“Y” shaped, 26 gauge aluminized steel. Opening Dims: 3″ O.D. x 4″ O.D. x 3″ O.D. Units connect at a 90˚ angle to the 3″ leg. 4″ vent connection is crimped for ease of vent hook-up. Adjustable elbows are not included. Heater must share common thermostat.