Connectors should be used to connect an appliance and equipment to the vertical chimney or vent, except were the chimney or vent is attached directly to the appliance. The vent connector should be as short as practical, and the appliance should be located as close as practical to the chimney or vent.
All vent pipes should be listed and labeled. The type and size of the vent for each fuel-gas appliance are dictated by the appliance manufacturer’s installation instructions. The design and installation instructions presented by the vent manufacturer should be followed when designing a vent system. Various types of vents are listed in the building standard, along with the corresponding types of appliances that can be served by the vent, which include, but are not limited to: Type B gas vents; single-wall metal pipes; Type L plastic vents for Category IV appliances; and Type BW vents. Where two or more vent connectors enter a common gas vent, chimney flue, or single-wall metal pipe, the smaller connector should enter at the highest level consistent with the available head room or clearance to combustible material. Vent connectors serving Category I appliances (those with a natural draft and no condensate produced) should not be connected to any portion of a mechanical draft system operating under positive static pressure, such as those serving Category III and IV appliances (those with a draft-inducer fan and producing condensate).
Vent connectors should be installed without dips or sags and should slope upward toward the vent or chimney at least a 1/4-inch per foot (21 mm/m). There’s an exception for mechanical draft systems.www.nachi.org www.apollohome.com