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The flow rate of water in a heating coil of an air handler is primarily controlled by the balancing valve on the heating loop. A balancing valve is specifically designed to adjust and regulate the flow of water, ensuring that each part of the heating system receives the necessary amount of water for optimal operation. This is particularly important in systems with multiple coils or branches, as it helps maintain consistent heating output and efficiency.

By manipulating the balancing valve, technicians can increase or decrease the water flow to specific coils. This process is crucial for achieving the desired temperature and ensuring that the entire air handling unit operates effectively. It enhances energy efficiency by preventing excessive flow, which could lead to overheating or inefficient heat transfer.

Other options such as an aquastat and mixing valves play different roles. The aquastat is primarily a temperature control device that regulates the temperature of the water in the heater, but it does not control the flow rate directly. Mixing valves, while useful in regulating temperature by mixing hot and cold water, are not specifically designed for controlling flow rate in the heating coil of an air handler. Therefore, the balancing valve is the most appropriate method for managing the flow rate within that specific context.