How to Size Air Handling Units (AHU) for Large Buildings
Updated: Oct 3
An air handling unit is not correctly sized simply because its catalogue airflow matches the required CFM or L/s value. A defensible air handling unit sizing process must satisfy supply airflow, outdoor-air ventilation, cooling and heating capacity, humidity control, fan pressure, filtration, acoustics, physical space and part-load operation at the same time.
This engineering guide explains the complete AHU sizing calculation in SI and I-P units. It covers airflow, ventilation, mixed-air conditions, cooling-coil duty, chilled-water flow, coil face area, fan static pressure and motor power, followed by a detailed commercial-building example.
Quick answer: Calculate supply airflow from the zone sensible load, verify ventilation and process-air requirements, determine the mixed-air condition, size the cooling coil from the total enthalpy change, establish the AHU cross-section from component face velocity and select the fan from the actual critical-path pressure loss. Finally, verify humidity, acoustics, energy use, controls, access and part-load performance.
Go straight to your next design step:
The duct system converts the AHU airflow requirement into a fan-pressure requirement. For a detailed explanation of duct sizing, fitting losses, critical-path analysis and fan ESP reporting, review the HVAC Duct Pressure Loss Design Suite.
IMAGE 1: AHU sizing workflow
Air Handling Unit Dimensions: Typical Sizes by Airflow
The AHU cross-section is set by coil face velocity. Most manufacturers design for about 2.5 m/s across the cooling coil, so the coil face area is A (m²) = V (m³/s) ÷ 2.5. Casing, insulation and frame allowances are then added to get the overall width and height. Indicative sizes:
1 m³/s (≈2,100 cfm): coil face ≈ 0.4 m²; unit roughly 1.0 m wide × 0.8 m high
2.5 m³/s (≈5,300 cfm): coil face ≈ 1.0 m²; roughly 1.5 × 1.1 m
5 m³/s (≈10,600 cfm): coil face ≈ 2.0 m²; roughly 2.0 × 1.5 m
10 m³/s (≈21,200 cfm): coil face ≈ 4.0 m²; roughly 2.6 × 2.2 m
15 m³/s (≈31,800 cfm): coil face ≈ 6.0 m²; roughly 3.2 × 2.5 m
Length depends on the sections fitted. A basic filter–coil–fan unit is often 2.5–4 m long, while units with heat recovery, mixing boxes, attenuators and humidifiers commonly reach 6–10 m. Confirm with the selected manufacturer, and keep coil-withdrawal and filter-access space about equal to the unit width on the service side.
What Does “AHU Size” Actually Mean?
AHU size is a group of coordinated performance requirements, not one number. Two AHUs with the same nominal airflow can have very different cooling coils, fan efficiencies, filter areas, pressure capabilities, casing leakage levels, sound performance and physical dimensions.
AHU parameter | What it controls | Consequence if selected incorrectly |
Supply airflow, L/s or CFM | Space sensible load and air distribution | Poor temperature control, drafts or insufficient circulation |
Outdoor airflow | Indoor air quality and building pressurization | Code noncompliance, excess humidity or unnecessary energy use |
Cooling-coil duty, kW or TR | Sensible and latent cooling | High room temperature or poor humidity control |
Heating-coil duty, kW | Winter heating and supply-air control | Low space temperature or unstable discharge air |
Fan pressure, Pa or in. w.g. | Ability to move air through the system | Low airflow, excessive noise or high energy consumption |
Filter efficiency and resistance | Air cleanliness and fan duty | Poor IAQ or falling airflow as filters become dirty |
Face area and casing dimensions | Velocity, carryover, pressure drop and footprint | Condensate carryover, high pressure loss or installation problems |
Sound power | Acoustic performance | Plantroom breakout noise or occupied-space complaints |
Controls and turndown | Part-load stability and efficiency | Hunting, excess reheat, poor ventilation or wasted fan energy |
Information Required Before Sizing an AHU
Collect the following information before using a manufacturer’s AHU selection program:
Outdoor design conditions: Summer and winter dry-bulb temperature, coincident wet-bulb temperature or humidity ratio, altitude and barometric pressure.
Indoor design conditions: Room temperature, relative humidity, permitted temperature variation and pressure requirements.
