Air Handling Unit (AHU)
CSI Division 23, HVAC
A larger, typically indoor mechanical unit that mixes outside and return air, filters and conditions it using chilled or hot water coils, and pushes it out through ductwork to the building, the workhorse of a central-plant HVAC system, distinct from a rooftop unit's smaller, fully self-contained packaged design.
Why it matters
An AHU's filtration and coil performance directly determine a building's indoor air quality, not just its temperature, so specifying and maintaining the wrong filter rating or letting coils go unmaintained can create a comfort and health issue long before anyone notices a temperature problem.
On a real project
A large office building's mechanical room houses an AHU that draws chilled water from the building's chiller plant, conditions a mix of outside and return air, and distributes it through ductwork to VAV boxes serving each floor's zones.
Who this matters most to
An HVAC Technician commissions and balances the AHU, verifying it delivers its designed airflow and temperature to each zone it serves. A Quality Control Manager confirms the unit's startup and balancing report matches the design airflow before accepting the system as complete.
Where this goes wrong
An AHU is started up and put into service before its air balancing report is actually reviewed against the design airflow numbers, under pressure to get cooling running for other trades finishing nearby work. Several zones end up under-ventilated for weeks before the balancing discrepancy is caught during commissioning, well after the unit was assumed to be performing correctly.