Approval from the managing agent, roof access, protection of walkways — everything must be planned. In an old house, penetrations through wooden floors require a compliant fire clearance. Plan for inspection hatches and possible access for sweeping.

Changing fuel: moving from an open fireplace to an insert or a stove while securing the flue

Moving from an open fireplace to an insert or a stove changes the temperatures and the draught. Sizing is checked, then the flue is lined continuously up to the outlet, per DTU 24.1, with airtight connections. A smoke test and a controlled first lighting validate safety. To gauge the impact of an open fireplace on the DPE, it's worth anticipating the logical next step: insert/stove and securing the flue.