Zone loads: Coincident sensible and latent cooling loads for each zone and the complete system.
Ventilation requirements: Occupancy, floor area, exhaust, make-up air, outdoor-air rates and air-distribution effectiveness.
System arrangement: CAV, VAV, single-zone, multizone, DOAS, 100% outdoor air, heat recovery, return fan or relief fan.
Airside route: Duct dimensions, fittings, VAV boxes, terminals, fire/smoke dampers, louvers, silencers and leakage class.
Coil data: Entering and leaving air conditions, chilled-water temperatures and allowable air- and water-side pressure drops.
Project constraints: Plantroom space, access clearance, delivery route, module size, maintenance side, electrical supply, redundancy and acoustic limits.
Do not begin final AHU selection with a rule such as “400 CFM per ton” or “six air changes per hour.” These are useful reasonableness checks, but they cannot replace load, ventilation and psychrometric calculations.
Step 1 — Calculate the Cooling and Heating Loads
The space cooling load contains sensible and latent components:
Qtotal = Qsensible + Qlatent
Sensible load changes the air temperature. It includes:
Envelope heat gain
Solar gain through glazing
Sensible heat from occupants
Lighting heat
Equipment and appliance heat
Sensible infiltration load
Latent load changes the moisture content. It includes:
Moisture released by occupants
Humid outdoor ventilation air
Infiltration
Cooking or washing processes
Open water surfaces
Industrial moisture sources
For a central AHU, distinguish between the following loads:
Space load: Heat and moisture generated inside the conditioned zones.
Ventilation load: The energy required to condition outdoor air.
System gains: Fan heat, duct heat gain, duct leakage and return-air heat gain.
Cooling-coil load: The total duty experienced by the AHU cooling coil.
These values are not interchangeable. Cooling-coil duty is often considerably higher than the room cooling load, particularly in hot-humid climates and systems with high outdoor-air percentages.
Step 2 — Calculate Supply Airflow from the Sensible Load
For comfort cooling, supply airflow is normally determined from the space sensible load and the difference between room and supply-air temperatures.
SI equation
V̇s = Qs ÷ [ρair × cp,air × (Troom − Tsupply)]
At approximately standard indoor conditions, ρ × cp is commonly taken as 1.20 kJ/(m³·K):
V̇s (m³/s) ≈ Qs (kW) ÷ [1.20 × ΔT (K)]
I-P equation
CFM ≈ Qs (Btu/h) ÷ [1.08 × ΔT (°F)]
Where:
V̇s = supply airflow
Qs = space sensible cooling load
ρair = air density at the design condition
cp,air = specific heat of air
ΔT = room temperature minus delivered supply-air temperature
The constants 1.20 and 1.08 are approximations. Correct for actual air density when altitude, temperature or project requirements make the difference material.
Airflow calculation example
Assume:
Zone sensible load = 72 kW
Room temperature = 24°C
Delivered supply-air temperature = 13°C
Therefore:
V̇s = 72 ÷ [1.20 × (24 − 13)]
V̇s = 5.45 m³/s
This is approximately:
5,450 L/s
19,600 m³/h
11,560 CFM
Why total cooling load must not be used
The temperature-difference equation carries sensible heat only. Substituting the total load, including latent heat, artificially increases the airflow and may still fail to provide adequate dehumidification.
Use the sensible load to calculate airflow. Handle latent load through the supply-air humidity ratio and cooling-coil enthalpy calculation.
Air changes per hour check
Some spaces are governed by ventilation, process, cleanliness or exhaust requirements.
For an ACH-based criterion:
V̇ (m³/s) = Room volume (m³) × ACH ÷ 3,600
In I-P units:
CFM = Room volume (ft³) × ACH ÷ 60
Compare the load-derived airflow with:
Minimum ventilation airflow
Mandatory air-change requirements
Exhaust make-up air
Pressurization requirements
Heating-air distribution requirements
Process-air requirements
Use the governing airflow and then recalculate the supply-air state and cooling-coil duty.
