Flue

The flue (or chimney system) is the structure that vents the combustion products of a boiler (gas, oil, biomass) or a hot air generator to the outside. It must ensure reliable draft (negative or positive pressure depending on the type of appliance), withstand temperatures, condensate (acidic condensation), and corrosion, while guaranteeing safety (airtightness, fire resistance) and durability. In condensing boilers, sealed systems (type C "balanced flue") are often used, operating under slight positive pressure, with condensate drainage and a slope towards the generator.

Flue: role, types and implementation

The ducts vary depending on the appliance and the method of air supply:

- Type C (sealed/balanced flue): combustion air intake and flue gas exhaust to the outside via a concentric flue (e.g., C13/C33) or two separate flues (C93). Suitable for gas condensing boilers; often operates under positive pressure → sealed flue (class P1/P2).

- Type B (atmospheric): air is taken from the room, fumes are evacuated by natural draft in a masonry or stainless steel duct (depression, class N1).

- Materials : Stainless steel (double wall insulated on the outside/sheath), PPs (polypropylene flue system) for condensation (lower flue gas temperatures, acidic condensates), lining in existing flues.

- Usual classifications (manufacturers): temperature (T200/T400…), pressure (N1 = depression, P1/P2 = overpressure), corrosion (W = humid/acid, D = dry), sealing.

- Installation details : slope towards the condensing generator (≈ 3%), siphon and neutralizer on the condensate drain, sections and lengths conforming to the instructions (elbows included), terminals on the facade/roof at regulatory distances from the bays/outlets, watertight connections and inspection hatches.

Flue: advantages, limitations and points to consider

Interests

• Safety: prevents backflow and CO; airtight system = no air intake from the room.
• Performance: limited losses, stabilized draft; in condensation, cooler fumes and ↑ efficiency.
• Adaptability: lining of existing conduits, concentric for compact construction sites, collective 3CEp in housing.

Boundaries

• Material/temperature incompatibilities**: PPs prohibited beyond its class T; stainless steel required at high temperatures (fuel oil/wood).
• Lengths/bends: beyond the limits specified, degraded draw (pressure losses).
• Acidic condensates: without slope or neutralization, risks of corrosion/odors.

Points to consider

• Sizing: follow the generator's instructions (Ø, equivalent lengths, losses/elbow) and the required class (T, P/N, W/D).
• Sealing: certified seals, smoke test/vacuum measurement; avoid unsealed drilling.
• Firestopping: floor/duct penetrations with intumescent collars/sleeves, installation report; respect fire clearances.
• Terminal location: roof heights/distances from bays and air intakes; avoid recirculations.
• Condensation: 2–3% slope towards the boiler, siphon full of water, neutralizer changed periodically.
• Maintenance: chimney sweeping/cleaning according to fuel type and annual inspection (condition of seals, fixings, siphon, cover); recording in the register.
• Collective housing: in residential buildings, favour 3CEp systems (multiple sealed boilers on a certified common flue).

Anecdote — “The suction cup that saved Orléans”

In Orléans, a condensing gas boiler was experiencing intermittent backflow: horizontal flue without a slope, dry trap, and terminal too close to a sill. A simple fix was implemented: a 3% slope towards the boiler, primed trap, addition of a neutralizer, and raising the terminal to comply with distance requirements. Since then, no more CO has been detected, the plume is reduced, and efficiency is stable—proof that a flue is as much about flue gas hydraulics as it is about compliance paperwork.