BOILER & DOMESTIC WATER HEATER VENTING
Venting stack design is broken down into basically three venting categories as determined by the appliance / boiler manufacture’s operating manual. The venting category is coordinated with the U.L. listing the type of flue product required. The most common categories are:
Characterized by atmospheric gas fired burners that have zero and or negative vent pressure. The flue product applied would be Metal – Fab B vent
Characterized by forced draft burners that fire natural gas and or No 2 oil, and have positive vent pressure. The flue product that would be applied is Metal – Fab PIC double wall with a 304 ss liner, U.L. listed, positive pressure rated joints and have 1 to 4 in of factory installed mineral wool between the inner and outer shell.
Characterized by appliances advertised as condensing with a positive vent pressure. The flue product that would be applied is the Metal – Fab Corr Guard with AL29-4C stainless steel liner and sealed joints that contain boiler pressurized flue gas and the condensed products of combustion . Some manufactures allow plastic for Category IV venting but there is a substantial risk of applying plastic that does not have a U.L. listing, the joints are glued, and no provisions for thermal expansion.