The Ontario Building Code | Additional Circuit Vents and Relief Vents
(1) Except as provided in Article 18.104.22.168. and in Sentence 22.214.171.124.(7), the minimum size of an additional circuit vent or relief vent installed in conjunction with a circuit vent is permitted to be one size smaller than the required size of the circuit vent, but need not be larger than 2 in.
(2) The size of the soilor waste pipe acting as a relief vent in accordance with Sentence 126.96.36.199.(4) shall be in conformance with Table 188.8.131.52., 184.108.40.206. or 220.127.116.11. or Article 18.104.22.168., whichever size is the largest considering the hydraulic load drained into the soil or waste pipe.