A 3/4" blow-off can be used on high-pressure boilers with less than how many square feet of heating surface?

The 3/4" blow-off valve is specifically designed for use on high-pressure boilers, and its application is limited by the size of the heating surface in the boiler system. In this context, the correct answer indicates that this type of blow-off is suitable for boilers with less than 100 square feet of heating surface. This limitation is primarily due to safety and efficiency considerations; a blow-off that is too small for a larger boiler could lead to insufficient drainage of sediment and scale, which could adversely affect the performance and safety of the boiler.

Thus, the specification of less than 100 square feet ensures that operators use appropriate equipment for blow-off applications, maintaining the integrity of the boiler operation and ensuring adherence to regulatory standards. The choice of 100 square feet serves as a benchmark to distinguish between different sizes and types of blow-off apparatus suitable for various boiler configurations.