The balancing report shows airflow below design. The fan is running. The specification says the numbers should match. Where do you start?
The most effective approach is to work from the quickest, cheapest checks to the more invasive ones, while keeping every finding documented. The sequence below is a practical starting point.
1. Confirm the measurement
Before chasing a shortfall, confirm it is real. Check that the instrument is calibrated, that the measurement location is suitable (adequate straight duct before and after the point), that the traverse method is correct and that the design figure being compared against is the right one.
2. Check the fan and its drive
Confirm rotation direction, speed, belt condition and tension, motor current and, for variable speed drives, the actual setpoint and output. Compare the operating point with the fan curve. Remember that airflow changes in proportion to fan speed, while pressure changes with the square of speed, so a small speed shortfall can have a larger effect on pressure than expected.
3. Look at filters and coils
Loaded filters and fouled coils add resistance without any visible duct problem. Check pressure drop across each and compare it with design and clean values.
4. Check dampers and obstructions
Volume dampers, fire dampers and smoke dampers that are closed, partly closed or incorrectly set are a frequent cause. So are blanked-off grilles, closed doors on return paths, and objects or debris inside the duct.
5. Inspect the duct condition
Look for crushed or kinked flexible duct, disconnected joints, collapsed liners, damaged insulation, open access doors and modifications made after installation.
6. Test for leakage
If the fan delivers close to design but terminals do not, leakage becomes a prime suspect. A duct leakage test quantifies how much air is lost and, combined with targeted inspection, helps locate where.
7. Measure the pressure profile
Static pressure readings along the main run reveal where abnormal pressure drops occur. A sudden loss across a fitting, transition or damper points directly at the cause.
8. Review the testing and balancing findings
Read the TAB report critically. Look for repeated patterns (for example, shortfalls only on the far side of the building), notes on conditions during testing, and whether any items were marked as outstanding.
9. Assess system cleanliness
Heavy dust or construction debris inside ducts, on coils or in the fan section can restrict flow. Inspection and, where justified, cleaning should be part of the diagnosis, not an afterthought.
Document as you go
Record readings, conditions and photographs at each step. A clear trail makes it far easier to agree on responsibility, choose the right correction and prove the outcome afterwards.
Matching symptoms to likely causes
Symptoms can help narrow the search before any instrument is used:
Symptom
Likely areas to check first
Low airflow everywhere
Fan speed and rotation, filters, coils, main damper positions, major leakage near the fan
Low airflow at far branches only
Leakage along the main run, undersized or restricted sections, balancing
Low airflow at one branch
Closed or partly closed damper, crushed or disconnected duct, blocked terminal
Good airflow but warm rooms
Supply temperature, coil performance, leakage into ceiling voids, control faults
High fan power with low airflow
Excess system resistance, dirty filters or coils, fan operating off its curve
Noise with low airflow
Damper throttling, high velocities through restrictions, loose or damaged duct
These are starting points, not conclusions. Measurement should confirm each suspicion.
Common mistakes during troubleshooting
Adjusting before diagnosing. Closing dampers or increasing fan speed without understanding the cause can hide the real problem and add energy cost.
Trusting a single reading. One airflow measurement can be wrong because of poor traverse position, turbulence or instrument error. Repeat and cross-check key readings.
Ignoring the return side. Return air paths, transfer grilles and return leakage are often overlooked, yet they strongly affect supply airflow.
Skipping documentation. Without a record of what was measured and changed, it becomes impossible to tell whether a correction worked or whether a new problem was introduced.
A hypothetical scenario
A floor of offices is reported to be warm. Total airflow at the air handler is measured at about 90% of design, while terminals show 70% on average. Static pressure along the main duct falls faster than the design calculation suggests, and a leakage test indicates significant loss in the supply main. The shortfall at the terminals is therefore not mainly a fan problem; it is a delivery problem. Sealing the identified leakage and clearing a partly closed damper, followed by a repeat airflow measurement, is likely to be far more effective, and far cheaper, than replacing the air handler.
When to escalate
If initial checks of the fan, filters, dampers and visible duct condition do not explain the shortfall, escalate to quantified testing: duct leakage testing, a static pressure profile along the main run, and a review of the original design calculations. At this stage it can also be useful to bring the designer, the TAB contractor and the installing contractor together to compare findings, because the cause often sits at the boundary between their responsibilities.
Rectify and verify
Once the cause is identified, correct it and verify the result using the same measurements that revealed the problem. Airflow that returns to design at the fan and terminals, with leakage below the agreed limit and pressures consistent with the design, is the evidence that the shortfall has been resolved, not just masked.
Conclusion
Systems rarely fail to reach design airflow for a single mysterious reason. They fall short because of a small number of identifiable, measurable causes. Checking them in a sensible order saves time, avoids unnecessary equipment changes and leads to corrections that last.
