Question 1

What size flue does a gas boiler need?

Accepted Answer

Most domestic gas boilers up to 70 kW use a concentric (coaxial) flue of 60/100 mm or 80/125 mm diameter. The exact size depends on the boiler manufacturer's specifications and the flue run length. Room-sealed (balanced flue) appliances are the standard for modern installations. AD-J Section 2 and the manufacturer's installation instructions specify minimum flue sizes and maximum flue lengths.

Question 2

How far must a flue terminal be from a window?

Accepted Answer

For room-sealed natural draught gas appliances, the flue terminal must be at least 300 mm from any openable window, air vent, or other opening. For fan-assisted appliances, the distance depends on the appliance input rating — typically 300 mm for appliances up to 7 kW, and 600 mm for those over 7 kW. Full terminal position requirements are shown in AD-J Diagram 25/26.

Question 3

What are the hearth requirements for a wood-burning stove?

Accepted Answer

A constructional hearth for a solid fuel appliance must be at least 125 mm thick (non-combustible), extend at least 150 mm beyond the sides and rear of the appliance, and extend at least 300 mm in front (or 225 mm if the appliance is raised 100 mm+). The hearth must be at least 840 x 840 mm overall. Superimposed hearths (on top of a constructional hearth) must be non-combustible and at least 12 mm thick.

Question 4

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

Accepted Answer

The chimney serving a solid fuel appliance must be at least 4.5 m measured from the top of the appliance to the top of the chimney pot or terminal. The chimney must also extend at least 1 m above the highest point of contact with the roof, and at least 600 mm above the ridge if within 600 mm horizontally of the ridge. See AD-J Diagram 20.

Appliance	Flue Type	Min. Diameter / Size
Gas boiler (room-sealed)	Concentric balanced flue	60/100 mm or 80/125 mm (per manufacturer)
Gas fire (open flue)	Conventional flue	125 mm diameter minimum
Oil boiler (room-sealed)	Twin-wall stainless or concentric	100-150 mm (per manufacturer)
Oil boiler (open flue)	Conventional flue or liner	150 mm diameter minimum
Wood stove (≤ 20 kW)	Twin-wall insulated or lined chimney	150 mm diameter (or 175 mm for > 20 kW)
Open fire	Masonry chimney or liner	200 mm diameter or 185 x 185 mm square min.

Dimension	Requirement
Constructional hearth thickness	≥ 125 mm non-combustible
Minimum overall size	840 mm x 840 mm
Extension in front of appliance	≥ 300 mm (or 225 mm if raised 100 mm+)
Extension to sides and rear	≥ 150 mm beyond appliance
Superimposed hearth	≥ 12 mm non-combustible on constructional hearth

Requirement	Dimension
Minimum height (appliance top to chimney top)	4.5 m (solid fuel)
Above highest roof intersection	≥ 1,000 mm
Above ridge (if within 600 mm horizontally)	≥ 600 mm above ridge
Oil boiler minimum height	As per manufacturer (typically 3-4.5 m)

Part J Flue Sizing & Chimney Requirements

Gas Flue Terminal Positions (AD-J Diagram 25/26)

Minimum Flue Sizes by Appliance Type

Solid Fuel Hearth Requirements (AD-J Section 2.26-2.33)

Chimney Height Requirements (AD-J Diagram 20)

Scotland & Northern Ireland

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Position	Natural Draught (mm)	Fan Assisted (≤ 7 kW, mm)	Fan Assisted (> 7 kW, mm)
Below openable window / air vent	300	300	300
To side of openable window / air vent	300	300	300
Above openable window / air vent	300	300	300
Below gutter / eaves	300	75	200
Below balcony or car port	600	200	200
From ground level	300	300	300
From internal / external corner	600	300	300
From adjacent terminal	300	300	300