Step 3 — Determine the Required Outdoor Air
For many non-residential occupancies using the ASHRAE 62.1 Ventilation Rate Procedure, the breathing-zone outdoor airflow begins with:
Vbz = Rp × Pz + Ra × Az
The zone outdoor airflow is then adjusted for air-distribution effectiveness:
Voz = Vbz ÷ Ez
Where:
Rp = outdoor airflow rate per person
Pz = zone population
Ra = outdoor airflow rate per unit floor area
Az = zone floor area
Ez = zone air-distribution effectiveness
For a multizone recirculating system, the AHU outdoor-air intake may not be the simple sum of all zone outdoor-air values. System ventilation efficiency, occupant diversity, zone primary-air fractions and the ventilation-critical zone may determine the final intake.
For VAV systems, verify outdoor-air compliance at both peak airflow and minimum-flow operation.
Always use:
The standard edition adopted by the project
Local building and mechanical codes
Health authority requirements
Fire and life-safety requirements
Client or employer requirements
Hospitals, laboratories, kitchens, cleanrooms and industrial processes may require additional or entirely different ventilation criteria.
Important: Outdoor air is generally a fraction of the total AHU supply airflow. It is not automatically added on top of an airflow that already represents total supply. However, outdoor air increases the cooling, heating and latent load at the AHU coil.
Step 4 — Calculate the Mixed-Air Condition
In a recirculating AHU, the cooling coil receives a mixture of outdoor air and return air.
Cooling-coil duty must therefore be based on the mixed-air condition, not the return-air condition alone.
For a dry-air mass balance:
hm = xoa × hoa + (1 − xoa) × hra
Wm = xoa × Woa + (1 − xoa) × Wra
Where:
xoa = outdoor-air dry-mass fraction
h = moist-air enthalpy
W = humidity ratio
m = mixed air
oa = outdoor air
ra = return air
Use a psychrometric chart or validated software to calculate:
Mixed-air dry-bulb temperature
Wet-bulb temperature
Relative humidity
Humidity ratio
Enthalpy
Dew-point temperature
A dry-bulb weighted average is insufficient for cooling-coil sizing when humidity and latent load are significant.
For a 100% outdoor-air AHU or DOAS, outdoor air is the coil-entering condition. If energy recovery is installed, use the air condition leaving the recovery device and include its pressure drop, leakage, frost-control and bypass effects.
IMAGE 2: Psychrometric mixing and cooling process
Step 5 — Size the Cooling Coil and Check Dehumidification
Total cooling-coil duty
Qcoil (kW) = ṁda (kg/s) × [hentering − hleaving] (kJ/kg)
Approximate dry-air mass flow can be obtained from:
ṁda ≈ ρair × V̇
In I-P units:
Qcoil (Btu/h) ≈ 4.5 × CFM × Δh (Btu/lb)
The 4.5 factor is based on standard-air assumptions. Use actual dry-air mass flow and project psychrometric properties for accurate engineering calculations.
Moisture removal and condensate
ṁcondensate = ṁda × (Wentering − Wleaving)
This moisture balance is essential in humid climates. A cooling coil may satisfy its sensible capacity while failing to achieve the required leaving-air humidity ratio.
The required supply humidity ratio can also be checked from the space latent load:
Wroom − Wsupply ≈ Qlatent ÷ (ṁda × hfg)
Where hfg is the latent heat of water vapour at the relevant condition.
Chilled-water flow
V̇water (L/s) ≈ Qcoil (kW) ÷ [4.186 × ΔTwater (K)]
The final coil must be selected using manufacturer software with:
Actual entering and leaving air conditions
Chilled-water supply and return temperatures
Airflow and density
Glycol concentration, if applicable
Allowable air-side pressure drop
Allowable water-side pressure drop
Corrosion and fouling requirements
Actual casing geometry
Cooling-coil input | Why it matters |
Entering air DB/WB or DB/RH | Establishes the sensible and latent entering condition |
Leaving air DB/WB or humidity ratio | Confirms temperature and dehumidification performance |
Airflow and density | Determines dry-air mass flow and face velocity |
Chilled-water temperatures | Determines water flow and required heat-transfer surface |
Coil rows, fins and circuiting | Affect capacity, pressure drop and cleanability |
Fin, tube and coating materials | Affect corrosion resistance and service life |
Maximum face velocity | Controls pressure drop and condensate carryover |
Drain-pan requirements | Protect downstream sections and ductwork from water carryover |
Do not select a cooling coil by the number of rows alone. Two six-row coils can have substantially different capacities, fin densities, circuiting arrangements and pressure drops.
