Flow switches are used to make sure that there is water flowing through the heater assembly. They are directional devices, whereas pressure switches are not. They are not as common as pressure switches though. The flow switch will work on either side of the pump whereas the pressure switch has to be installed on the pressure or output side. As long as the heater is flowing with water, the direction it flows doesn't matter. They have a small cord that comes off of them and connects to the controller. They use a little paddle looking piece that activates the switch by water flow. If the switch comes out, it must be replaced. Your GFI would be tripping and there would be no heat if the hot tub flow switch wasn't operating properly. When spa flows fail to work, they close up and it's hard to tell if they are doing their job.
