Abstract: Air switches operated by ventilator pressure are used in combination with an electric circuit to cause an indicating light to be energized as long as a selected volume of air is moving through the ventilator per unit of time. A red light and buzzer are employed to indicate a low pressure, and a meter is also employed for visual inspection of the condition of the system. Time delay relays are employed in the circuit in an arrangement to allow the system to reach operating condition and also to prevent functioning of the indicating means by temporary undesirable pressure conditions in the ventilator. The system also includes indicating means in the area of the fan to allow the workman to determine the condition of the system at the fan.