Abstract: A flue cap is provided comprising a body defining a plurality of attachment surfaces adapted for selectively matingly engaging a selected flue from a plurality of different size flues. The flue cap comprises a polymer material defining an attachment surface that is adapted for a close mating engagement with an outer diameter of the selected flue and defines an aperture establishing a passageway in fluid communication with the flue and passing through the cap. A flue assembly is constructed by steps comprising providing an open end of the flue; providing a flue cap comprising a polymer material defining an attachment surface that is adapted for a close mating engagement with an outer diameter of a flue, bonding the flue cap to the flue with an adhesive; and flowing a fluid out the open end of the flue and through the flue cap.