Details3" diameter. Clamps and seals on both ends to clamp to heater and first piece of vent pipe. You will need this piece if you are purchasing Z-Vent pipe for your Hot Dawg heater. Z-Vent is a factory built, engineered AL29-4C special gas vent pipe, suitable for positive or negative pressure. Z-Vent is UL tested and listed to UL1738/S636. CAT II, III, IV venting approved.