These water-sensing bilge pumps work on the principle of impeller resistance. The pump turns on every 2-1/2 minutes for about one second to sense for water. If water is present, the pump will sense the resistance of the water against the impeller and continue to run until all the water is removed. After that, it will resume checking for water every 2-1/2 minutes. A separate float switch is not required.
The Rule model 25S pump has been discontinued and replaced by model 25SA.