An Air Handling Unit (AHU) is a central piece of HVAC equipment that conditions and circulates air: it draws return and fresh air, filters it, heats or cools it through coils, sometimes humidifies or recovers energy, and then delivers the treated air through ductwork. AHUs range from small rooftop packages to large custom units moving 1,000 to over 200,000 m³/h, and they form the heart of commercial, hospital, and industrial climate systems by keeping temperature, humidity, and air quality within tight setpoints.
Core Components of an AHU
A standard AHU contains a mixing box (return plus fresh air), one or more filter stages (G4 pre-filter and F7 bag filter typical), a cooling coil (4 to 8 rows of chilled water at 5–7°C), a heating coil (hot water at 40–60°C or electric), a fan (plug or centrifugal), and a drain pan. Larger units add a heat-recovery wheel, humidifier, and sound attenuators. Each component adds external static pressure, usually 150 to 600 Pa in total for the fan to overcome.
How an AHU Conditions Air
Return air mixes with fresh air (often 10% to 30% outside air for economy), passes the filters, then crosses the cooling coil where it is dehumidified and lowered to the supply temperature. In winter the heating coil raises it, and a humidifier trims moisture to 40%–60% relative humidity. The supply fan pushes the air out; a return fan pulls it back, and an energy wheel can recover 60% to 85% of the exhaust energy.
Sizing and Selection Numbers
Capacity follows the load: a medium office AHU at 20,000 m³/h with a 10°C cooling rise handles about 230 kW of sensible load using roughly 40 kW of fan power at 400 Pa. Coil rows, face velocity (1.5 to 3 m/s), and fin density set the pressure drop; more rows mean better heat transfer but higher fan energy. Choose the smallest coil that meets the load to balance first cost against operating hours.
AHU Configuration Comparison
| Configuration | Airflow range | ESP | Typical use |
|---|---|---|---|
| Packaged rooftop | 2,000–40,000 m³/h | 150–300 Pa | Shops, schools |
| Modular AHU | 5,000–100,000 m³/h | 200–500 Pa | Offices, hospitals |
| Custom AHU | 50,000–200,000+ m³/h | 300–600 Pa | Factories, airports |
| AHU + ERV | 1,000–80,000 m³/h | 250–550 Pa | Energy-saving buildings |
Selection Checklist
- Choose a packaged rooftop unit when airflow is under 40,000 m³/h and roof space exists.
- Choose a modular AHU when you need mixed coils, high filtration, or a hospital-grade setup.
- Add an energy-recovery wheel when outside-air fraction exceeds 20% and climate is extreme.
- Specify F7 or better filtration for healthcare and clean-process applications.
- Confirm total ESP (150–600 Pa) matches the selected supply and return fan curves.
AHU Maintenance
Filters are the first consumable: G4 pre-filters last 3 to 6 months, F7 bags 6 to 12 months, and a clogged filter can raise ESP by 100 to 300 Pa, pushing the fan past its curve and cutting airflow by 15% to 30%. Coils need annual cleaning to keep heat-transfer within 5% of design, and drain pans must be kept clear to avoid microbial growth and odour. Belts, where used, are checked every 2,000 to 4,000 hours; a slipping belt loses 5% to 10% of delivered airflow before it visibly fails and alerts staff.
Controls and BMS Integration
A modern AHU talks to the building management system over BACnet or Modbus, with sensors for supply temperature (±1°C), humidity (40% to 60% RH), and static pressure. The controller modulates the cooling and heating valves and trims the fan with a VFD to hold setpoint while saving 15% to 40% of energy versus constant-speed operation. CO&sub2; demand control can further cut fresh-air flow at night, and alarm limits on filter differential pressure tell staff exactly when to change media instead of on a fixed calendar that may be wrong.
Energy Recovery Options
When outside-air fraction exceeds 20%, an enthalpy or sensible wheel recovers 60% to 85% of exhaust energy, while a plate exchanger reaches 50% to 75% with no cross-contamination risk. A run-around coil loop suits spaces that forbid air mixing, at 40% to 60% effectiveness. Payback is typically 1 to 4 years in extreme climates, and many green codes now require recovery above a threshold airflow of about 1,000 m³/h, making it standard rather than optional on most new builds and major retrofits today.
Siting and Airflow Path
Place the AHU where return and fresh-air intakes are clear of exhaust and contamination by at least 3 to 5 m, and keep the supply plenum short to limit duct loss below 200 Pa. A rooftop unit needs a level curb and weatherproof transitions; an indoor AHU needs a service aisle of 0.8 to 1.2 m on the filter side. Poor siting adds 10% to 20% hidden pressure that the fan must overcome, quietly raising power and noise beyond the original design intent and the approved equipment budget for the project.
Summary for Specifiers
An AHU is only as good as its filters, coils, and controls. Size it to the load, pick filtration to the space risk, and add energy recovery whenever outside air exceeds 20%. Plan service access and a short duct path, then commission the BMS loops so the unit holds setpoint while saving 15% to 40% of energy. A well-specified AHU delivers stable comfort and air quality for 15 to 20 years with only routine filter and coil care required.
Quick Reference Numbers
Useful AHU figures: 1,000 to 200,000 m³/h of flow, 4 to 8 coil rows, 5 to 7°C chilled water, 40 to 60°C heating water, 150 to 600 Pa total ESP, and 60% to 85% energy recovery when outside air exceeds 20%. Filter stages of G4 plus F7 protect coils and air quality, and face velocity of 1.5 to 3 m/s balances transfer against pressure drop. Holding these ranges steady during design keeps the unit efficient, quiet, and within the fan's published operating envelope without surprises at commissioning time on the project.
Keruite manufactures this equipment at an 80,000 m² facility in Dezhou, China, with an in-house AMCA-210-style test laboratory; every unit is balanced to G2.5 and performance-tested before shipment.
FAQ
What is the difference between an AHU and an FCU?
An AHU is a large central unit that conditions and distributes air for a whole zone or building through ducts, typically moving 1,000 to 200,000 m³/h. A Fan Coil Unit is a small local device serving one room with a fan and coil and no duct network, usually 200 to 2,000 m³/h. AHUs handle filtration and fresh air centrally; FCUs provide local temperature control only.
How much outside air should an AHU use?
Outside-air fraction is usually 10% to 30% of total flow, set by occupancy and ventilation codes, with a minimum based on persons (about 20 to 30 m³/h each). Wheel or plate energy recovery is added when that fraction exceeds 20% in climates with large indoor-outdoor temperature gaps, recovering 60% to 85% of exhaust energy.
Why specify F7 filtration in an AHU?
F7 bag filters capture fine particles down to about 1 micron at 70% to 90% efficiency, protecting coils from fouling and improving indoor air quality for sensitive spaces like hospitals and labs. Lower G4 panels catch only coarse dust, so F7 or better is the standard where health, precision, or cleanroom-adjacent duty is required.
What does ESP mean on an AHU schedule?
ESP is External Static Pressure, the resistance the AHU fan must overcome to push air through filters, coils, and ducts, typically 150 to 600 Pa. Underestimating ESP starves the space of air; overestimating wastes fan power. The selected fan curve must deliver the design flow exactly at the stated ESP for the unit to perform as specified.