When must two or more safety valves be used on a steam heating boiler?

The requirement for using two or more safety valves on a steam heating boiler is based on the need to ensure sufficient safety and pressure relief capacity. When a boiler has a size that necessitates a safety valve larger than 4 1/2 inches in diameter, it indicates that the potential pressure and volume of steam produced could exceed what a single valve can safely manage in the event of a malfunction or pressure buildup.

Using multiple safety valves provides a redundant safety mechanism, ensuring that even if one valve fails or becomes obstructed, there are still other valves available to relieve the pressure. This is a critical aspect of boiler safety regulation and design, as it prevents the risk of overpressure, which can lead to catastrophic failure and accidents.

The context regarding other options illustrates that merely having a valve larger than 3 inches or focusing on temperature does not inherently address the capacity requirements for safe operation. Similarly, stating that all domestic boilers require multiple safety valves disregards the specific capacity needs governed by the boiler's design and operation specifications. Therefore, the necessity for multiple safety valves is primarily dictated by the size and design of the boiler related to its pressure relief needs.