Final answer:
The middle tab on a dial of a conventional fan/limit control sets the temperature at which the fan will turn off in a forced air heating system, for efficient energy use and proper heat distribution.
Step-by-step explanation:
The middle tab on a dial of a conventional fan/limit control is typically the setting for the fan off temperature in a forced air furnace. This control is responsible for turning the blower fan on and off during the heating cycle based on the air temperature within the furnace. When the furnace heats up to a certain temperature, this control turns the fan on to blow warm air throughout the dwelling.
Once the furnace cools down after the burner shuts off, the fan continues to run until it reaches the temperature set at this middle tab, at which point it will turn off, ensuring all the remaining heat is effectively distributed without wasting energy.