Built-Up Air Handling Units

A built-up AHU is not a packaged unit. It has no self-contained refrigerant circuit. Cooling comes from chilled water piped in from a central chiller plant. Heating comes from hot water from a boiler. The AHU is just a cabinet: a fan, coils, filters, and dampers assembled in a mechanical room.

Note: AHUs come in an enormous range of styles and configurations. What this lesson covers is a fairly standard built-up unit with chilled water cooling, hot water heating, an economizer, and a VFD. Your job will throw different combinations at you: single-coil units, DX coils, steam heat, no economizer, face-and-bypass dampers instead of modulating valves. The sequence logic and the troubleshooting approach are the same; the hardware varies. Learn the standard example first, then adapt.

The key difference from an RTU: the RTU stages compressors and gas heat, all or nothing in steps. A built-up AHU modulates. The chilled water valve opens 0 to 100 percent. The hot water valve opens 0 to 100 percent. The OA damper modulates. The fan runs at variable speed. Everything is continuous, not staged.

You will find built-up AHUs in medium to large commercial buildings: office towers, hospitals, universities, government facilities. Anywhere the mechanical plant is centralized and the air system needs to handle large volumes with precision.

The sequence is more complex than an RTU, but it follows the same logic: maintain supply air temperature setpoint, bring in the right amount of outdoor air, and move the right volume of air through the system.