How do you calculate AHU supply airflow?

AHU supply airflow (Q) = Total sensible cooling load / (1.2 × ΔT). Where ΔT is the temperature difference between room temperature and supply air temperature (typically 8–12°C for chilled water AHUs). This gives airflow in m³/s. Multiply by 3600 for m³/h, or by 2119 for CFM.

What is the difference between AHU and FCU?

An Air Handling Unit (AHU) is a large central unit that conditions and distributes air via ductwork to multiple zones — typically serving an entire floor or building section. A Fan Coil Unit (FCU) is a small self-contained unit that recirculates room air through a coil — installed locally in each room or zone. AHUs handle fresh air treatment, filtration, and humidification at a centralised location; FCUs are decentralised and simpler.

What chilled water temperatures are used for AHU coil selection?

Standard chilled water design temperatures for AHU coil selection are: supply (flow) at 7°C and return at 12°C — a 5°C differential (ΔT). Some high-efficiency systems use 6°C/14°C or 7°C/14°C for larger ΔT, reducing chilled water flow rate and pump energy. Always verify with the chiller manufacturer's minimum flow requirements.

How do I calculate AHU fan power?

AHU fan power (kW) = (Q × ΔP) / (η_fan × η_motor × 1000). Where Q is airflow in m³/s, ΔP is total static pressure in Pa, η_fan is fan efficiency (typically 0.65–0.75), and η_motor is motor efficiency (0.88–0.93). Typical AHU external static pressures are 200–600 Pa for standard commercial systems.

What face velocity should I use for AHU cooling coil selection?

The recommended face velocity for AHU cooling coils is 2.0–2.5 m/s. At face velocities above 2.5 m/s, moisture carryover from the coil into the ductwork becomes a risk — causing wet insulation, mould, and supply air temperature problems. At below 1.8 m/s, the coil becomes oversized and uneconomical.

AHU Sizing Guide — How to Calculate Airflow, Coil Capacity and Fan Power

The Air Handling Unit (AHU) is the central component of any ducted HVAC system. It conditions outside and recirculated air — cooling, dehumidifying, filtering, and distributing it through the building ductwork. Selecting the right AHU requires calculating four key parameters: supply airflow, cooling coil capacity, chilled water flow rate, and fan power. This guide walks you through each step with worked examples.

AHU Components — What You Are Sizing

A typical chilled water AHU consists of:

Filter section: G4 pre-filter + F7 bag filter (for commercial); HEPA for healthcare
Cooling coil: Chilled water or DX coil to cool and dehumidify air
Heating coil: LPHW or electric for winter or reheating
Supply fan: Centrifugal or axial fan to overcome duct system resistance
Fresh air damper + return air damper: To mix outdoor and recirculated air
Drain pan: To collect condensate from the cooling coil

Step 1 — Calculate Supply Airflow

Supply airflow is determined from the sensible cooling load and the supply air temperature:

Supply Airflow (Sensible Only): Q = Qs / (ρ × Cp × ΔT) Q = Qs / (1.2 × ΔT) [simplified, in m³/s] Where: Qs = Total sensible cooling load (kW) ρ = Air density ≈ 1.2 kg/m³ at sea level Cp = Specific heat of air = 1.006 kJ/kg·K ΔT = Room temperature − Supply air temperature (°C) Typical: 12°C room temp difference Room = 24°C, Supply = 12°C → ΔT = 12°C For total heat (sensible + latent) using enthalpy method: Q = Total load (kW) / (ρ × Δh) Where Δh = enthalpy difference from room to supply air condition (kJ/kg)

Minimum Ventilation Airflow Check

Always verify the calculated airflow meets minimum ventilation requirements per NBC/ASHRAE 62.1:

Office spaces: minimum 10 L/s per person
Retail: 10 L/s per person + 3 L/s·m² area
Meeting rooms: 10 L/s per person

The supply airflow must satisfy both the cooling load requirement and the minimum ventilation requirement — whichever is larger governs.

Step 2 — Size the Cooling Coil

Coil Entering and Leaving Conditions

Coil Entering Air (Mixed Air) Condition: T_mix = (Q_fresh × T_outdoor + Q_return × T_room) / Q_total Typical design conditions: Outdoor air: 42°C DB / 28°C WB (Mumbai summer) Room air: 24°C DB / 17°C WB Fresh air %: 20-30% for typical commercial Coil Leaving Air (Supply Air) Condition: Cooling to 12°C DB / 11.5°C WB (approximately) This achieves the required room conditions Cooling Coil Capacity (Total): Q_coil = m_air × (h_entering - h_leaving) Q_coil = ρ × Q_supply × (h_entering - h_leaving) = 1.2 × Q_supply × Δh Where Δh = enthalpy difference across coil (kJ/kg) Typical: 30–50 kJ/kg for mixed air to supply air condition

Coil Face Area

Face Velocity: V_face = 2.0 to 2.5 m/s (recommended) Coil Face Area = Q_supply (m³/s) / V_face (m/s) Example: Q = 4 m³/s supply airflow V_face = 2.3 m/s Face Area = 4 / 2.3 = 1.74 m² Standard coil size: 1400mm × 1250mm = 1.75 m² ✓

Step 3 — Chilled Water Flow Rate

Chilled Water Flow Rate: Q_chw = Q_coil / (ρ_water × Cp_water × ΔT_chw) = Q_coil / (4.187 × ΔT_chw) [in L/s, Q_coil in kW] Standard chilled water temperatures: Flow (supply): 7°C Return: 12°C ΔT_chw = 12 - 7 = 5°C Example: If Q_coil = 120 kW: Q_chw = 120 / (4.187 × 5) = 120 / 20.93 = 5.73 L/s = 20.6 m³/h

Use our AHU Sizing Calculator to compute supply airflow, coil capacity, chilled water flow rate, and fan power in one step. Also check the HVAC Heat Load Calculator to first determine your building cooling load.

