Technical Booklet L Combustion Appliances and Fuel Storage Systems·Page 38·3.4
Flue pipe diameter
A flue pipe must have the same diameter or equivalent cross-sectional area as the appliance flue outlet and should not be smaller than the size recommended by the appliance manufacturer.
A flue pipe should have the same diameter or equivalent cross-sectional area as that of the appliance flue outlet and should be not smaller than the size recommended by the appliance manufacturer.
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