What is an inefficient height

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The inefficient height describes the height, mouth, and heel of the chimney. This is the difference in the heights between the sophoos axis and the chimney flue. The ineffective height of the narrow or middle flue duct for solid fuel consumers is at least 1/10 of its effective height, for the wood-burning appliances of 1/20 effective height, the liquid and gaseous fuel appliance in the narrow chimney flue must be 150 mm, and an overhanging 250 mm.