Common airflow problems in commercial buildings

When tenants complain about temperature, stuffiness or noise, the cause is usually an airflow problem rather than an equipment problem. The plant is often running exactly as it should. What has gone wrong is how much air reaches each space, and that cannot be diagnosed by looking at a system or reading a screen. It needs airflow testing: measurement at each outlet, compared against the design figure.

Below are the four problems that account for most of the commercial HVAC complaints we are called to investigate, what causes each one, and how measurement separates the real fault from the assumed one.

Problem 1: Hot and cold spots across a floor

The most common complaint on a commercial floor is that one area is uncomfortable while another is fine. Occupants describe it as a plant capacity problem. It rarely is.

The usual cause is unbalanced distribution. The total air leaving the unit may be close to design, but it is not divided correctly between zones. Some outlets are over-supplied and others are starved. This happens where dampers were never set, were set for a previous layout, or have been adjusted informally over the years to solve one complaint at the expense of another.

How it is diagnosed: measure airflow at every accessible supply and return outlet and compare each reading against the design value. The pattern shows immediately. A floor where the total is correct but individual outlets range widely is a distribution problem, not a capacity problem.

The fix is damper adjustment through air balancing, redistributing the available air until each outlet is within tolerance of design.

Problem 2: Stuffy or stale air

Complaints about stuffiness, odours or air that feels heavy usually point at the outside air and exhaust side of the system rather than the supply side.

The usual cause is insufficient outside air reaching the space, or exhaust that is not doing its job. An outside air damper may be closed or partially seized, a filter may be loaded, or an exhaust fan may be running but moving far less than its rated duty. Toilet and kitchen exhaust are frequent culprits because they are rarely checked once installed.

How it is diagnosed: measure the outside air intake rate and the exhaust airflow rates separately. Outside air is measured at the intake duct, usually by traverse. Exhaust is measured at each grille and at the fan.

The fix depends on the finding: adjusting the outside air damper, rectifying the fan or drive, or clearing an obstruction. The point is that the measurement tells you which, before anything is replaced.

Problem 3: Noise from diffusers

Noise complaints are often treated as a diffuser selection problem and answered by replacing the diffuser. That works only when selection was actually the issue.

The usual cause is velocity. Too much air is being pushed through a given outlet, either because the system is unbalanced and that outlet is taking more than its share, or because a damper upstream is partially closed and the air is accelerating through the restriction. Undersized diffusers cause the same symptom, but they are the less common cause on a system that was quiet when first installed.

How it is diagnosed: measure airflow and velocity at the noisy outlet and at its neighbours. If the noisy outlet is running well above design while others are below, the problem is distribution, not the diffuser.

The fix is to redistribute airflow across the branch, and only resize the outlet if measurement shows the selection genuinely cannot pass the design volume quietly.

Problem 4: Systems not meeting design after a fit-out

This one is specific to buildings that have been altered, and it is the most predictable of the four.

The usual cause is that a fit-out changed the duct layout, added or removed outlets, or redrew the zones, and the system was never re-balanced afterwards. The original balance was correct for a floor plan that no longer exists. Every subsequent complaint is treated as a new fault when the system is simply set for the wrong building.

How it is diagnosed: a full airflow survey comparing actual against the updated design, not the original one. This is also where the absence of current documentation shows up, and where an as-built record earns its keep.

The fix is to re-balance the system to the current design intent, and to document the result so the next tenancy change starts from a known baseline.

A note on BMS readings

One pattern is worth calling out because it cuts across all four problems. A building management system reports what its controllers and sensors believe is happening. Where a sensor has drifted, an actuator has stuck, or a terminal has been registered against the wrong address, the value on the screen and the air in the room diverge, sometimes substantially.

If a system is being investigated on BMS data alone, the investigation can run a long way in the wrong direction. Comparing reported values against physical measurement at the terminal is often the step that ends a long-running complaint. That comparison is the core of VAV and system diagnostics.

When to bring in airflow testing

Maintenance adjustments are worth trying first, and they resolve a reasonable share of complaints. Bring in professional airflow testing when:

• adjustments have been made and the complaint has returned
• the same area is repeatedly reported and nobody can say why
• the building has been altered and the system was not re-balanced
• there is no current documentation of what the system should be delivering
• you need evidence rather than an opinion, for a consultant, a certifier or a client

Testing with calibrated instruments turns a subjective complaint into a set of numbers: what each outlet is delivering, what it should be delivering, and where the gap is. That is the difference between fixing the problem and replacing parts until it goes away.

For carparks, tunnels and other mechanically ventilated spaces, the equivalent work is ventilation system testing, which follows the same principle at system level.

Get the measurements

ENGIC provides HVAC airflow testing and diagnostics across Sydney for mechanical contractors, builders, consultants and facility managers. Every accessible outlet is measured individually with calibrated instruments, compared against design, and documented in a structured report delivered within 24 hours of site testing.

If you have a complaint you cannot close out, get in touch and we will tell you what the system is actually doing.