What prevents water from draining from a boiler if a return line is broken?

The Hartford loop is a critical component in boiler systems designed to prevent water from draining back from the boiler when the return line is compromised. It is essentially a piping configuration that creates a loop that maintains a water seal in the system. This design serves to keep water within the boiler and prevent any potential loss of water through the broken return line.

When the return line is broken, the Hartford loop helps to ensure that there is always a sufficient water level in the boiler, providing both safety and functionality. The water in the loop effectively prevents air from entering the boiler and maintains pressure within the system, which is essential for the boiler to operate correctly.

Other options like the backflow valve, expansion tank, and purge valve play roles in the overall system but do not specifically prevent water from draining in response to a broken return line as effectively as the Hartford loop does. The backflow valve primarily prevents reverse water flow in a plumbing system, the expansion tank accommodates water volume changes due to heating and cooling, and the purge valve is used to remove air from the system but doesn't serve the function of water retention in the case of a broken return line. Therefore, the Hartford loop is specifically designed for this scenario and is the correct answer.