Every HVAC system is designed around a relatively simple principle: conditioned air leaves the air handling unit (AHU), travels through the ductwork, and reaches the spaces where it is needed.
On paper, the process looks straightforward.
In an operating building, however, the airflow measured at the AHU does not always match the airflow measured at the supply terminals. When that happens, the difference can create an important question for engineers, facility managers and building owners:
Where did the air go?
The answer is not always a faulty fan or insufficient cooling capacity. The distribution system itself may be contributing to the problem.
Duct leakage, poor balancing, excessive pressure losses, incorrect damper positions, equipment conditions and measurement differences can all affect the relationship between AHU airflow and terminal airflow.
Understanding that relationship is important because an HVAC system does not deliver value simply by producing conditioned air. It delivers value when the required amount of air reaches the intended space under the required operating conditions.
For buildings operating in Saudi Arabia’s demanding cooling environment, maintaining the integrity and performance of the air distribution system can therefore be an important part of HVAC performance management.
The Journey From AHU to Terminal
The AHU is only one part of the air distribution system.
After conditioned air leaves the unit, it normally passes through a network of main ducts, branches, fittings, dampers, flexible connections and terminal devices before reaching occupied areas.
At every stage, the system has to maintain the conditions required for the air to reach its intended destination.
There are three broad areas where performance can differ from the original design.
1. The AHU May Not Be Operating at Design Conditions
Before assuming there is a leakage problem, the actual operating condition of the AHU should be established.
Fan speed, static pressure, filter condition, coil pressure drop, control settings and system resistance can all influence the amount of air being delivered.
For example, a partially loaded filter can increase resistance. A control sequence may limit fan speed. A change in system pressure can also affect the operating point of the fan.
This means that comparing an AHU airflow measurement with a design value is only meaningful when the operating conditions are properly understood.
A reliable investigation should therefore record relevant operating information rather than looking at airflow as an isolated number.
2. The Distribution System May Be Out of Balance
Even when the AHU is producing approximately the expected airflow, individual branches may not receive the correct amount.
This can happen because of incorrect damper positions, poor balancing, excessive pressure losses, unsuitable transitions or installation issues.
One branch may receive more air than intended while another receives less.
In a large commercial building, the effect can become significant because a relatively small distribution problem can be repeated across many branches and occupied zones.
The result may be inconsistent room temperatures, inadequate airflow at certain terminals or complaints from occupants even though the central HVAC equipment appears to be operating normally.
3. Air May Be Escaping Through Leakage Paths
The third possibility is duct leakage.
Ductwork is assembled from numerous components and connections. Depending on the construction and installation, potential leakage paths can include:
- Longitudinal seams
- Transverse joints
- Access doors
- Flexible connections
- Equipment connections
- Poorly sealed penetrations
- Flanged connections
- Construction-related openings
- Damaged or deteriorated sealing materials
A duct system does not need to have one dramatic defect for leakage to become relevant.
Multiple smaller leakage points distributed throughout a network can collectively affect system performance.
This is why duct integrity should be considered as part of the overall airflow investigation rather than treating every airflow discrepancy as an equipment problem.
How Do You Measure the Gap Between AHU and Terminal Airflow?
A useful investigation starts with a reliable baseline.
The AHU supply airflow should be measured and documented under a clearly defined operating condition. Depending on the system, relevant information can include:
- Supply airflow
- Fan operating point
- Fan speed
- Static pressure
- Filter condition
- Control settings
- Coil condition
- Relevant damper positions
Terminal airflow should then be measured using an appropriate testing, adjusting and balancing (TAB) method.
The measurements should be taken under comparable operating conditions.
This point is important because it is tempting to simply subtract terminal airflow from AHU airflow and describe the difference as duct leakage.
That conclusion may be premature.
The difference between two measurements can also be affected by measurement uncertainty, system configuration, balancing conditions, pressure losses and the number and location of measurement points.
Therefore, an airflow difference can be a signal that further investigation is required, but it is not automatically proof of duct leakage.
Where leakage is suspected, a dedicated duct leakage test provides a more controlled way of evaluating the integrity of a defined duct section.
The test can establish how much air is escaping from the tested section under a specified pressure condition.
Why Duct Leakage Testing Matters
Duct leakage testing changes the conversation from assumptions to measurable performance.
Instead of saying:
“The rooms are not receiving enough air.”
the project team can investigate:
- What airflow was designed?
- What airflow is actually being delivered?
- What pressure is the duct operating under?
- What leakage is measured?
- What leakage is permitted by the project requirements?
- Which section was tested?
- What corrective action is required?
This creates a much clearer path toward rectification.
A proper test report should identify the tested section, test conditions, measured leakage, applicable acceptance criteria and any corrective action or retesting requirements.
The result is useful not only for construction quality control but also for facility management and future troubleshooting.
Why This Matters in Saudi Arabia
Air distribution performance is particularly relevant in Saudi Arabia because many buildings operate under substantial cooling demand for extended periods.
