What is the function of the middle tab on a dial of a conventional fan/limit control?

The middle tab on a dial of a conventional fan/limit control is designed to turn the fan on. This component is crucial for regulating the operation of the fan in heating and cooling systems. When the temperature in the system exceeds a preset level, the fan is activated to ensure appropriate heat dissipation or air circulation, thus maintaining the desired temperature range.

In essence, by turning the middle tab to the appropriate position, the technician ensures that the fan operates as needed based on the conditions in the system. This function is vital for effective system performance, as it helps manage air flow, supports temperature regulation, and protects the equipment from overheating.

The other options, while relevant to fan control, do not specifically pertain to the functionality of the middle tab. Setting a high limit involves adjusting the maximum temperature threshold that can be reached before the system shuts off, while controlling the temperature generally refers to the broader operation of the thermostat or control system as a whole, not just the individual switch for the fan.