ULC 636

The Standard covers the design, construction, and performance of gas venting systems intended for negative or positive pressure venting of gas-fired appliances producing flue gases having temperatures under the following: (1) Class I venting systems are suitable for gas-fired appliances producing flue gas temperatures of more than 135°C but not more then 245°C; (2) Class II venting systems are suitable for gas-fired appliances producing flue gas temperatures of 135°C or less; (3) Class II venting systems are further classified into four temperature ratings as follows: (A) Up to and including 65°C; (B) Up to and including 90°C; (C) Up to and including 110°C; and (D) Up to and including 135°C.