An air handler, or air handling unit (often abbreviated to AHU), is a device used to condition and circulate air as part of a heating, ventilating, and air-conditioning (HVAC) system. The PRB has four air handlers. AHU-1 and AHU-2 serve the lab side, AHU-3 serves the clean rooms, and AHU-4 serves the office side. AHU-1 and AHU-2 use 100% outside air and AHU-3 and AHU-4 recirculate the air with only a small outside air intake. Click the highlighted text to learn more courtesy of wikipedia.