Flue Liner Calculator — What Size Flue Liner Do I Need?

This calculator determines the correct flue liner diameter based on your stove output and fuel type, then calculates the total liner length needed (including allowance for bends), and estimates vermiculite insulation bags.

Flue liner sizing (Approved Document J, Table 2)

• Closed appliance up to 30kW, any fuel: minimum 150mm (6") diameter
• Closed appliance above 30kW and up to 50kW, any fuel: minimum 175mm (7") diameter
• Above 50kW: not tabulated in AD J — size by design to BS EN 13384-1
• Clean Air Act / DEFRA-exempt appliance up to 20kW (smokeless/low-volatiles): minimum 125mm (5") diameter
• Gas fire/stove (AD J Table 5): minimum 125mm (5") diameter

The flue must never be smaller than the appliance outlet collar (AD J paras 2.4–2.5), and the appliance manufacturer's installation manual always takes priority.

Liner length calculation

Total liner length = Chimney height + Bend allowances

• Each 45-degree bend adds approximately 0.5m
• Each 90-degree bend adds approximately 1.0m
• Approved Document J: flues should be as straight and vertical as possible, with any bends angled no more than 45° to the vertical
• No more than four changes of direction in total (each up to 45°) — a 90° factory-made bend, elbow or tee counts as two 45° bends (AD J, Bends in flues / Diagram 15)

Vermiculite insulation

Vermiculite is poured into the void between the flue liner and the chimney wall. The volume needed is the annular cross-section (chimney void area minus the round liner area) multiplied by the chimney height, with a small allowance for settling and taper. There is no single "metres per bag" figure — it depends on the void size and liner diameter — so the calculator derives the bag count from this geometry. Tell it whether the void is round (clay-lined) or square/rectangular (brick), because a square flue holds about 27% more vermiculite than a round one of the same width.

Chimney height requirements

A minimum total flue height of about 4.5m can give adequate draught (AD J para 2.8). The flue must terminate at least 600mm above the ridge if it passes through the roof at or within 600mm of the ridge; elsewhere it must be at least 2.3m horizontally from the roof and either 1m above where the chimney meets the roof or as high as the ridge (Approved Document J, Diagram 17).

Standards

UK Building Regulations Approved Document J (Combustion appliances and fuel storage systems). Flue liners per BS EN 1856-2 (metal liners). Flue system design per BS EN 15287.