Final answer:
A feed water regulator in a boiler controls the water level by regulating the flow of water into the boiler.
Step-by-step explanation:
The function of a feed water regulator in a boiler is to control water level by regulating the flow of water into the boiler. It ensures that the water level is maintained at the desired level, preventing the boiler from overheating or running dry. The regulator monitors the water level and adjusts the water flow accordingly, maintaining a stable operating condition of the boiler.