What will happen if the water level in the boiler is too low?

When the water level in the boiler is too low, the boiler can overheat. This condition occurs because water in the boiler serves as both a heat transfer medium and a coolant for the heating surfaces. If there is insufficient water, the metal surfaces of the boiler can become exposed to direct heat, leading to overheating. This overheating can result in damage to the boiler, including warping, cracking, or even a catastrophic failure due to the high temperatures the metal can reach without the moderating effect of the water.

The other choices do not directly address the primary risk associated with low water levels in a boiler. For example, while it's true that low water might influence steam pressure, the immediate and most critical concern is overheating, which can lead to severe safety risks and equipment failure. Fuel consumption might decrease initially if there isn't enough water to generate steam efficiently, and the safety valve typically operates based on pressure rather than directly influenced by low water levels.