What device prevents combustion from occurring inside the return line?

The device that prevents combustion from occurring inside the return line is a flame arrestor. Flame arrestors are critical safety components designed to allow the flow of air and vapor while preventing the ignition of flames from advancing into areas where fuel and air could potentially accumulate. They achieve this by using a series of baffles or mesh that cools and extinguishes the flame front, effectively stopping it from propagating through the return line.

In the context of heating systems, particularly those involving oil burners, ensuring that combustion does not travel backward into the return line is essential for safety and system integrity. Combustion within a return line can lead to explosive situations, making flame arrestors vital for preventing such occurrences. This prevents hazards that can arise in the event of leaks or other failures in the system.

The other devices mentioned serve different purposes; for example, a backflow preventer is designed to stop the backward flow of liquids, primarily in water supply systems, while a check valve allows for one-way flow in a piping system, and a temperature gauge is used to monitor the temperature of substances in a system but does not prevent combustion. Understanding the specific role of each component helps to appreciate why a flame arrestor is the appropriate answer.