System designed to exhaust natural gas or propane burning, category II appliances, category III appliances, or category IV appliances, or Type BH systems.
Designed to handle high efficient appliances.
This type of system is listed to UL 1738, ULC S636, or UL 1777.
For more information on special gas vents: https://duravent.com/products/commercial-products/special-gas-vent/