Flue Sizing

To clear the smoke and fumes from a fireplace or appliance the flue must be correctly sized. If it is too small an insufficient volume of air will pass through it – resulting in a smoky fire. Too large a flue will cause cooling of the flue gasses, slowing down the flow with similar results.

All the above should be increased by 25mm if there is a bend in the flue.

Type of FireFlue SizeTaken from:
400 / 450 mm open fire225 mm (min. 200 mm)Building Regulations 2002
Larger open fire15% of opening size (WxH)and BS 6461 1984
Closed appliance150 mm

The above table gives recommended flue sizes for most types of fireplace and appliances. For chimneys less than 5 metres high use one size larger than quoted. Always refer to manufacturers installation instructions for their recommendations.

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