When conditioned air does not reach its intended destination, the building may still consume energy to produce and move that air.
The issue can become more complicated when operators attempt to compensate for poor distribution.
For example, if certain spaces are warmer than expected, the response might be to increase fan speed.
If cooling remains inadequate, operators may lower supply-air temperature.
If one area continues to experience comfort problems, additional equipment may eventually be considered.
These actions can sometimes address symptoms without identifying the original cause.
If the underlying problem is leakage, imbalance, excessive pressure loss or a control issue, simply increasing system output does not necessarily correct the distribution problem.
The result can be a system that consumes more energy while still failing to deliver the intended airflow to specific spaces.
This is particularly relevant for buildings with demanding operational requirements, including:
- Commercial buildings
- Hotels and hospitality facilities
- Healthcare facilities
- Industrial facilities
- Large residential developments
- Retail environments
- Mission-critical facilities
- Data centres
In these environments, airflow is not only about occupant comfort. It can also influence ventilation, pressurisation and environmental control.
Common Signs That an Air Distribution Investigation Is Needed
Facility teams may encounter several symptoms before the underlying cause is identified.
Common warning signs can include:
Uneven temperatures between zones
Some rooms may remain warmer or cooler than neighbouring spaces even though they are served by the same general HVAC system.
Low terminal airflow
A terminal may consistently deliver less airflow than the design requirement.
High fan operation
Operators may need to run fans at higher speeds to achieve acceptable conditions.
Persistent comfort complaints
Occupants may report that certain areas are difficult to cool despite apparently normal central equipment operation.
Unexpected pressure behaviour
Changes in static pressure or pressure relationships can indicate that the distribution system requires further investigation.
Repeated balancing problems
If balancing teams repeatedly struggle to achieve the required terminal airflow, the problem may extend beyond individual damper adjustments.
None of these symptoms automatically proves that duct leakage is the cause. They are indicators that the complete air distribution system should be evaluated.
Closing the Gap: Testing, Rectification and Sealing
Once the project team has established that duct leakage may be contributing to the problem, the next step is controlled testing.
The process should begin by identifying the relevant duct sections and reviewing the approved design documents and project specifications.
The test section is then prepared and tested under the applicable pressure condition.
If the measured leakage exceeds the project’s acceptance requirement, the affected section should be investigated and rectified.
The repair strategy depends on the actual defect.
Some leakage may be associated with accessible joints or connections. Other situations may require more extensive inspection of the duct construction and sealing system.
After rectification, retesting is important.
The purpose of retesting is not simply to produce another document. It verifies whether the corrective work has actually improved the performance of the tested section.
This creates a simple quality loop:
Measure → Identify → Rectify → Retest → Document
That approach is much more useful than relying on visual inspection alone.
Duct Leakage Testing Is Not the Same as TAB
Another important distinction is between duct leakage testing and testing, adjusting and balancing.
TAB focuses on measuring and adjusting airflow distribution so that the system operates according to the design requirements.
Duct leakage testing focuses on the airtightness performance of the tested duct section under specified conditions.
The two activities support each other, but they answer different questions.
If a terminal has low airflow, TAB may identify an incorrect damper position or distribution imbalance.
If the duct system itself is losing air, leakage testing can provide evidence of that condition.
For this reason, a complete HVAC performance investigation may need to consider both distribution balancing and duct integrity.
Why Building Owners Should Look Beyond the AHU
One of the easiest mistakes in HVAC troubleshooting is to focus too heavily on the equipment that produces the air.
The AHU may be operating correctly while the distribution system prevents the building from receiving the expected benefit.
A building owner therefore needs to look at the complete path:
AHU → Main Duct → Branch Duct → Terminal → Occupied Space
Every part of this path contributes to final performance.
If the system is measured only at the AHU, important information can be missed.
A more complete performance assessment combines equipment measurements, airflow measurements, pressure information, balancing data and, where appropriate, duct leakage testing.
Building a More Reliable HVAC Performance Baseline
For existing buildings, documenting HVAC performance provides a useful baseline for future troubleshooting.
A facility team can maintain records of:
- AHU airflow
- Static pressure
- Fan operating conditions
- Terminal airflow
- Balancing results
- Duct leakage test results
- Corrective actions
- Retest results
Over time, this information can help the facility team understand whether performance is stable or whether certain systems are developing recurring problems.
For new construction, the same approach can support quality assurance before handover.
The earlier a distribution problem is identified, the easier it may be to correct before ceilings, insulation and finishes make access more difficult.
Conclusion
The difference between AHU airflow and terminal airflow should not automatically be treated as a fan problem or as proof of duct leakage.
The correct approach is to investigate the complete air distribution system.
First, establish the actual operating condition of the AHU. Then evaluate airflow distribution, balancing, pressure conditions and controls. Where leakage is suspected, use dedicated duct leakage testing to obtain measurable evidence about the integrity of the duct system.
For buildings in Saudi Arabia, where cooling demand can be substantial, maintaining controlled air distribution can be an important part of overall HVAC performance.