Hot-humid and Middle East design note
In Gulf and coastal climates, outdoor-air latent load can dominate the difference between room load and AHU coil load.
Use a coincident outdoor dry-bulb/wet-bulb or dry-bulb/dew-point condition from the project weather source. Do not combine peak dry-bulb temperature with an unrelated peak relative humidity.
Also verify:
Outdoor-air intake leakage
Building pressurization
Condensate quantity
Negative-pressure drain-trap depth
Wet-coil pressure drop
Fin and casing corrosion protection
Fan heat
Supply-duct heat gain
Energy-recovery leakage
Part-load humidity control
Step 6 — Calculate Coil Face Area and AHU Cross-Section
The preliminary coil face area is:
Aface = V̇air ÷ vface
If:
Design airflow = 5.45 m³/s
Preliminary cooling-coil face velocity = 2.2 m/s
Then:
Aface = 5.45 ÷ 2.2 = 2.48 m²
A nominal clear face of approximately 1.8 m × 1.4 m provides 2.52 m² before allowing for frames, blank-off plates and the manufacturer’s actual free area.
For preliminary comfort-cooling studies, designers often begin around 1.8–2.5 m/s or 350–500 fpm across a wet cooling coil.
Use the lower part of the range where:
Latent performance is important
Condensate carryover is a concern
Low pressure drop is required
Noise criteria are strict
Coil airflow distribution may be uneven
This is a preliminary range, not a universal limit. Verify the selected coil, drain pan, eliminator, filters and heat-recovery device at their actual rated velocities.
The final AHU width and height may be governed by the component requiring the largest face area—not necessarily the cooling coil.
The AHU length must include:
Mixing and transition sections
Filter sections
Coil sections
Access doors
Coil withdrawal space
Fan service clearance
Sound attenuators
Humidifier absorption distance
Drain-pan and trap arrangements
Controls and electrical panels
Delivery and module-splitting requirements
IMAGE 3: AHU components and airflow path
Step 7 — Calculate Static Pressure and Select the Fan
The AHU fan must deliver design airflow at the calculated system resistance.
Trace the critical index run from the relevant inlet or return point, through the AHU and along the most resistant supply path to the terminal device.
Do not add the pressure losses of parallel branches.
Fan pressure equation
ΔPfan = ΔPexternal critical path + ΔPinternal AHU + ΔPsystem effect + justified allowance
External-system losses | Internal-AHU losses |
Straight supply and return duct friction | Filters at scheduled final resistance |
Elbows, tees, transitions and offsets | Cooling and heating coils |
VAV or CAV terminal units | Heat-recovery device |
Fire, smoke and balancing dampers | Internal dampers and transitions |
Diffusers, grilles and louvers | Internal sound attenuator |
Applicable intake or exhaust path | Internal airflow-measurement station |
External system effect | Casing and plenum losses |
Clarify the AHU manufacturer’s terminology.
Some AHU schedules request external static pressure, because the manufacturer’s software calculates internal component losses separately. Other schedules require the complete fan static or total-pressure duty.
Never enter internal component pressure losses twice.
Dirty-filter pressure
Fan selection must consider the filter’s specified final resistance, not only its clean pressure drop.
As the filter loads with dust:
System resistance rises
Airflow may fall
Fan speed may increase
Fan power may increase
Noise may change
Cooling capacity may reduce if airflow is not maintained
Fan electrical input power
Pinput (kW) = V̇ (m³/s) × ΔPfan (Pa) ÷ [ηtotal × 1,000]
Where ηtotal is the combined efficiency applicable to the calculation boundary.
For separate efficiencies:
Pelectrical = V̇ × ΔP ÷ (ηfan × ηdrive × ηmotor)
Do not multiply efficiencies again if the manufacturer’s published value already represents complete fan-system electrical efficiency.
Specific fan power
SFP = Pinput ÷ V̇
Compare the result with the energy code adopted by the project.
