To adjust the air ratio for different firing rates on an adjustable head burner, what must be done?

Adjusting the air ratio for different firing rates on an adjustable head burner involves changing the position of the draw assembly. The draw assembly is responsible for controlling the mixture of air and fuel that enters the burner. By adjusting its position, you can effectively change how much air mixes with the fuel, which is crucial for achieving optimal combustion at various firing rates.

When the firing rate of the burner changes, the amount of air needed for complete combustion also changes. By repositioning the draw assembly, the technician can manipulate the airflow, ensuring the burner receives sufficient air to maintain the desired combustion efficiency and performance. This is particularly important in oil burner applications, where maintaining the right air-to-fuel ratio is critical for efficiency, emissions control, and safe operation.

The other options do not effectively address the need for precise air and fuel mixture adjustments in relation to firing rates. Changing the fuel supply would not necessarily alter the air-fuel ratio directly; while increasing the burner size could potentially change the firing rate capability, it does not inherently allow for finer adjustments of the air ratio. Modifying the oil viscosity impacts how the oil burns, but it does not directly relate to adjusting the air ratio for different firing rates.