High Pressure Two Ways to Kill a Sunk Sunk Heaters are designed to vent out the top of the heater using gravity flow. High Pressure air (wind) _Acupressure Pressure at the top of the heater can cause the venting process to reverse causing damage to the front grille, melting of the control and destruction of the bummer. The air pressure above the heater must equal the air pressure below the heater for the heater to work properly. The drawing illustrates two common conditions where reverse venting can occur.