Sizing of Air Handling Unit

by Newgram

This five-step action plan for the implementation of Air Handling Unit (AHU) project

The AHU is part of the heating, ventilating, and air-conditioning (HVAC) system for industrial or large commercial building. The HVAC system is a specialized system that requires variable inputs from the end user, engineering personnel, equipment supplier and/or consultant.

Air Handling Unit (AHU) makes use of vapor compression cycle to condition air entering and leaving the space in a specific area; and at the same time controls and maintains the required space’s temperature, humidity, flow and cleanliness.

It is important for the end user to understand the process and steps on working with the project team on AHU and/or HVAC system project. The five successful factors in implementing the AHU are;

  • Well organized project team.
  • Defined design criteria and indoor environmental parameters.
  • Cooling and heating load calculation.
  • Proper Schematic layout, piping and ductwork for M&E work.
  • Identify problems/opportunities to improve energy usage for AHU.

Organized Project Team

A complete project team involves not just the project initiator/coordinator but also external parties such as building designer, engineering consultant, contractor, and between mechanical engineer, electrical engineer, process engineer, facility operator, and architect within the same organization.

Projects that hire engineering consultant usually involved sole for the design of the AHU whereas the installation works fall under separate Mechanical & Electrical contractors, there should be coordinated involvement between the consultant and contractors form the initial stage of the project. The responsibility to ensure that the project can proceed smoothly falls under the owner/project initiator.

In a “Design & Build” project, the design and installation work is carried out by the same contractor or consultancy firm. The project initiator spends lesser time to address concerns arising from the installation work as he/she needs only works with the consultant/contractor for this project. The project team is able to rectify issues immediately during project works.

Quick checklist for the Air Conditioning Surveys should include the following;

  1. Electrical and water supply
  2. Refrigeration
  3. Drainage
  4. Space conditions
  5. Cooling and heating loads
  6. Location and layout of the system
  7. Air distribution and duct system
  8. Scope of the contractors

Defined design criteria and indoor environmental parameters

 The area of the space, volume of air required for conditioning and processing, the temperature and condition of the processed air in term of relative humidity has to be defined during the design stage. Air change rate and air velocity are also required for sizing of the duct and fan required.

The indoor air quality should comply with the local government regulation.

Climate Design Information

Take climate conditions at the 0.4% annual percentile and to be about 35 hours a year hotter than these design conditions for extreme conditions. Climate Design Conditions should be based on the values for 0.4% WB (7th column from the left) and MDB (8th column from the left)

Estimate the rH [%] correspond to the 0.4% WB figure using psychometric chart.


 Cooling Loads Calculations.

 For countries in South East Asia, mostly cooling is required for the conditioned spaces. Heat gains through conduction, convection, radiations should be estimated in determining the requirement for the cooling load,

 Sensible Heat Gains – (added by conduction, convection and/or radiation.) Calculate surface areas and determine the materials of walls, partitions, windows and roofs installed.

Heat Gains from Utilities such as hot water piping and electrical panels.

Heat Gains from Lightings – should be calculated per surface area.

Heat Gains from Motor/Electrical Equipment – Estimation for load, usage, motor efficiency can be referenced to ASHRAE Fundamentals 28.9. For quick estimation, the total installed power can be ranged from 35% to 50 % and multiply with the heat gain factor for a rough estimate.

Latent Heat Gains and Internal Heat Load – (occurred when moisture is present in conditioned space.)  Is not required for residential or warehousing/storage spaces but should be considered for certain building construction without vapor barrier.

Solar Heat Gains (Sensible) – Determine solar exposure of the roof and with walls exposed to west or east and the peak load.

Outside Air Calculation

Air changes option: it is calculated by multiplying a pre-defined (according with different national codes and the kind of room selected) number of air changes.

As percentage of supply air option: the necessary supply air is calculated with the formula:

SA = {[QS + QL] – [1.86 * (TSAmin + 2500) * (QL / 2500)]}


QL = Total latent load

QS = Total sensible load


TRA    = Return air temperature

TOCC  = Off cooling coil temperature

Calculation of heat loads. 

  • Summer or winter (For temperate countries)
  • Specific relative humidity requirement.

Proper Schematic layout, piping and ductwork for M&E work

Good design layout of the air handling system includes the following,

  • Proper fresh air intake and exhaust air system.
  • Supply air enter the conditioned space on high point. Exhaust system at the low point in the room
  • Modulating control for the louvers for makeup air, speed of supply fan and flow rate of the chilled water (refrigerant)
  • Arrangement and location of temperature sensors.

Piping and ductworks in the air distribution system.

Air motion in occupied space

Air quality

Identify problems/opportunities to improve energy usage for AHU

 The most common concern is that the desired temperature and quality of the air inside the conditioned space cannot be met or maintained. As such, the duct, ventilation fan, cooling capacity and makeup air are often sized much greater than the designed (required) in the air handling unit system. Once the peak heat load is known, the maximum cooling capacity should be able to remove the heat from the specific space within a period of time.

Instrumentation and controls

With the advancement of technology, instrumentation and logic control are being used to operate AHU efficiently. Maintaining the room temperature should be dependent on temperature control and air velocity. Temperature control helps to modulate the cooling system in the AHU. Measurement of air velocity ensure that conditioned air is being recycled back to the AHU and thus reducing the cost of treating outside air.

Recycling heating and cooling medium helps to further reduce overall energy consumption in the AHU. Successful energy efficient optimization requires continuous monitoring and changes to the control parameter for the AHU system.

Minimize losses

Minimize the loss of the treated air in the conditioned space seems easy but it is not usually done in practical way. Some example can be installation of automatic door and relocation of the supply and return air duct.


Information provided in this article are provided “as is” without warranty of any kind, either express or implied, including without limitation warranties of merchantability, fitness for a particular purpose and non-infringement. The author provides, with reasonable effort, accurate and up-to-date information in this article. It does not, however, make any warranties or representations as to its accuracy or completeness. It periodically adds, changes, improves or updates the information and documents in this article without notice. The author assumes no liability or responsibility for any errors or omissions in the content of its article. You are using the article at your own risk. Under no circumstances shall the author or any other party involved in creating, producing or delivering the contents of this article be liable to you or any other person for any direct, indirect, special, incidental or consequential damages arising from your access to, or use of, this article.

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