5.1 Air-System Configurations and System Relationships
Key Takeaways
- Trace each airstream from source to destination before selecting measurement planes or comparing totals.
- Supply, return, relief, exhaust, outdoor, transfer, infiltration, and exfiltration are different paths in the building mass balance.
- Constant-volume and variable-volume systems require different operating-state and diversity assumptions during TAB.
- Heat-recovery and induction systems contain coupled airstreams, so a change on one side can affect pressure or performance on the other.
- Active chilled beams separate ventilation primary air from much of the sensible room load; primary airflow is not the total induced room circulation.
Air-System Configurations and System Relationships
Start with the air path
The current NEBB TAB technician seminar names air-system relationships as a manual chapter and specifically reviews supply, return, exhaust, constant-volume, variable-volume, heat-recovery, induction, active chilled-beam, and makeup-air systems. Those configurations cannot be balanced from one generic checklist. First trace where air enters, where fans add pressure, where paths split or join, and where air leaves the building.
| Airstream | Function | Common measurement concern |
|---|---|---|
| Outdoor air (OA) | Ventilation or process replacement | Intake stratification, wind, damper position |
| Supply air (SA) | Conditioned air delivered to zones | Main/branch continuity, leakage, diversity |
| Return air (RA) | Air routed back to equipment | Plenum leakage, transfer paths, door position |
| Exhaust air (EA) | Air intentionally removed | Hood capture, negative spaces, discharge condition |
| Relief air | Air released from an air-side economizer/building | Relief-damper control and building pressure |
| Makeup air (MAU) | Replaces exhausted process or kitchen air | Interlock and operating schedule |
| Transfer air | Moves between rooms without a dedicated supply path | Door undercuts, grilles, pressure relationships |
Define the measurement boundary before writing a continuity equation. At an air handler, outdoor plus return air approximates mixed air, and supply air after the fan may differ because of leakage. At the building boundary, outdoor intake plus infiltration must balance relief, exhaust, and exfiltration over a stable period. A mismatch can reflect a missing path or measurement uncertainty, not necessarily a violation of physics.
Constant-volume systems
A constant-volume (CV) fan is intended to deliver approximately steady airflow while thermal control changes temperature, cycles equipment, or uses reheat. Branch dampers and terminal devices establish distribution. Proportional balancing works well when fan speed and system resistance remain stable.
Verify fan mode, filters, coils, dampers, doors, and terminal conditions before comparing repeated sweeps. A cycling bypass, uncommanded economizer damper, or pressure-dependent control device can make a “constant” system vary.
Variable-volume systems
A variable-air-volume (VAV) system changes central and terminal airflow with load. The sum of every terminal maximum may exceed the central fan's diversified design value because all zones may not peak together. The project sequence tells the technician how to establish test modes, whether boxes may be commanded to maximum, how diversity is represented, and how the fan static-pressure loop is controlled.
Do not force all boxes open and then condemn a fan for failing to deliver the arithmetic sum unless that is the defined design test condition. Record overrides and restore normal control after testing.
Return, exhaust, makeup, and pressure
Return systems may use ducted returns, ceiling plenums, or both. A ceiling plenum introduces paths through doors, shafts, lighting openings, and partitions; pressure conditions and ceiling completion therefore matter. Exhaust systems must be evaluated with their makeup and transfer paths. Closing a door can reduce exhaust flow or make a room more negative if the transfer opening is undersized.
For a kitchen or laboratory, makeup air may be introduced directly to the room, through the hood, or through the general HVAC system. Measure the specified operating mode and required door condition. Room or building pressure targets come from the design and applicable requirements; there is no safe universal positive or negative number for every occupancy.
Heat-recovery systems
Energy-recovery wheels, fixed-plate exchangers, runaround coils, and heat pipes transfer heat between outdoor and exhaust airstreams. Wheels can also transfer moisture. Check both airstreams, fan arrangements, pressure relationships, bypass dampers, rotation, and cross-leakage risk. A dirty wheel changes pressure drop and flow on both sides.
When calculating effectiveness, confirm which temperatures or enthalpies are entering and leaving each side. Do not confuse recovered energy with airflow transfer.
Induction units and active chilled beams
An induction unit uses a high-velocity primary-air jet to entrain room air through a secondary coil or cabinet. An active chilled beam similarly uses conditioned primary ventilation air to induce room air across a water coil. The diffuser discharge contains primary plus induced air, but the duct delivers only primary air. A capture-hood reading at the room discharge may not represent primary airflow.
Use the manufacturer's method, primary-air nozzle pressure or calibrated relation, and water-side measurements as applicable. Condensation control is crucial: chilled-water temperature and room dew point must satisfy the design sequence.
A system map before measurements
For every system, mark:
- Fan and airflow direction.
- Outdoor, return, relief, exhaust, transfer, and makeup paths.
- Constant or variable control points.
- Design flows and diversity basis.
- Valid main and branch measurement planes.
- Operating modes and interlocks.
- Boundaries used for continuity checks.
The exam-ready question is not “Which instrument do I like?” It is “Which quantity at which boundary answers the system question?”
Why may the sum of all VAV box maximum setpoints exceed the central fan design airflow?
At an active chilled beam, what does the primary duct airflow represent?
What should establish a laboratory room-pressure target and test condition?