Use to add length to a straight length of DirectVent Pro. 8 1/2" pipe adds 3"-7" of length, 16" pipe adds 3"-14 1/2" of length, when used with rigid sections of pipe. DirectVent lengths feature a fully unitized, twist-lock construction. Use of sealant is optional, unless specified by the appliance manufacturer. Also available in galvalume (both feature laser-welded outer wall seams). Does not fit DirectVent GS pipe.