How to Size Air Handling Units (AHU) for Large Buildings

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.

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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

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:

1. Outdoor design conditions: Summer and winter dry-bulb temperature, coincident wet-bulb temperature or humidity ratio, altitude and barometric pressure.

2. Indoor design conditions: Room temperature, relative humidity, permitted temperature variation and pressure requirements.

3. Zone loads: Coincident sensible and latent cooling loads for each zone and the complete system.

4. Ventilation requirements: Occupancy, floor area, exhaust, make-up air, outdoor-air rates and air-distribution effectiveness.

5. System arrangement: CAV, VAV, single-zone, multizone, DOAS, 100% outdoor air, heat recovery, return fan or relief fan.

6. Airside route: Duct dimensions, fittings, VAV boxes, terminals, fire/smoke dampers, louvers, silencers and leakage class.

7. Coil data: Entering and leaving air conditions, chilled-water temperatures and allowable air- and water-side pressure drops.

8. 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

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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

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:

1. Calculate coincident zone sensible and latent loads.

2. Select an initial delivered supply-air condition.

3. Calculate supply airflow from the sensible load.

4. Verify ventilation, ACH, exhaust make-up and pressurization.

5. Calculate outdoor- and return-air mixing conditions.

6. Determine cooling-coil duty, leaving humidity ratio and condensate.

7. Calculate chilled-water flow.

8. Size the cooling-coil face and AHU casing.

9. Calculate external and internal pressure losses.

10. Select the fan at the required airflow, pressure and density.

11. Verify motor power, sound and stable operation.

12. Check filters, heat recovery, casing and accessories.

13. Verify controls and minimum-flow operation.

14. Coordinate dimensions, access, utilities and maintenance requirements.

15. 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.

GET THE HVAC DUCT PRESSURE LOSS DESIGN SUITE →

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

• HVAC Fan Selection Guide with Static Pressure Calculations

• How to Select an AHU Cooling Coil: Rows, Fins and Face Velocity

• How Dirty Filters Change Airflow, Fan Power and Cooling Capacity

• Dehumidification Using Cooling Coils: Calculation Guide

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

• AHRI Standards 430/431 — Central-Station AHU Supply Fans

• Eurovent — How to Select an Air Handling Unit

• AMCA International — Fan and System Education

If you like to read more..............................

Properly sizing an Air Handling Unit (AHU) is critical for maintaining indoor air quality, thermal comfort, and energy efficiency in large commercial and industrial buildings. An undersized AHU will struggle to maintain temperature and ventilation requirements, while an oversized system increases capital costs and energy consumption.

For HVAC engineers, MEP consultants, and facility managers, AHU sizing involves analyzing cooling load, airflow requirements, ventilation standards, and system pressure losses.

This guide explains the step-by-step method used by HVAC professionals to size AHUs for large buildings such as hospitals, airports, malls, office towers, and data centers. (Air Handling Units (AHU) for Large Buildings)

1. Understand the Building Cooling Load

The first step in AHU sizing is calculating the total cooling load of the building.

Cooling load represents the total heat that must be removed from the space to maintain desired indoor conditions.

Major Load Components

Sensible Heat Load

• Heat from occupants

• Lighting systems

• Electrical equipment

• Solar radiation

Latent Heat Load

• Moisture from occupants

• Outdoor air humidity

• Infiltration

Cooling Load Formula

Total Cooling Load:

Q = Sensible Heat + Latent Heat

Large commercial buildings often require hundreds of tons of refrigeration (TR) capacity.

Example:

Office Tower Cooling Load = 800 TR

2. Calculate Required Airflow (CFM)

After determining cooling load, the next step is calculating the required airflow rate.

Standard HVAC Airflow Formula

CFM = (Cooling Load × 12,000) / (1.08 × ΔT)

Where:

• CFM = Airflow in cubic feet per minute

• ΔT = Temperature difference between supply and return air

Typical ΔT values:

• Offices: 18°F – 22°F

• Hospitals: 16°F – 18°F

• Data centers: 20°F – 25°F

Example Calculation

Cooling Load = 800 TR

ΔT = 20°F

CFM = (800 × 12,000) / (1.08 × 20)

CFM ≈ 444,000 CFM

This airflow determines the required AHU fan capacity and coil size.

3. Determine Ventilation Air Requirements

Large buildings must meet ventilation standards specified by ASHRAE Standard 62.1.

