What device is commonly found on the cold water fill line alongside the pressure reducing valve?

The device commonly found on the cold water fill line alongside the pressure reducing valve is a backflow preventer. This device is essential for protecting the potable water supply from contamination. The backflow preventer ensures that water flows in one direction, preventing any potentially contaminated water from flowing back into the clean water supply system. This is crucial in maintaining the safety and integrity of the drinking water.

While a pressure gauge is useful for monitoring the system's water pressure, it does not provide backflow protection. A drain valve is typically used for emptying the system or performing maintenance, and a flow control valve is used to manage the rate of water flow but does not offer backflow prevention. Therefore, the backflow preventer is integral to ensuring that the water supply remains uncontaminated in conjunction with the pressure-reducing valve.