Technical Booklet L Combustion Appliances and Fuel Storage Systems·Page 59·4.14
Flue pipe sizing requirements
The flue pipe must be the same size as the appliance flue outlet and have at least the same cross-sectional area as the appliance outlet.
The flue pipe should be the same size as the appliance flue outlet and the flue should have at least the same cross-sectional area as that of the appliance flue outlet.
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