Our necessarily high-level description of energy-recovery operation didn’t provide control details. Suffice it to say that an air handler with total-energy-wheel preconditioning poses a control challenge in both dedicated-outdoor-air and mixed-air systems.

Factory-installed controls with preprogrammed control sequences can minimize controls engineering in the field and assure a system that delivers optimum performance.

When bypass dampers control wheel capacity, air pressure in the mixing box fluctuates; therefore, outdoor airflow must be measured and the intake damper must be modulated to maintain the minimum outdoor airflow. Building pressure must be controlled as well.

Note that the return-air pressure also changes with wheel capacity when the central mixed-air unit includes bathroom exhaust. In these applications, bathroom-exhaust airflow must be sensed and maintained at constant flow.

Consider these important control alternatives the next time you design an HVAC system that preconditions outdoor air using total-energy recovery:

• Control wheel capacity using bypass dampers to minimize fan energy and increase control stability.
• Use preheat for freeze protection to minimize the first cost of the heating plant.

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