One advantage of the low-pressure flexible watertube boiler is that:

The advantage of low-pressure flexible watertube boilers lies in their design, which allows them to accommodate the thermal expansion and contraction that occurs during operation. Traditional rigid boilers may experience significant stresses due to these temperature changes, which can lead to structural issues over time. However, low-pressure flexible watertube boilers are engineered to be more adaptable, allowing the tubes to flex without sustaining damage or stress. This flexibility reduces the risk of leaks and failures associated with thermal expansion, ultimately leading to enhanced durability and reliability in the boiler's performance.

In contrast, the other options imply features that are not accurate regarding the nature and maintenance of low-pressure flexible watertube boilers. While tubes may have a long lifespan, they will eventually need replacement due to wear and tear. Maintenance is still necessary to ensure the boiler functions efficiently and safely, and proper feedwater treatment is essential to prevent scaling and corrosion, making the assumption of unnecessary treatment misleading.