Chimney & Flue Sizing Calculator — Document J Compliant

Approved Document J Table 2 sizes a non-exempt closed appliance (such as a wood or multi-fuel stove) at a minimum 150mm (6-inch) diameter flue right up to 30kW rated output. A DEFRA/Clean Air Act-exempt appliance up to 20kW — which covers most modern wood stoves — may use a 125mm (5-inch) flue, as may any pellet appliance. Only above 30kW and up to 50kW does the minimum rise to 175mm (7-inch). The 125mm allowance is tied to exemption status, not simply to being under 5kW. The flue must never be smaller than the appliance spigot or the size the manufacturer specifies (AD J para 2.4), and a minimum flue height of 4.5m from the top of the stove could be satisfactory (AD J para 2.8). Always check the stove manufacturer’s installation instructions as they take precedence.

Approved Document J para 2.8 states that a minimum flue height of 4.5m — measured from the appliance outlet (or hearth level for an open fire) to the flue terminal — could be satisfactory for a solid-fuel appliance, provided the outlet position guidance is followed. It is guidance rather than an absolute mandate; the height needed also depends on the building, flue type, number of bends and local wind. Per AD J Diagram 17, a terminal at or within 600mm of the ridge must be at least 600mm above the ridge; elsewhere on the roof it must be at least 2300mm horizontally from the weather surface and either 1000mm above the point where the chimney passes through it or as high as the ridge. Heights below 4.5m risk poor draught and smoke spillage.

Yes. When installing a wood burner or stove into an existing masonry chimney, a flue liner is required under Approved Document J. Unlined chimneys are too large (typically 225mm square) and have rough, porous walls that accumulate creosote and allow flue gas leakage. A 316-grade stainless steel flexible liner (for wood) or 904L grade (for multi-fuel/coal) should be installed the full length of the chimney.

Balanced flue (room-sealed) gas boilers use the manufacturer’s specific terminal, which is supplied with the appliance. Terminal positioning must comply with the Table to Approved Document J Diagram 34: for a fanned-draught balanced flue this includes a minimum 300mm from an openable window or air vent, 75mm below gutters or drainpipes, and 200mm below eaves, plus set clearances from corners and boundaries. Use Diagram 35 where the terminal is near a roof window. A plume management kit may be needed if the terminal is close to windows, doors, or public pathways.

Approved Document J para 1.49 advises limiting changes of direction between the appliance outlet and the flue outlet to no more than four, each up to 45 degrees, with no more than two between points of access for sweeping. Where bends are essential they should be angled no more than 45 degrees to the vertical (AD J para 1.48). Bends reduce the effective chimney draught and increase the risk of soot accumulation, so use the minimum number and ensure all are accessible for sweeping. A factory-made 90-degree bend, elbow or tee counts as two 45-degree bends (AD J Diagram 15).