Which of the following is NOT a possible cause of low water level in a boiler?

In the context of boiler operation, low water level can lead to serious operational issues and potentially dangerous conditions. Each of the first three options directly contributes to a decrease in water level within the boiler system.

Leaks caused by breakage or corrosion allow water to escape from the system, directly reducing the water level. Excessive blowdown, which involves intentionally removing water from the boiler to maintain water quality, can also lead to lowered water level if not properly managed. Boiler feed pump failure would stop or reduce the replenishment of water into the boiler, which can quickly lead to a low-water condition as steam is generated.

Inadequate insulation, on the other hand, does not directly affect the water level in the boiler. While poor insulation can lead to heat loss and inefficiencies in the boiler's operation, it does not cause a reduction in the amount of water contained in the system. Thus, it is not a cause of low water level, making it the correct choice in this context.