Size of Chimney Flues
(1) Except for chimneys serving fireplaces, the size of a chimney flue shall conform to the requirements of the solid fuel-burning appliance installation standard referenced in Sentence 6.2.1.4.(1) and Article 9.33.1.2.
(2) Where a chimney flue serves only one solid fuel-burning appliance, the flue area shall be at least equal to that of the flue pipe connected to it.
Article 9.21.2.4 Ontario Regulation 332/12 Building Code, O. Reg. 332/12: BUILDING CODE (ontario.ca) © Queen’s Printer for Ontario, 2021.