Fireplace liner
Liner shape
Measure the front opening of the firebox (width × height).
Appliances on this liner
Height: from the appliance draft hood or flue collar to the top of the liner.
Appliance
Appliance 2
Extra elbows in the liner
Count only bends beyond the one long-radius 90° at the bottom of the liner (already allowed for) and the two 90s included with any lateral.
GPH is on the burner nozzle or rating plate. Outlet is the appliance flue collar.
Existing flue (optional check)
Sizes come from the 2021 International Residential Code (R1003.14–R1003.15), International Fuel Gas Code / NFPA 54 (Section 504 and Tables 504.2(1) and 504.3(1)), and International Mechanical Code (801.2.1, 801.6, 801.7), which sends oil venting to NFPA 31. Table values are not interpolated: height rounds down and lateral rounds up to the next table row, which always gives the safer answer. Appliance and liner manufacturer instructions and the local code official have the final say.