A low water cut-off device on an automatically fired hot water heating boiler rated at 350,000 Btuh is:

A low water cut-off device is crucial for ensuring the safe operation of hot water heating systems, especially for automatically fired boilers. The requirement for such a device is primarily to prevent the boiler from running when there is insufficient water to cover the heat exchanger, which could lead to overheating and potential damage.

In the case of the question, the statement that the device is required when any portion of the circulating system is located below the boiler is correct. This is because if any part of the system is below the boiler, it may be susceptible to losing water due to gravity or negative pressure, which could create a hazardous condition. The low water cut-off device helps to maintain a safe operational level by shutting off the boiler in case the water level drops dangerously low.

This safety measure is a standard requirement in various boiler applications to prevent accidents and ensure that the heating system operates within safe parameters. Therefore, having a low water cut-off device in situations where portions of the circulating system are lower than the boiler is essential for maintaining safety and operational integrity.