Terminology

• Semi-custom (modular) AHU: pre-engineered standard modules
• Custom AHU: built to project dimensions and features; common for very large airflows
• Fan array: multiple small direct-drive fans in parallel instead of one large fan
• Mixing box: section blending outdoor and return air
• Coil section: chilled water, hot water, steam, or DX coils
• Filter section: pre-filters and final filters by MERV rating
• Energy recovery section: wheel, plate, or run-around coil
• Single-zone / VAV / DOAS: unit serving one zone, variable air volume system, or ventilation only
• Casing: double-wall insulated panels; thermal break ratings for condensation control
• Coil pull space: clearance to remove coils for replacement

Standards

Ratings

• Semi-custom units suit light to medium duty; custom units often selected above about 20,000 cfm
• Casing leakage, deflection, and thermal performance per AHRI 1350 and ASHRAE 111 testing where specified
• Filter efficiency by MERV per ASHRAE 52.2

Test standards

• AHRI 430: central-station air handling unit supply fans
• ASHRAE 52.2: filter efficiency (MERV)
• AMCA 210: fan performance
• ASHRAE 62.1: ventilation; ASHRAE 90.1 or IECC energy requirements

Components

• Fans: plenum fans or fan arrays with variable-frequency drives
• Coils: water or refrigerant coils with drain pans
• Filters: pre-filter and final filter banks
• Dampers: outdoor, return, relief, and isolation
• Energy recovery: wheels or plates
• Controls: DDC controllers, sensors, airflow measuring stations

Detailing

• Size mechanical rooms for unit length plus coil pull, filter access, and door clearance
• Locate outdoor air intakes away from exhausts, loading docks, and cooling towers
• Provide condensate traps sized for fan static pressure and floor drains nearby
• Coordinate shafts and duct mains from AHUs to floors
• Provide vibration isolation and acoustic treatment near occupied spaces
• Plan housekeeping pads, structural supports, and replacement routes

Uses

Specific uses

• Offices with VAV systems
• Hospitals and laboratories with high outdoor air
• Schools and universities
• Auditoriums and large single zones
• Data centers and process spaces (custom units)

Pros and cons

Pros

• Centralized maintenance and filtration
• High outdoor air and humidity control capability
• Energy recovery and economizer integration
• Long service life with quality casings

Cons

• Large mechanical rooms and shafts
• Requires central plant or DX source
• Custom units have long lead times
• Duct distribution takes ceiling space

Manufacturing and cost

Construction

• Units built in sections at the factory; shipped in pieces and assembled on site
• Connected to ducts, piping, power, and controls; tested and balanced

Main manufacturers and suppliers

• Trane
• Carrier
• Daikin Applied
• York (Johnson Controls)
• Greenheck

Trade associations: ASHRAE, AHRI, and AMCA.

Typical costs

• AHU equipment: roughly $1,200 to $4,000 per 1,000 cfm for basic units; $2,000 to $6,000 per 1,000 cfm for mid-range configurations

Sources

• University of Michigan: AHU design guideline (custom vs. semi-custom), https://umaec.umich.edu/desguide/tech/23/DG230013.pdf
• University of Missouri: 23 73 23 custom air handling units specification, https://www.umsystem.edu/sites/default/files/media/documents/general/237323-custom-air-handling-units_aod.pdf
• University of Maryland: 23 73 13 modular indoor central-station AHU specification, https://www.umaryland.edu/media/umb/af/dc/documents/division-23/237313P---Modular-Indoor-Central-Station-Air-Handling-Units-03-07-22.pdf
• New Rising Sun Arts Centre: air handling unit cost overview, 2026, https://new.risingsunartscentre.org/news/air-handling-unit-cost-overview-and-pricing-2026.html
• Wikimedia Commons: Air handling unit.JPG, by P199 (Public domain), https://commons.wikimedia.org/wiki/File:Air_handling_unit.JPG