Flow switch
Also called: flow sensor, pressure switch, no flow light, low-flow lockout.
A flow switch is the sensor on a salt cell or pool heater that stops the device from running until enough water is moving through it. A salt cell stops generating chlorine below its flow switch, and a heater refuses to fire or locks out below its minimum flow.
Why it matters
The expensive mistake is on a salt pool. Run a variable-speed pump below the cell's flow switch and the pump is quiet, the bill is tiny, and the chlorine has silently stopped. A schedule set on a spotless filter can fall below the flow switch by the time the filter is due, because a loading filter lowers flow at the same speed. On a heater, a flow-switch lockout is the #1 no-heat cause, usually a dirty filter, a closed valve or a pump set too slow. After cleaning a salt cell, put it back the right way round, because most cells and their flow switches are directional.
In numbers
A heater wants roughly 10 GPM per 100,000 BTU, so a 400,000 BTU heater wants about 40 GPM. To find your own floor, drop 100 RPM at a time until the device complains, then go back up 200 RPM.