What size flue does a wood burning stove need?

Most wood burning stoves up to 10kW require a 150mm (6-inch) internal diameter flue per Approved Document J. Smaller stoves under 5kW may use 125mm (5-inch) if the manufacturer specifies it. Larger stoves of 10-20kW typically need 175-200mm (7-8 inch) flues. The minimum chimney height is 4.5m from the top of the stove to the flue terminal. Always check your stove manufacturer's installation instructions as they take precedence.

What is the minimum chimney height for a solid fuel appliance?

Approved Document J requires a minimum chimney height of 4.5m measured from the top of the appliance (hearth level for open fires) to the top of the flue terminal. This ensures adequate draught for safe combustion. The terminal must also extend at least 600mm above the ridge if within 600mm of the ridge, or be level with the ridge if 600mm-2.3m from it. Heights below 4.5m result in poor draught and potential smoke spillage.

Do I need a flue liner in an existing chimney?

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.

What terminal do I need for a balanced flue gas boiler?

Balanced flue (room-sealed) gas boilers use the manufacturer's specific terminal, which is supplied with the appliance. Terminal positioning must comply with ADF J Diagram 25: minimum 300mm from an openable window or air vent, 75mm below gutters or drain pipes, and specific clearances from corners and boundaries. A plume management kit may be needed if the terminal is near windows, doors, or public pathways.

How many bends are allowed in a flue?

Approved Document J allows a maximum of 4 bends in a flue, and each bend must not exceed 45 degrees from the vertical. Bends reduce the effective chimney draught and increase the risk of soot accumulation. Where possible, use the minimum number of bends and ensure all bends are accessible for sweeping. Offsets (two 45-degree bends forming an S-shape) count as two bends.

Chimney & Flue Sizing Calculator — Document J Compliant

Calculate the correct chimney or flue diameter, minimum height, and terminal type for stoves, open fires, gas fires, and boilers per Approved Document J.

Appliance Type

Determines minimum flue size and height

Fuel Type

Affects liner grade and terminal requirements

Appliance Output

Rated heat output to room

Chimney / Flue Height

From appliance to top of terminal

Flue Type

Existing chimney or new installation

How We Calculate This

This calculator determines the correct flue diameter, minimum chimney height, and terminal requirements based on Approved Document J of the UK Building Regulations. It covers solid fuel (wood and coal), gas, and oil appliances.

Flue diameter

The minimum flue diameter is determined by the appliance type and heat output. Open fires require a minimum 200mm diameter regardless of size. Stoves are sized based on kW output: up to 5kW = 125mm, 5-10kW = 150mm, 10-20kW = 175mm, over 20kW = 200mm. Gas and oil appliances follow manufacturer specifications.

Chimney height

Minimum chimney height ensures adequate natural draught for safe combustion and flue gas dispersal. Solid fuel appliances require at least 4.5m from hearth to terminal. The calculator checks your specified height against the requirement.

Terminal requirements

The flue terminal must prevent rain entry, resist downdraught, and allow free passage of flue gases. For solid fuel, it must also prevent bird nesting. Terminal position relative to the roof ridge and nearby structures is governed by ADF J Diagrams 21-25.