Flue Sizes

Document J recommends a minimum flue size of 200 mm diameter or 185 mm nominal square for a fireplace recess with an opening up to 500 mm x 550 mm, for an open, solid fuel fire. For larger recesses, the flue requires a free opening of 15% of the area of the recess.

The Building Regulations documents specify a range of minimum flue sizes according to the type and rated output of the installed appliance. It should be noted that low output appliances connected to flues sized for greater outputs will be likely to generate acidic condensates. The use of clay liners for these flues will resist this corrosion.

Recess Opening
The fire recess is measured as AxB.

An alternative method of calculating the size of the flue is to use the flue sizing chart.

Height
In order to obtain an adequate draw to remove flue gases, a flue should be a minimum of 4.5m high from the top of the fire throat unit to the chimney outlet.

For stability, the height of a chimney stach (H) should not exceed 4.5 times its minimum horizontal width (W). H is measured from the level of the highest point of the intersection of the roof and the outer wall of the chimney and a datum level 0.15 m max.. up the terminal. The top of the terminal may be higher than this datum level.

Flue OutletsFlue outlets for flues serving solid fuel appliances should be finished above the shaded areas shown in the diagram below so that the flue gases are carried away from the building.

Bends
Ideally, flues should be straight. The use of offsets is not recommended in the Building Regulations documents but, when needed, they can be achieved by the use of bends, typically 22½°, 30°, 37½° and 45°. Only one offset is allowed for each flue.

The requirements of the Building Regulations documents allow a maximum 45° bend for both gas and solid fuel appliances.The use of different lengths of straight flue between bends allows flexibility in obtaining an offset to avoid any structural obstacle.

Only complete factory made liners should be used for both straight lengths and bends so that the joint rebates remain intact.

Installation of flue liners

All liners should be laid with the female rebate uppermost. This ensures that condensates formed from the flue gases are not able to flow out from the flue into the surrounding masonry where they would cause structural deterioration and staining of walls as well as providing a path for the possible escape of dangerous flue gases.

The joints between liners must be made with a fireproof mortar or manufacturer’s proprietary fireproof sealant.As work progresses any mortar ingress into the flue way should be wiped clean. This ensures a smooth passage for the flue gases and for sweeping.

It can be achieved by using a coring ball, as shown below. This operation should not be delayed until the chimney is finished as the mortar may have hardened and be difficult to remove.

The space between the outside of the flue liner and the chimney stack should be filled with insulating material such as a weak lime mortar or lightweight insulating concrete.

Installation of Terminals

The chimney terminal (or “pot”) is an important component of a chimney system. It takes the flue gas outlet away from the turbulence created by the bulk of the chimney stack and can reduce the likelihood of water ingress. Also, it can put the flue outlet above the high-pressure zone, which causes down draught problems.

The terminal should be embedded into the top of the stack by 125 mm or one quarter of the height of the terminal, whichever is the greater.

A straight-sided terminal should be butted against the top of the last liner as shown below.

A tapered terminal should be fitted over the last liner as shown below.
Note that the top of the last liner lies within the corralled brickwork in this
case.