Gas fire flue sizes, liners and fluepipes: Approved Document J

Note 7 of 11 · free to read

Minimum flue sizes for gas fires, fluepipe sizing and clearance, how a flexible liner is fitted, and when a debris collection space is needed.

Key points

Table 5 gives default flue sizes before the gas appliance is chosen; check its instructions once the model is known. For a radiant or convector fire, a new flue is 125 mm circular, or 16,500 mm² rectangular with no side under 90 mm.
An existing rectangular flue for the same fire needs at least 12,000 mm², with no side under 63 mm
For an ILFE or DFE fire in an opening up to 500 × 550 mm, the default minimum flue dimension is 175 mm; some models specify a circular flue of at least 125 mm.
The connecting fluepipe matches the appliance outlet. Measure its 25 mm combustible-material separation from the outer flue wall, or from the outer surface of the inner wall for a multiwall product.
A flexible liner goes in as one jointless length, and an unlined masonry flue needs a 12-litre debris space

Approved Document J, Section 3, gives guidance for gas-fire flues in England. Once an appliance is chosen, check the flue against its installation instructions. The default sizes below apply when a flue is provided or refurbished before the appliance is known (paragraphs 3.17 and 3.19).

Minimum flue sizes (Table 5)

Radiant or convector gas fire, new flue: circular at 125 mm diameter, or rectangular at 16,500 mm² with no dimension under 90 mm.
Radiant or convector gas fire, existing flue: circular at 125 mm diameter, or rectangular at 12,000 mm² with no dimension under 63 mm.
Inset live fuel effect (ILFE) or decorative fuel effect (DFE) fire in a fireplace opening up to 500 mm × 550 mm: a minimum flue dimension of 175 mm, circular or rectangular. Some ILFE and DFE appliances need a circular flue of at least 125 mm instead, and their instructions say so.
DFE fire in a larger opening: sized by calculation, as paragraph 2.7 describes.

Connecting fluepipes and chimneys

The connecting fluepipe is the same size as the appliance flue outlet, in diameter or equivalent area. The chimney flue has at least the same cross-sectional area as that outlet (paragraph 3.20).
For a connecting fluepipe, Table 7 measures the minimum 25 mm separation from the outer surface of the flue wall; for a multiwall product, it uses the outer surface of the inner wall. Through a combustible wall, floor or roof that is not a fire compartment boundary, a non-combustible sleeve can provide that space to the relevant flue wall. The space may contain suitable non-combustible insulation. Fire compartment boundaries need their fire separation maintained.
A masonry chimney has at least 25 mm of masonry between the flue and combustible material, and flueblock walls are at least 25 mm thick.

Flexible metal liners

A flexible metal flue liner goes in as one complete length, with no joints within the chimney (paragraph 3.37).
Apart from sealing at the top and the bottom, the space between the chimney and the liner is left empty, unless the liner maker's instructions say otherwise. Double-skin liners follow their maker's instructions.
Flexible metal liners are for relining an existing chimney, never for the primary liner of a new chimney. They may also connect a gas back boiler in a fireplace recess to its chimney (paragraph 1.40).
A new or replacement liner counts as building work. In England, use the appropriate building-control route: normally notify before starting, or use the applicable self-certification route where an authorised installer and the work are covered. That route still requires the completion certificate and notification. Sweep the flue before relining it. A suitable liner can also reduce an oversize flue to the size the appliance needs.

Debris collection space

A flue has a debris collection space at its base unless it is lined, built of flue blocks, or a factory-made metal chimney with a flue box (paragraph 3.38).
Where one is needed, it holds at least 12 litres, is at least 250 mm deep below the point where the flue gases enter the chimney, and is readily accessible for clearing, for example by removing the appliance.
An unlined masonry chimney is the case it exists for: loose material falls down it and collects at the fire's connection.
For radiant convector fires and inset live fuel effect fires that discharge through a spigot, keep at least 50 mm between the end of the spigot and any surface (paragraph 3.38).

Terminals and the flue route

A natural-draught open flue of up to 170 mm diameter is fitted with an outlet terminal. For a larger flue the blockage risk is judged locally; where jackdaws or squirrels nest, a cage made for solid fuel, with a mesh between 6 mm and 25 mm, may be acceptable if its outlet openings total at least twice the flue's cross-sectional area (paragraph 3.25).
A natural-draught flue runs straight and vertical. An appliance with a rear outlet may connect through a horizontal section of no more than 150 mm, and any essential bend is no more than 45° from the vertical (paragraph 1.48).
So that it can be swept, a flue has no more than four changes of direction, each up to 45°, and no more than two from a sweeping access point to the next access point or the flue outlet. A 90° factory-made bend, elbow or tee counts as two (paragraph 1.49).
Seal any gap between a fireplace surround, cladding or dry lining and the fireplace opening, so flue gases cannot escape into the void behind (paragraph 1.53).

Notice plate

Where a hearth, fireplace, flue or chimney is provided or extended, a robust, indelibly marked notice plate is fixed in an obvious place inside the building, such as next to the consumer unit, the chimney or hearth, or the water stopcock. It records where the hearth or flue is, the category of flue and the appliance types it can take, the type and size of flue or liner with its maker's name, and the installation date (paragraphs 1.57 and 1.58).