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Fireplace opening size relative to flue size

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Fireplace opening : flue size ratio

A fireplace should have a minimum round flue diameter of 200mm. If that condition is met then the flue size should be 15% (or more) of the fireplace opening size.

You can use our fireplace flue calculator to work out the flue size needed for a particular fireplace just click the link below.

Fireplace flue size calculator

Workings out for a round liner

The area of a circle is Π r²

Π r² ≥ area of the fireplace opening * 0.15

r² ≥ area of the fireplace opening * 0.15 / Π



r ≥ √area of the fireplace opening * 0.15 / Π

We want the diameter which is simply 2 x r

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