Review on DiversiTech CVENT 2 Concentric Vent 2: Efficient Airflow for Optimal Performance by Sean Bouchard

I installed this with a Navien NPE 210A without a tank. Installed great. At full load the water heater works fine, but at low load my unit starts to create a harmonic resonance with a bang against the wall, causing the unit to light up and giving you a COLD shower! As soon as I disconnect the inlet channel, the device is restored immediately. It is not necessary to draw in outside air, so I no longer use the intake duct on this vent. This was a very annoying issue that needed to be fixed! I wonder if a 3" version would have worked, but I don't spend money anymore trying to get it right with a coax vent.