Select the fan using certified performance data and verify:
The duty point is in a stable and efficient region
Absorbed power does not overload the motor
Air-density corrections are applied
Inlet and outlet system effects are included
VFD turndown is acceptable
Motor cooling is maintained
Sound power meets the acoustic design
Fan-array redundancy is defined where required
Backflow through inactive fans is controlled
Maintenance access is available
The HVAC Duct Pressure Loss Design Suite helps organize duct sections, calculate straight-duct and fitting losses, correct for air density, identify the critical path and prepare a recommended fan ESP report.
Step 8 — Select Filters, Heat Recovery, Casing and Accessories
Filtration
Choose filter efficiency according to the applicable:
Indoor-air-quality requirement
Mechanical code
Healthcare standard
Cleanroom classification
Client specification
Equipment protection requirement
Obtain both clean and final pressure drops at the actual face velocity.
Provide:
Differential-pressure indication
Dirty-filter alarm
Adequate replacement access
Proper filter seals
Sufficient filter-bank area
Safe maintenance clearance
A MERV or ISO classification indicates filtration efficiency; it does not define pressure drop. Use the selected filter manufacturer’s actual resistance data.
Energy recovery
Heat or energy recovery can reduce outdoor-air coil load, particularly in systems with high outdoor-air fractions.
However, it also introduces:
Additional supply-side pressure drop
Additional exhaust-side pressure drop
Fan energy
Maintenance requirements
Cross-leakage risk
Frost-control requirements
Bypass-control requirements
Possible contamination transfer
Compare recovered energy against additional fan power, maintenance and leakage risks.
AHU casing
Specify casing performance for:
Strength and deflection
Air leakage
Thermal transmittance
Thermal bridging
Acoustic insulation
Condensation resistance
Corrosion resistance
Outdoor weather protection
Confirm base height and condensate-trap depth against the fan suction pressure.
Coordinate:
Service-door swing
Coil and filter withdrawal space
Internal lighting
Viewing ports
Safe access
Drain connection and fall
Module splits
Lifting points
Door and elevator dimensions
Roof access
Flexible connections
Vibration isolation
Controls panels and cable routes
Acoustics
Request octave-band sound-power data rather than relying only on a single dBA value.
Assess:
Fan inlet sound
Fan discharge sound
Casing-radiated sound
Duct breakout and break-in
VAV-box regenerated noise
Diffuser and grille noise
Sound-attenuator insertion loss
Sound-attenuator pressure drop
Low-frequency fan noise
Structure-borne vibration
Step 9 — Verify Controls and Part-Load Operation
An AHU selected only at peak duty may perform poorly during most of the year.
Typical control functions include:
Supply-air temperature reset
Duct static-pressure reset
Minimum outdoor-air control
Outdoor-airflow measurement
Demand-controlled ventilation where permitted
Cooling-coil leaving-air control
Heat-recovery bypass
Economizer sequence
Freeze protection
Dirty-filter alarm
Condensate high-level alarm
Fan proof and airflow monitoring
Fan-array staging
VFD speed control
At minimum airflow, recheck:
Outdoor-air fraction
System ventilation efficiency
Cooling-coil face velocity
Dehumidification performance
Diffuser throw
Fan stability
Motor cooling
Room pressurization
Worked AHU Sizing Example
The following warm-humid-climate example demonstrates the calculation sequence for a recirculating chilled-water AHU.
The example is illustrative. Final equipment must be selected with project weather data, applicable codes and certified manufacturer software.
Design inputs
Input | Value |
Zone condition | 24°C, 50% RH |
Zone sensible load | 67 kW |
Zone latent load | 23 kW |
Zone total load | 90 kW |
Outdoor airflow | 0.90 m³/s |
Outdoor condition | 35°C DB / 26°C WB |
Final supply condition | 12°C, W ≈ 0.0076 kg/kg dry air |
Cooling-coil leaving condition | Approximately 10°C, near saturation |
Chilled-water temperatures | 6/12°C |
Preliminary coil face velocity | 2.2 m/s |
The zone load excludes the outdoor-air load. Ventilation must therefore be included in the AHU cooling-coil calculation.
