Technical Booklet L Combustion Appliances and Fuel Storage Systems·Page 39·3.5
Flue sizing based on appliance type
A flue must be at least the size shown in Table 3.2 for the relevant appliance, or the size of the appliance flue outlet or manufacturer recommendation, whichever is larger.
A flue should be not less than the size shown in Table 3.2 that is relevant to the particular appliance, and not less than the size of the appliance flue outlet or that recommended by the appliance manufacturer.
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