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The flue is mandatory for a gas-boiler heating system. Safety, efficiency and passing the gas inspection all depend on it. Flue mistakes are a leading cause of carbon-monoxide poisoning and connection refusals.
Coaxial flue (pipe-in-pipe) — for wall-hung sealed-combustion boilers. Inner pipe (60 mm) exhausts, outer annulus (100 mm) draws combustion air from outside. 60/100 mm total. Installed horizontally through the wall, 0.5–3 m long. No vertical chimney needed — simple and cheap.
Pros: cheap install, doesn't burn room air. Cons: length limit 3–5 m, tip icing in severe frost (solved with anti-ice nozzle), must respect windows and vents on the facade.
Stainless sandwich flue — double wall: inner pipe, 25–50 mm basalt wool, outer pipe. Vertical through roof or up the external wall above the ridge. Used for open-combustion floor boilers and when coaxial isn't possible. Service life 15–25 years. More expensive, needs fire-rated penetrations.
Brick flue — only acceptable if lined with a stainless insert. Acid condensate destroys unlined brick in 3–5 years. Lining cost 30 000–70 000 ₽.
Minimum 5 m from burner to tip for steady draft. Tip relative to roof: within 1.5 m of ridge — at least 0.5 m above; 1.5–3 m — no lower than ridge; beyond 3 m — no lower than a 10° line from ridge. Coaxial: at least 2 m above ground and 0.5 m from windows/vents.
Must match the boiler outlet — neither more nor less. Never reduce — kills the draft.
Boiler type | Flue diameter |
|---|---|
Wall-hung (coaxial) | 60/100 mm |
Wall-hung (separate pipes) | 80 mm (each) |
Floor ≤ 30 kW | 130 mm |
Floor 30–50 kW | 150 mm |
Floor 50–80 kW | 180–200 mm |
Gas combustion produces water vapour and CO₂. On cooling (especially in condensing boilers) condensate forms with pH 3–4 — a weak acid that attacks most metals.
AISI 316 (molybdenum, acid-resistant) — best for condensing boilers, 20–30 years, d150 sandwich 3 500–6 000 ₽/m
AISI 304 — fine for conventional boilers, 15–20 years, d150 sandwich 2 500–4 500 ₽/m
Galvanised steel — only as the outer sleeve. Forbidden for the inner pipe: zinc corrodes in 1–3 years releasing toxic compounds
Through a wooden ceiling: 200 mm clear to combustibles using a basalt-filled penetration box (2 000–5 000 ₽). Through the roof: master-flash or metal flashing (3 000–8 000 ₽). Condensate trap at the bottom is mandatory (1 500–4 000 ₽). Inspection tee (2 000–5 000 ₽). Wall brackets every 2–2.5 m (800–2 000 ₽ each).
Galvanised instead of stainless — dangerous, fails in 1–3 years
No condensate trap — condensate drips into boiler, corrodes heat exchanger (15 000–40 000 ₽ repair)
Too short — below 5 m or below ridge, poor draft
Horizontal runs over 1 m — total horizontal ≤ 3 m, each section ≤ 1 m
Leaky joints — always use high-temp sealant
Type | Materials | Install | Total |
|---|---|---|---|
Coaxial (through wall) | 5 000–12 000 ₽ | 5 000–10 000 ₽ | 10 000–22 000 ₽ |
SS sandwich d130 (6 m) | 35 000–60 000 ₽ | 20 000–35 000 ₽ | 55 000–95 000 ₽ |
SS sandwich d150 (8 m) | 45 000–80 000 ₽ | 25 000–40 000 ₽ | 70 000–120 000 ₽ |
Sandwich along exterior (8 m) | 50 000–90 000 ₽ | 25 000–40 000 ₽ | 75 000–130 000 ₽ |
Brick-flue lining (8 m) | 25 000–50 000 ₽ | 20 000–35 000 ₽ | 45 000–85 000 ₽ |
StroyInvest24 installs flues to SNiP and boiler-maker specs — guaranteed gas-inspection approval and decades of safe operation.
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