What may be present in the return line to ensure the oil is maintained at the proper temperature for easy circulation?

Maintaining the proper temperature of fuel oil in the return line is crucial for ensuring efficient circulation and optimal performance of the heating system. Heaters are employed for this purpose, as they warm the fuel oil to a temperature that facilitates its flow, especially in colder environments where viscosity can be a concern. By keeping the oil at an appropriate temperature, heaters prevent issues such as clogging or inconsistent flow, which could lead to operational problems in the burner system.

Filters, while important for removing contaminants from the oil, do not directly affect the temperature of the oil. Pumps are responsible for moving the oil through the system but also do not have a role in regulating its temperature. Mixers, on the other hand, would primarily be used to blend different oils or additives but are not employed specifically to maintain the temperature in the return line. Thus, heaters are the correct answer as they directly ensure that the oil is at the desired temperature for effective circulation.