A ratio relay is primarily used to _____.

A ratio relay is primarily used to provide sequence control of more than one actuator. This functionality is crucial in HVAC systems where multiple components must work together in a coordinated fashion to maintain optimal performance and efficiency.

For instance, in a system where both a fan and a damper need to operate in synchronization to control airflow effectively, a ratio relay ensures that the actions of these actuators are proportioned correctly relative to one another. This sequencing helps maintain balance in the system, preventing issues such as over or under-ventilation.

The other options do not capture the primary role of a ratio relay. While controlling temperature changes can be a function in HVAC systems, it typically falls under different control strategies involving thermocouples or thermostats. Manual control of airflow focuses more on operator intervention rather than automated sequencing. Lastly, switching on emergency systems is a critical function but is generally handled by dedicated emergency relays or systems designed specifically for safety rather than the sequential control that a ratio relay provides.