Professional Flue Systems: Ducts, Liners & Accessories

Professional flue systems represent all the components and techniques for smoke extraction in industrial and tertiary heating installations. At Climatik, we support French professionals – installers, heating engineers, design offices – in selecting flue solutions that comply with current standards for fixed combustion equipment.

Our catalog includes insulated double-wall ducts, single-wall ducts, flexible and rigid liners, connection accessories, and roof terminals from leading European manufacturers. All components meet the NF DTU 24.1 requirements and guarantee the safety and performance of your professional installations.

Insulated double-wall ducts: the professional standard

Insulated double-wall ducts are the essential technical solution for any installation passing through premises, floors, or attics. Composed of two concentric tubes of AISI 316L or 304 stainless steel separated by high-density rock wool (density 100 to 120 kg/m³), they provide optimal thermal insulation and an external surface temperature not exceeding 50°C during operation.

This technology allows for passing through all combustible materials while respecting reduced regulatory safety distances (generally 8 cm between the outer wall and combustible material). Double-wall ducts are essential for professional installations of hot air generators, boilers, and any combustion appliance with a power output greater than 30 kW.

Available in diameters from 80 mm to 300 mm, with insulation thicknesses of 25 mm, 30 mm, or 50 mm depending on flue gas temperatures, these ducts have a lifespan of 15 to 25 years in an industrial environment. Connections are made by male-female interlocking with high-temperature gaskets and stainless steel clamping bands.

Single-wall ducts: connection and distribution

Single-wall ducts in black steel or stainless steel are used exclusively for connecting heating appliances to the double-wall duct or chimney, only within the heated volume. Their use is strictly regulated by DTU 24.1, which requires significant safety distances from combustible materials (minimum 3 times the duct diameter).

These ducts, with a minimum steel thickness of 2 mm, are available in a matt black or brushed stainless steel finish, with diameters from 80 mm to 200 mm. Straight elements of 25 cm, 50 cm, or 1 meter are assembled with 15°, 30°, 45°, or 90° elbows, a sweeping tee, wall rosettes, and ceiling plates.

For professional installations of hot air generators, the total length of single-wall connection must not exceed a maximum of 3 meters, with a maximum of 2 elbows. Beyond this, the use of insulated double-wall becomes mandatory to maintain flue gas temperature and ensure draft.

Flexible and rigid lining for existing ducts

Lining involves inserting a metal duct inside an existing masonry flue to bring it up to current standards. This operation is systematically required when replacing an old heating system with modern, high-efficiency equipment.

Flexible liner in 316L stainless steel

The flexible liner allows for lining old ducts with deviations, offsets, or non-constant sections. Made of double-skin AISI 316L stainless steel (minimum 0.12 mm thickness per skin), it resists acidic condensates from condensing boilers and has a lifespan of 20 to 30 years.

Available in diameters from 80 mm to 250 mm, the flexible liner is unrolled over the entire height of the duct in a single piece, thus eliminating intermediate joints that are sources of leaks. Connection at the top and bottom is done with stainless steel clamping bands and sealing sleeves.

Rigid liner in 304 or 316L stainless steel

The rigid liner is suitable for straight ducts or those with slight deviations. Composed of 1-meter interlocking elements in 304 or 316L stainless steel with a thickness of 0.4 to 0.5 mm, it offers superior rigidity and better mechanical resistance than the flexible liner.

This solution is essential for high-power installations (>100 kW) where the flue gas evacuation speeds require a perfectly smooth inner wall. Rigid lining also facilitates subsequent mechanical sweeping.

Flue sizing

The correct sizing of the flue is crucial for the proper functioning of the installation. An undersized flue causes smoke spillage, while an oversized one generates excessive cooling and destructive condensation.

Simplified calculation method

For a combustion hot air generator, apply the rule: **1 cm² of duct cross-section per kW of nominal power**, with a minimum diameter of 150 mm regardless of power.

A 70 kW generator therefore requires 70 cm² of cross-section, i.e., a diameter of 150 mm (actual cross-section 177 cm²). A 150 kW generator requires 150 cm² of cross-section, i.e., a diameter of **200 mm** (actual cross-section 314 cm²).

This simplified method applies to straight vertical ducts with a height of 4 to 12 meters. For specific installations (horizontal ducts, great height, multiple deviations), a complete thermal calculation according to NF EN 13384-1 becomes mandatory.

Standards and regulatory compliance

The installation of professional flue systems is strictly regulated by DTU 24.1 (flue work) and DTU 24.2 (hearths and flues). These unified technical documents specify the rules for design, installation, safety distances, and authorized materials.

Key compliance points include: minimum outlet height 40 cm above the ridge or any part of a building less than 8 meters away, prohibition of horizontal ducts except for a maximum of 1 meter immediately after the appliance, mandatory sweeping traps every 4 meters vertically and at each change of direction.

The CE marking of ducts according to standards EN 1856-1 (metal ducts) and EN 1856-2 (liners) attests to compliance with European requirements. Professional ducts display their chimney fire resistance classification (G for resistant, N for non-resistant), their corrosion resistance (W for wet, D for dry), and their maximum operating temperature (generally 450°C to 600°C).

Installation and maintenance professional

The installation of flue systems requires the intervention of a qualified installer proficient in connection, sealing, and wall penetration techniques. Each installation is subject to a declaration of conformity given to the client and kept for 10 years.

Mechanical sweeping remains mandatory at least once a year (twice for solid fuels), performed by an approved professional who issues a sweeping certificate essential for insurance. Between two sweepings, a quarterly visual inspection of visible elements (connections, clamps, roof terminals) prevents problems.

Insulated double-wall ducts require an annual check of joint seals, tightening of clamping bands, and the condition of wall penetrations. Any external blackening of a double-wall duct indicates a smoke leak and requires immediate replacement of the faulty element.

Climatik's support for your flue projects

At Climatik, our expertise focuses on technical advice and the supply of professional flue components. We help you correctly size your installation according to the type of appliance, building configuration, and regulatory constraints.

We guide you towards suitable technical solutions – double-wall ducts, flexible or rigid lining, connection accessories – from European manufacturers recognized for the reliability of their products and the availability of their after-sales service.

All our flue components are available in stock with fast delivery anywhere in France. Complete technical documentation is provided with each order to ensure compliant installation.