Why is a pressurized furnace generally not recommended for large boilers?

A pressurized furnace is generally not recommended for large boilers primarily because it can lead to flue gases leaking into operational areas, which poses significant health hazards. In a pressurized system, any malfunction can cause an uncontrolled release of flue gases, which are toxic and can create dangerous conditions for personnel in the vicinity. This risk of exposure to harmful gases is particularly critical in larger installations where the volume of gases can be substantial.

Moreover, large boilers typically operate under considerable variations in pressure and temperature, which can exacerbate vulnerabilities in the system. Ensuring that flue gases remain contained is essential for maintaining a safe working environment. Techniques like natural draft or atmospheric burners are often utilized in large boiler systems because they reduce the risks associated with pressurized environments.

While it's true that other aspects, such as maintenance requirements and the potential for mechanical failure, are important considerations when evaluating different furnace systems, the safety implications associated with flue gas leakage in pressurized systems make it the most critical factor in this context.