The relief valve for a residential low-pressure hot water boiler system should be?

The relief valve for a residential low-pressure hot water boiler system is designed to protect the system from excessive pressure that can lead to hazardous situations. The correct pressure setting for these types of systems is typically 30 psig, which corresponds to a pressure of 30 pounds per square inch gauge. This is a standard safety measure to ensure that the relief valve opens at a pressure that is safe for the boiler operation, allowing excess pressure to vent safely, thus preventing potential malfunctions or explosions.

The pressure at which a relief valve operates is critical for maintaining system safety. A setting of 30 psig is appropriate for residential systems, reflecting common practice within the industry. This value indicates the pressure at which the relief valve will begin to open, allowing steam or water to escape if the system's pressure exceeds that limit.

In contrast, the other options listed represent pressures that do not align with standard safety practices for low-pressure hot water boiler systems. For instance, pressures expressed in psia (gauge pressures plus atmospheric pressure) or lower absolute pressures do not meet the required specifications for such systems. Therefore, a relief valve set at 30 psig effectively provides the necessary safety threshold for safe operation of residential low-pressure hot water boilers.