[1] A sail switch typically operates through the use of a paddle or a diaphragm which gets displaced due to the force of fluid or air moving past it.
Sail switches find application in the detection of fluid flow and measurement of fan speeds.
[2] A sail switch might be used to protect a fan forced pellet stove, central heating system, electric heating element from being energized before the air flow from the blower is established.
[2] 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.