Gas Fitter Test 2025 – 400 Free Practice Questions to Pass the Exam

The flow switch is designed to monitor the flow of water within a hydronic heating system and respond to changes in that flow, which typically indicate a load change in the system. When there is a load change—such as a radiator demanding more or less heat—the flow of water through the system will either increase or decrease. The flow switch detects this change and can signal the system to adjust accordingly, ensuring that sufficient water flow is maintained to provide the necessary heating or cooling to the desired areas.

In contrast, the low water cut-off is primarily a safety device that shuts off the system if the water level becomes too low, preventing damage to the boiler. The expansion tank is used to accommodate the expansion of water as it heats up and maintain system pressure, but it does not directly respond to load changes. The aquastat monitors the temperature of the water and controls the boiler operation based on temperature settings, but it does not directly respond to flow changes created by load variations. Thus, for its responsiveness to load changes in a hydronic heating system, the flow switch is the correct choice.