A flow sensor for a pool heater monitors the passage of water through the pipe to the heater. If water flow is interrupted, the sensor signals the heater to shut off. Pool heaters are designed to operate only when water flows through the system. Running a pool heater dry can damage or break the heating element. If the heater is not activating, a faulty flow sensor may be to blame. Running a bypass is a quick way to test the flow sensor, but is not a solution for a defective sensor.
Shut off the circuit breaker that controls electricity to the pool heater and the pool pump that sends water through the heater. If the heater operates on gas, close the gas valve in the line connected to the heater.
Use a screwdriver to take out the screws holding the housing over the heater. The flow sensor is mounted at the point the water pipe attaches to the heater. Two wires connect from the sensor to the heater.
Detach the two wires from the sensor. On some heaters, the wires may terminate with slip-on connectors that pull off the sensor contacts. On other models, the sensor contacts have two screws that loosen to take out the wires.
Splice the two wires together with a piece of electrical tape.
Turn on the circuit breaker for the pool pump and activate the pump normally. Once water is flowing to the heater, turn on the heater as well. If the heater activates and warm water recirculates back to the pool, the flow sensor is defective. Shut off the equipment and do not operate the heater until the sensor is replaced.