Installed immediately above the appliance, the test port allows plumbing and heating professionals to sample flue gases and ensure the appliance is operating at optimum efficiency. Centrotherm Eco Systems InnoFlue® Test Port, For Use With: InnoFlue® Polypropylene Vent Systems, 6 in Dia Nominal, Polypropylene, Gray, 7.2 in OD x 12.7 in L, Domestic