Step 4 — Fan Sizing (Static Pressure and Power)

Total External Static Pressure

The AHU fan must overcome the resistance of the entire duct system — supply ductwork, fittings, diffusers, return ductwork, and filters. Typical values for commercial buildings:

System Component	Typical Pressure Drop
G4 pre-filter (clean)	50–80 Pa
F7 bag filter (clean)	80–120 Pa
Cooling coil (4-row)	100–150 Pa
Supply ductwork (main)	80–150 Pa
Diffuser and terminal	20–50 Pa
Return ductwork	50–100 Pa
Typical Total ESP	350–650 Pa

Fan Motor Power: P_fan = (Q × ΔP) / (η_fan × η_motor × 1000) Where: Q = Airflow (m³/s) ΔP = Total static pressure (Pa) η_fan = Fan efficiency ≈ 0.65–0.75 (centrifugal) η_motor = Motor efficiency ≈ 0.88–0.93 Example: Q = 4 m³/s, ΔP = 500 Pa, η_fan = 0.70, η_motor = 0.90 P = (4 × 500) / (0.70 × 0.90 × 1000) = 2000 / 630 = 3.17 kW Select: 3.7 kW (5 HP) fan motor with VFD control

Worked Example — Full AHU Sizing

Project: Office floor, 800 m², Mumbai Cooling load: Sensible 95 kW, Latent 28 kW, Total 123 kW Occupancy: 80 people, Fresh air: 10 L/s/person = 800 L/s = 0.8 m³/s Step 1 — Supply Airflow: Q_supply = 95 / (1.2 × 12) = 95 / 14.4 = 6.6 m³/s Min ventilation airflow = 0.8 m³/s (80 persons × 10 L/s) Governs: 6.6 m³/s (cooling load governs) Step 2 — Coil Capacity: Mixed air enthalpy: 20% fresh air (42°C / 80 kJ/kg), 80% return (24°C / 48 kJ/kg) h_mixed = 0.20 × 80 + 0.80 × 48 = 16 + 38.4 = 54.4 kJ/kg Supply air enthalpy ≈ 32 kJ/kg (12°C, 95% RH) Δh = 54.4 - 32 = 22.4 kJ/kg Q_coil = 1.2 × 6.6 × 22.4 = 177.4 kW ≈ 50.4 TR Step 3 — Chilled Water Flow: Q_chw = 177.4 / (4.187 × 5) = 177.4 / 20.93 = 8.47 L/s = 30.5 m³/h Step 4 — Fan Power: ΔP_total = 500 Pa (assumed for this system) P_fan = (6.6 × 500) / (0.70 × 0.90 × 1000) = 3300/630 = 5.24 kW Select: 5.5 kW motor with VFD AHU Selection Summary: Airflow: 6.6 m³/s = 23,760 m³/h Coil load: 178 kW (50.5 TR) CHW flow: 30.5 m³/h (7°C / 12°C) Fan motor: 5.5 kW Face area: 6.6 / 2.3 = 2.87 m² → 1700 × 1700mm coil

AHU vs FCU — When to Use Which

Parameter	AHU	FCU
Typical capacity	10–200+ kW	0.5–10 kW
Fresh air handling	Yes — central OA treatment	Limited (via separate DOAS)
Filtration	High efficiency (F7, HEPA)	Basic G4 only
Zone control	Complex (VAV boxes needed)	Simple (on/off or 3-speed)
Best for	Large open offices, malls, hospitals	Hotel rooms, small offices

Common AHU Sizing Mistakes

Sizing only for sensible load and ignoring latent: The cooling coil must handle total heat (sensible + latent). Using only sensible load for coil selection results in an undersized coil that cannot dehumidify adequately in Indian humidity conditions.
Using too high a face velocity: Face velocities above 2.5 m/s cause moisture carryover — condensate droplets are blown into the ductwork, causing wet insulation and mould growth downstream.
Not sizing for dirty filter pressure drop: Fan ESP must be calculated at dirty filter conditions. Clean filter pressure drop × 2 is often used for the dirty condition to size the fan correctly.
Ignoring duct system balance: If the AHU serves multiple zones with different pressure drops, the longest and highest-resistance branch governs. Always size the fan for the critical path.

Conclusion

AHU selection is a four-step process: calculate supply airflow from cooling load and temperature difference, size the cooling coil for total heat removal and appropriate face velocity, calculate chilled water flow based on coil capacity and temperature differential, and finally size the supply fan for the system's total static pressure. Getting all four correct ensures your AHU performs reliably and efficiently throughout the building's lifecycle.

Speed up your AHU selection with the MEPMate AHU Sizing Calculator. Also use the Psychrometric Calculator to accurately determine mixed air and supply air enthalpies for precise coil duty calculations.