Which device ensures that an oil burner has an adequate air supply for combustion?

The correct answer is the vaporstat, as it is designed to monitor and control the amount of air and oil mixing to maintain the efficiency of combustion within the burner. In an oil burning system, adequate air supply is crucial for proper combustion as it helps in achieving the right fuel-to-air ratio. The vaporstat works by sensing the vapor pressure and ensuring that there is enough air to facilitate complete combustion, which helps prevent issues like soot formation, incomplete burning, and inefficient energy use.

In contrast, while the fan assembly is also critical for providing air to the combustion process, its primary function is to move air into the combustion chamber rather than regulate the air supply. The manifold is responsible for distributing the oil to the burners and does not play a role in controlling the air. The flame guard serves as a safety device to prevent backfire and protect the system but does not directly influence the air supply necessary for combustion.