What device monitors water flow in a pool heater?

The device that monitors water flow in a pool heater is the flow switch. A flow switch is specifically designed to detect whether water is flowing through the heater. It ensures that the heater only operates when there is adequate water flow, which is crucial for preventing overheating and potential damage to the system. By monitoring the flow, it helps maintain efficiency and safety in the operation of the pool heater.

In contrast, an aquastat is primarily used for temperature control and regulation, not specifically for monitoring flow. A low water cut-off serves to shut off the heater when the water level drops below a certain point, but it does not measure the flow rate directly. A pressure switch may detect water pressure but is not specifically focused on monitoring flow rates either. Thus, the flow switch stands out as the correct choice for this particular function in a pool heater.