A fan interlock is required to prevent heater operation when what condition is not met?

The requirement for a fan interlock in a heating system is crucial for ensuring safe and efficient operation. In this case, the correct answer highlights the need for the fan to be operational before the heater can activate. This is important because many heating systems, particularly those that involve forced air circulation, rely on the fan to distribute heated air evenly throughout a space.

If the fan is not operating, there is a significant risk that the heater could operate without distributing heat properly, which may lead to overheating and potential damage to the system or the environment. Thus, the interlock serves as a safety mechanism to prevent the heater from turning on unless the fan is functioning properly, ensuring that heat is effectively and safely circulated throughout the area.

In summary, the interlock prevents the heater from operating unless the fan is running, which is crucial for maintaining the integrity and safety of the heating system.