So Many Unit Ventilators, So Little Time...
Unit Ventilators are designed to provide precise temperature control for classrooms by keeping the air circulating in the room while adding fresh air and maintaining a comfortable temperature for students. The problem is that in many cases these units are sized based on peak design conditions (which include occupancy, solar load, and thermal gain and ventilation). Changes to any of these variables, such as occupancy or renovation will cause the system to be oversized. This creates a number of issues, the most important being poor humidity control. There are often additional complaints about airflow noise off the unit ventilator in the classroom disrupting the learning environment.
Recently, Nodaway Valley School District, made renovations to their facilities which created some issues on their existing R-22 (direct expansion) Unit Ventilators with hot water reheat coils. The school district faced a number of problems. The most significant was that the renovations caused a majority of the units to be oversized. These oversized units were short cycling and causing high humidity in the classrooms. This meant they would either have to replace the units or find a better way to control the capacity. Replacing the units would have been a massive and expensive undertaking, so they took an alternative route and installed the APR Control, which gave the systems "continuous capacity modulation" and the ability to match the changing load. This resulted in extending the runtime, while keeping the active portion of the coil below dew point for better dehumidification. Additional benefits the APR Control will provide include system and circuit protection from coil frosting, liquid slugging , and inadequate maintenance budgets.
Before the renovations, the Nodaway Valley School District had encountered noise from the unit ventilators. To address this problem a fan speed control was installed to reduce airflow. The fan speed control prevented teacher complaints about noise in the classroom because the air was no longer whistling through the grate. However, an additional problem would have been created if the APR Controls had not been installed: discharge temperature off the evaporator coil would have dropped significantly, causing the (inactive) hot water heating coils to freeze and burst.
By installing the APR Control, they were able to eliminate and prevent a broad spectrum of possible complications that arise from using direct expansion unit ventilators in classroom and school environments.