Clause (8.12.2) - Before replacing an existing appliance or connecting a vent connector to a chimney, the chimney flue shall be examined to ascertain that the chimney
(a) is properly constructed;
(b) is lined with a tile or metal liner;
(c) is clear and free of soot, creosote, or obstructions;
(d) will effectively conduct the products of combustion outdoors; and
(e) is sized in accordance with Clause 8.13.
HVAC TECH GROUP - Director's Advisory FS-110-07 https://hvactechgroup.com/article.php/20071130200551500