Which two elements are used to control combustion air flow in a fire heater?

The correct choice emphasizes the essential components involved in regulating the combustion air flow within a fire heater. The register on each burner serves a vital function by allowing for the precise control of the air-fuel mixture entering the burner, which is crucial for maintaining efficient combustion and preventing issues such as incomplete burning or excessive emissions. The stack damper contributes by managing the draft within the flue system, ensuring that the combustion gases exit the heater effectively while also allowing for adjustable air flow depending on the operational needs of the heater. Together, these elements ensure that combustion occurs efficiently and safely, optimizing performance in heating systems.

Other options, while they include important components of heating systems, do not specifically focus on the air flow control aspect seen in this context. For instance, the gas valve and flame sensor are primarily aimed at managing the fuel supply and ensuring safety rather than air flow. Similarly, the blower fan and draft regulator can influence air and combustion processes, but they serve broader functions and might not directly control air flow in the same way that registers and dampers do. Circuit breaker and pressure gauges pertain to electrical and pressure monitoring but do not directly affect air flow management.