Reline a deteriorated masonry flue with a UL 1777-listed stainless steel liner sized and insulated to the connected appliance

Steps

1. Perform a Level II inspection (including video scan) to confirm the existing masonry flue's condition and establish that relining is the correct remedy
2. Select a liner product listed to UL 1777 for the fuel type in use; note that UL 1777-listed liners are intended for Category I gas, Type L vented oil, and solid-fuel appliances with continuous flue-gas outlet temperatures not exceeding 1000°F, and are not intended for Category II–IV gas appliances or systems with positive chimney pressure or condensing acidic flue gas
3. Size the liner diameter/cross-section to the connected appliance's outlet per the appliance manufacturer's instructions and the liner manufacturer's sizing tables, not simply to fit the existing masonry flue
4. Insulate the liner per the liner manufacturer's installation instructions (wrap or pour insulation) where required for the application and clearance-to-combustibles needs
5. Install per the liner manufacturer's listed instructions and applicable code (NFPA 211, NFPA 54/National Fuel Gas Code, and the locally adopted mechanical/residential code), including proper top and bottom terminations
6. Complete a post-installation Level II-equivalent inspection/video scan to confirm the liner is fully seated, unobstructed, and properly terminated before returning the appliance to service