Fit, prime, and glue the coupling pipe to the 4-inch x 2-foot pipe. Fit, prime, and glue the 4-inch to 2-inch reducer to the other end of the combustion pipe.

Insert both glued ends into either end of the PVC coupler [5]. Ease the O-ring [6] over the 2-inch piece of pipe, then glue one of the 1¼-inch caps [4] to the 2-inch piece.

Acquire an empty 5-gallon drywall bucket, 1- and 4-inch-diameter PVC pipe, a 4-inch flat PVC end cap, pipe fittings to reduce the 4-inch diameter down to 1 inch, a 1-inch tee fitting, PVC solvent ...