1. Calculate supply airflow
At the selected supply humidity ratio:
cp,m = 1.006 + 1.86W
cp,m ≈ 1.020 kJ/(kg dry air·K)
The required dry-air mass flow is:
ṁda = 67 ÷ [1.020 × (24 − 12)]
ṁda ≈ 5.47 kg dry air/s
At a supply-air specific volume of approximately 0.818 m³/kg dry air:
V̇ = 5.47 × 0.818
V̇ ≈ 4.48 m³/s
Equivalent airflow:
4,480 L/s
16,100 m³/h
Approximately 9,500 CFM
2. Check the latent load
At 24°C and 50% RH:
Wroom ≈ 0.0093 kg/kg dry air
Selected supply humidity ratio:
Wsupply ≈ 0.0076 kg/kg dry air
Therefore:
Qlatent ≈ 5.47 × 2,500 × (0.0093 − 0.0076)
Qlatent ≈ 23 kW
This confirms that the selected supply humidity ratio can meet the zone latent load.
3. Check the total zone capacity
Air state | Humidity ratio | Enthalpy |
Return air, 24°C and 50% RH | 0.0093 kg/kg | 47.8 kJ/kg dry air |
Outdoor air, 35°C DB / 26°C WB | 0.0175 kg/kg | 80.2 kJ/kg dry air |
Final supply air, 12°C | 0.0076 kg/kg | 31.3 kJ/kg dry air |
Coil leaving air, approximately 10°C | 0.0076 kg/kg | 29.3 kJ/kg dry air |
The total zone capacity is:
Qzone = 5.47 × (47.8 − 31.3)
Qzone ≈ 90 kW
The selected supply condition therefore satisfies both the 67 kW sensible load and 23 kW latent load.
4. Calculate mixed-air conditions
At the outdoor-air condition, the specific volume is approximately 0.898 m³/kg dry air.
Outdoor dry-air mass flow:
ṁoa = 0.90 ÷ 0.898
ṁoa ≈ 1.00 kg dry air/s
Outdoor-air dry-mass fraction:
xoa = 1.00 ÷ 5.47
xoa ≈ 0.183 or 18.3%
Mixed-air enthalpy:
hm = 0.183 × 80.2 + 0.817 × 47.8
hm ≈ 53.7 kJ/kg dry air
Mixed-air humidity ratio:
Wm = 0.183 × 0.0175 + 0.817 × 0.0093
Wm ≈ 0.0108 kg/kg dry air
The corresponding mixed-air dry-bulb temperature is approximately 26°C.
5. Calculate cooling-coil duty
Qcoil = 5.47 × (53.7 − 29.3)
Qcoil ≈ 134 kW
This equals approximately:
38 TR
457,000 Btu/h
The 134 kW cooling-coil duty is considerably higher than the 90 kW zone load because it includes:
Zone sensible and latent load
Outdoor-air treatment
Heat added between the coil and delivered supply condition
This demonstrates why matching an AHU cooling coil only to the room TR is unsafe.
6. Calculate chilled-water flow
For a 6 K chilled-water temperature rise:
V̇water = 134 ÷ (4.186 × 6)
V̇water ≈ 5.3 L/s
Equivalent flow:
19.1 m³/h
Approximately 84 US gpm
7. Calculate condensate
ṁcond = 5.47 × (0.0108 − 0.0076) × 3,600
ṁcond ≈ 63 kg/h
This is approximately:
63 L/h
16.6 US gal/h
The drain pan, connection and trap must be designed for the calculated condensate rate plus a suitable drainage allowance.
8. Calculate coil face area
At a preliminary face velocity of 2.2 m/s:
Aface = 4.48 ÷ 2.2
Aface ≈ 2.04 m²
A nominal 1.5 m × 1.5 m coil provides 2.25 m² gross face area. The manufacturer must verify the net active area after allowing for frames and blank-off sections.
9. Calculate fan pressure
Assume the following verified pressure budget:
Pressure item | Design loss |
Governing inlet branch | 120 Pa |
Critical supply duct, fittings and terminals | 550 Pa |
Filters at scheduled final resistance | 220 Pa |
Wet cooling coil | 180 Pa |
Internal dampers, transitions and casing effects | 80 Pa |
Explicit system-effect and commissioning allowance | 50 Pa |
Required fan pressure | 1,200 Pa |
Outdoor- and return-air branches are parallel before the mixing box. Their pressure losses must not automatically be added together. Use the governing balanced inlet path.