Ventilation air ensures adequate indoor air quality (IAQ) by diluting pollutants and CO₂ levels.

Ventilation Calculation (Air Handling Units (AHU) for Large Buildings)

Outdoor Airflow = Rp × People + Ra × Area

Where:

• Rp = Outdoor airflow per person

• Ra = Outdoor airflow per floor area

Example:

Office Space:

• Rp = 5 CFM/person

• Ra = 0.06 CFM/ft²

For a floor with:

• 200 occupants

• 20,000 ft² area

Outdoor Air = (5 × 200) + (0.06 × 20,000)

Outdoor Air = 2,200 CFM

This value must be integrated into the AHU design.

4. Size the Cooling Coil

The cooling coil removes heat from supply air.

Cooling coil capacity must match the cooling load and airflow.

Coil Capacity Formula

Q = 4.5 × CFM × (h1 − h2)

Where:

• Q = Cooling capacity (BTU/hr)

• h1 = Return air enthalpy

• h2 = Supply air enthalpy

Coil sizing also considers:

• Chilled water temperature

• Water flow rate

• Coil rows

• Fin spacing

Large buildings often use 6–8 row chilled water coils for high efficiency.

5. Calculate Fan Static Pressure

AHU fans must overcome system pressure losses.

Sources of Pressure Loss

• Air filters

• Cooling/heating coils

• Dampers

• Ductwork

• Sound attenuators

• VAV boxes

Typical Static Pressure Range

Large building AHUs typically operate at:

3 – 6 inches WG

Fan power requirement:

Fan Power = (CFM × Static Pressure) / (6356 × Fan Efficiency)

High-efficiency systems use EC fans or VFD-controlled centrifugal fans.

6. Select the AHU Configuration

Large buildings rarely use a single AHU. Instead, engineers divide loads into multiple AHUs serving zones.

Common AHU Configurations

Single Zone AHU

• One AHU per floor

• Used in offices and retail spaces

VAV AHU Systems

• Variable Air Volume control

• Reduces energy consumption

Dedicated Outdoor Air Systems (DOAS)

• Separates ventilation from cooling

• Improves humidity control

Selecting the right configuration improves:

• energy efficiency

• maintenance access

• zoning flexibility

7. Account for Future Expansion

Engineers often add 10–15% capacity margin when sizing AHUs.

Reasons include:

• Tenant changes

• Equipment additions

• Future floor expansions

• Increased occupancy

Oversizing beyond this range should be avoided to prevent energy waste.

8. Consider Energy Efficiency Strategies

Modern AHU designs incorporate energy-saving features.

Common AHU Efficiency Improvements

Energy Recovery Wheels

Recover energy from exhaust air to reduce cooling load.

EC Fan Arrays

Improve efficiency and redundancy.

High-Efficiency Filters

MERV 13+ filtration improves indoor air quality.

Demand Controlled Ventilation

Adjusts ventilation based on CO₂ levels.

These strategies significantly reduce operational costs in large buildings.

Common AHU Sizing Mistakes

Engineers frequently encounter issues caused by improper sizing.

1. Oversizing AHUs

Results in:

• short cycling

• poor humidity control

• higher energy consumption

2. Ignoring Ventilation Requirements

Leads to poor indoor air quality and non-compliance with ASHRAE standards.

3. Underestimating Static Pressure

Causes insufficient airflow and comfort complaints.

4. Poor Zoning Strategy

Reduces system efficiency and temperature control.

Example AHU Sizing for a Large Office Building

Building Area: 500,000 ft²

Estimated Cooling Load: 900 TR

Calculated Airflow: ~500,000 CFM

AHU Design:

• 10 AHUs

• 50,000 CFM each

• VAV distribution

• 6-row chilled water coil

• Fan static pressure: 4.5 in WG

This configuration provides efficient zoning and redundancy.

Final Thoughts

Sizing an Air Handling Unit for large buildings requires a detailed analysis of:

• cooling load

• airflow requirements

• ventilation standards

• coil capacity

• fan static pressure

• system configuration

A well-designed AHU system ensures thermal comfort, energy efficiency, and regulatory compliance.

For complex projects such as hospitals, airports, data centers, and high-rise buildings, HVAC engineers typically rely on load simulation software and ASHRAE guidelines to optimize AHU sizing.

Proper engineering during the design phase can significantly reduce energy consumption, operational costs, and maintenance issues throughout the building lifecycle.

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.