Stove Flue Calculator — Approved Document J

Approved Document J 2022, Table 2 sets the minimum flue size for closed solid-fuel appliances: 150mm for an appliance up to 30kW burning any fuel (this can drop to 125mm for a DEFRA / Clean-Air-Act-exempt appliance up to 20kW burning smokeless fuel), and 175mm for appliances above 30kW and up to 50kW. The 200mm figure people often quote is for a fireplace opening up to 500×550mm, not a closed stove. Always check the stove installation manual — never reduce the flue below the stove outlet size.

Approved Document J para 2.8 indicates that a minimum flue height of 4.5 metres (measured from the appliance outlet to the flue terminal) can be satisfactory if the outlet positions in paras 2.10–2.12 are followed; it is guidance, not an absolute requirement. Shorter flues should be justified by a BS EN 13384-1 draught calculation. Per Diagram 17, where the flue exits at or within 600mm of the ridge it must terminate at least 600mm above the ridge; elsewhere on the roof it must be at least 2300mm horizontally from the roof surface and at least 1000mm above where it passes through (or as high as the ridge).

It depends on how airtight the home is. Approved Document J 2022, Table 1 (stove with no flue draught stabiliser) requires 550 mm² of permanent free ventilation per kW of rated output ABOVE 5kW in a leaky dwelling (design air permeability over 5.0 m³/h·m²), but 550 mm² per kW of TOTAL rated output — with no 5kW deduction — in a modern airtight dwelling (5.0 m³/h·m² or less). So an 8kW stove needs about 1,650 mm² in a leaky house but 4,400 mm² in an airtight one. A dedicated direct air supply to the stove can be used instead. If a draught stabiliser is fitted, the figures rise to 300 mm²/kW for the first 5kW plus 850 mm²/kW for the balance.

Flexible liner (316 or 316L stainless steel) is used when an existing chimney is available but needs relining. Twin wall insulated flue is used when there's no existing chimney. Twin wall provides better insulation (better draft) but costs more. Both must comply with Document J. Use our Flue Liner Calculator to estimate the liner length and materials needed.

Building Regulations require a carbon monoxide (CO) alarm in any room where a solid fuel burning appliance is installed. The alarm must be ceiling-mounted (or high on a wall) and comply with BS EN 50291. This is mandatory for all new installations and strongly recommended for existing ones.