10. Calculate fan power
Air power:
Pair = 4.48 × 1,200
Pair ≈ 5.38 kW
Assume:
Fan efficiency = 72%
Direct drive
Motor efficiency = 92%
Electrical input:
Pelectrical = 5.38 ÷ (0.72 × 0.92)
Pelectrical ≈ 8.1 kW
If the motor and fan losses enter the airstream:
ΔTfan ≈ 8.1 ÷ (5.47 × 1.02)
ΔTfan ≈ 1.45 K
This supports the assumed temperature rise from approximately 10°C coil leaving air toward the 12°C delivered supply condition after fan and duct heat.
An 11 kW motor may be appropriate as the next standard size, but it must be confirmed against:
Manufacturer’s fan curve
Absorbed power across the operating range
Worst-density operating condition
VFD range
Motor overload condition
Stable operating region
Preliminary AHU selection summary
Parameter | Preliminary result |
AHU supply airflow | 4.48 m³/s / approximately 9,500 CFM |
Outdoor airflow | 0.90 m³/s / approximately 1,900 CFM |
Cooling-coil duty | 134 kW / 38 TR |
Chilled-water flow | 5.3 L/s / 84 gpm |
Approximate condensate | 63 L/h / 16.6 US gal/h |
Minimum preliminary coil face area | 2.04 m² / 22.0 ft² |
Required fan pressure | 1,200 Pa / 4.82 in. w.g. |
Estimated fan electrical input | 8.1 kW / 10.9 hp |
IMAGE 4: AHU fan pressure path and duty point
The complete cooling-load design workflow
This example connects the room load, delivered supply condition and cooling-coil duty. The Cooling Load Design Handbook takes that calculation further, from building information and psychrometrics to supply airflow, system loads and preliminary equipment selection.
The 157-page PDF includes 18 chapters and three complete worked projects: a bedroom, a commercial office and a multi-room villa.
The preview request form asks for your first name and email.
Educational design reference. Use the project's adopted codes, design conditions and certified manufacturer data for final selection.
Common AHU Sizing Mistakes
Mistake | Correct approach |
Using total cooling load in the sensible-airflow equation | Use sensible load for airflow and enthalpy for total coil duty |
Selecting by 400 CFM/TR alone | Use CFM/TR only as a reasonableness check |
Adding outdoor airflow twice | Establish whether outdoor air is part of the total supply or a separate DOAS stream |
Calculating coil duty from return air only | Use mixed-air or post-energy-recovery conditions |
Using ACH alone for comfort cooling | Compare ACH with load-derived airflow and use the governing condition |
Selecting the fan from a typical ESP | Calculate actual critical-path pressure losses |
Using only clean-filter resistance | Verify airflow at scheduled final filter resistance |
Adding 10–20% to every quantity | Apply one documented allowance only where justified |
Ignoring air density | Correct airflow, ventilation and fan selection for actual conditions |
Ignoring fan and duct heat | Include downstream heat gains in the coil leaving-air requirement |
Ignoring minimum VAV airflow | Recheck ventilation, humidity and diffuser performance at turndown |
Using gross casing area as coil area | Use the net active coil face area |
Ignoring delivery and maintenance | Check module size, access, weight and service clearance |
AHU Selection Schedule Checklist
Before approving a vendor selection, confirm that the AHU schedule or submittal identifies:
Supply, return, exhaust and outdoor airflow
Peak and minimum operating conditions
Entering and leaving air DB/WB or humidity ratio
Total, sensible and latent coil capacities
Chilled-water or refrigerant conditions
Water flow and pressure drop
Clean and final filter pressure drops
Filter efficiency and classification
External and internal pressure-loss boundaries
Fan type, speed and efficiency
Fan absorbed power
Motor and VFD rating
Inlet, outlet and casing sound power
Casing leakage and strength
Thermal and acoustic casing performance
Heat-recovery efficiency and pressure drop
Heat-recovery leakage and bypass arrangement
Controls, sensors and alarms
Communication protocol
Overall dimensions and weight
Module splits
Service clearance and maintenance side
Drain-pan construction and trap requirements
Corrosion protection
Applicable certifications and standards
Frequently Asked Questions
How is AHU airflow calculated?
