9.2 VAV Terminal Types, Airflow Sensors, and Calibration

VAV Terminal Types, Airflow Sensors, and Calibration

Read the sequence before naming the mode

Variable-air-volume (VAV) terminals regulate primary airflow to a zone. The box type, reheat arrangement, fan location, controller application, and approved sequence determine what should happen in cooling, deadband, heating, warm-up, and unoccupied modes. Common arrangements include:

These are common behaviors, not substitutes for the project sequence. A series fan may cycle in an unoccupied mode, and a parallel fan may be commanded for ventilation or another project-specific condition. Backdraft devices, coils, discharge arrangements, and airflow targets vary by product.

At the box, verify tag, model, inlet size, flow-sensor orientation, tubing, damper stroke, terminal-fan rotation and speed, coil and valve arrangement, downstream outlets, controller program, and the actual sequence. A BAS point name such as “cooling max” is not proof that the physical box has reached that state.

Differential-pressure airflow sensors

Many VAV terminals use a multi-point pickup connected to a differential-pressure transducer. The pickup creates a signal related to inlet velocity pressure. A common controller relationship is:

Q = K x square root of differential pressure

Q is the controller airflow value and K is a product-specific factor. The factor may incorporate inlet area, pickup gain, units, and controller conventions. Do not derive K from a generic amplification range when the manufacturer or approved control data supplies it. