What type of appliance would typically require a safety relief valve?

A low-pressure steam boiler typically requires a safety relief valve because it operates under conditions that could lead to dangerous overpressure situations. The purpose of the safety relief valve is to automatically open and relieve excess pressure, thus preventing the risk of an explosion or damage to the boiler system. These safety devices are crucial in maintaining the operational integrity of steam boilers, where steam generation can rapidly increase pressure if not properly controlled.

In contrast, the other appliances listed may have different safety mechanisms or do not operate under the same pressures that necessitate a safety relief valve. For example, condensing boilers and high-efficiency furnaces may operate within different design safety margins and may use other forms of safety devices such as limit switches or combustion safety controls. Standard electric heaters generally do not operate under pressure conditions that would require a safety relief valve, as they heat water through electrical resistance rather than generating steam. Thus, low-pressure steam boilers stand out as appliances that inherently require this type of safety valve for safe operation.