For comfort cooling, calculate airflow from the space sensible load and the difference between room and supply-air temperatures:
V̇ = Qs ÷ (ρ × cp × ΔT)
Then compare the result with ventilation, ACH, exhaust make-up, pressurization and process-air requirements.
How many CFM are required per ton of cooling?
Approximately 400 CFM/TR is a familiar comfort-cooling rule of thumb. However, the actual value changes with sensible heat ratio, supply-air temperature, climate and system type.
Calculate airflow from the sensible load and calculate coil tonnage from the total mixed-air enthalpy change.
What supply-air temperature should be used?
Approximately 12–14°C or 54–57°F is common for many comfort-cooling systems, but it is not universal.
Humidity control, diffuser performance, fan heat, duct gains, system type and zone sensible heat ratio may require a different temperature.
Can an AHU be sized using air changes per hour?
ACH is appropriate when ventilation, process or code criteria govern, such as in some hospitals, laboratories, cleanrooms, kitchens and industrial spaces.
For normal comfort cooling, ACH alone does not prove that sensible and latent loads are satisfied.
How is cooling-coil capacity calculated?
Use the dry-air mass flow multiplied by the enthalpy difference across the coil:
Qcoil = ṁda × (hentering − hleaving)
Use mixed air or air leaving the energy-recovery device as the entering condition.
How much spare capacity should be added?
There is no universal safety factor.
Use realistic coincident loads and add only documented allowances for:
Calculation uncertainty
Leakage
Fouling
Future tenant loads
Planned expansion
Coordination changes
Avoid adding independent safety factors to the load, airflow, coil, pressure and motor.
What static pressure should be used?
There is no reliable universal AHU ESP.
Calculate the critical route through:
Ducts
Fittings
Terminals
Filters
Coils
Dampers
Louvers
Sound attenuators
Heat-recovery devices
State clearly whether the manufacturer requires external static pressure or total fan pressure.
What is the difference between AHU airflow and tonnage?
Airflow primarily carries the space sensible load and supports ventilation and air distribution.
Cooling-coil tonnage represents the total sensible and latent heat removed from the entering air. Two AHUs with the same airflow can therefore require very different cooling capacities.
Final AHU Sizing Workflow
A professional AHU selection follows this sequence:
Calculate coincident zone sensible and latent loads.
Select an initial delivered supply-air condition.
Calculate supply airflow from the sensible load.
Verify ventilation, ACH, exhaust make-up and pressurization.
Calculate outdoor- and return-air mixing conditions.
Determine cooling-coil duty, leaving humidity ratio and condensate.
Calculate chilled-water flow.
Size the cooling-coil face and AHU casing.
Calculate external and internal pressure losses.
Select the fan at the required airflow, pressure and density.
Verify motor power, sound and stable operation.
Check filters, heat recovery, casing and accessories.
Verify controls and minimum-flow operation.
Coordinate dimensions, access, utilities and maintenance requirements.
Review certified manufacturer selection data.
Correct airflow is only the beginning. A successful AHU must deliver that airflow through the actual duct system while controlling temperature, humidity, ventilation, noise and energy use throughout its complete operating range.
Use the suite to organize duct sections, calculate straight-duct and fitting losses, identify the critical path, correct for air density and prepare a recommended fan ESP report.
You can also review the HVAC Duct Pressure Loss Design Suite features and workflow before purchasing.
Related HVAC Design Guides
Technical References
ASHRAE Standards 62.1 and 62.2 — Ventilation and Acceptable Indoor Air Quality
ASHRAE Standard 55 — Thermal Environmental Conditions for Human Occupancy
ASHRAE Standard 90.1 — Energy Standard for Buildings Except Low-Rise Residential Buildings
AHRI Standard 410 — Performance Rating of Forced-Circulation Air-Cooling and Air-Heating Coils
Author note: Nexora Design Lab publishes practical engineering guidance on HVAC design, MEP systems, air distribution, pressure-loss calculations and sustainable building technologies.
Engineering disclaimer: This article is an educational design guide. It does not replace project-specific load calculations, governing codes, authority requirements or certified manufacturer selection data.
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