Sail switch
A sail switch, vane switch or flow switch is a mechanical switch that is switched on or off in response to the flow or non-flow of a fluid such as air or water. A sail switch typically operates through the use of a paddle which gets displaced due to the force of fluid moving past it. Sail switches find application in the detection of fluid flow and measurement of fan speeds. A sail switch might be used to protect a central heating system electric heating element from being energized before the air flow from the blower is established.[1] Sail switches might also be used to alarm if a ventilation fan in a hazardous location fails and air flow has stopped. For some HVAC systems, a sail switch can activate an electronic air cleaner, a humidifier, or other equipment in response to airflow from the system fan.[2][3]