On a hydronic heating system, what control responds to a load change?

In a hydronic heating system, the control that responds to a load change is the flow switch. The flow switch is designed to detect changes in the flow of water through the system. When there is a change in demand for heating—such as when a thermostat activates due to a temperature drop—the flow switch can sense this alteration in water flow. If the system's demand increases, the flow will also increase, which can trigger the system to adjust accordingly, either by activating pumps or altering heat-generating equipment to meet the new load requirements.

Other components play important roles in the system as well but do not directly respond to load changes. The low water cut off is primarily a safety device that shuts down the system if the water level drops below a safe threshold, preventing potential damage or unsafe conditions. An expansion tank is used to absorb excess pressure fluctuations in the system due to water expansion and does not react to load changes. The aquastat is a temperature control device that regulates the temperature of the water in the system but does not directly manage the response to load changes in the same way that a flow switch does. Hence, the flow switch is the correct choice for responding to load changes in a hydronic heating system.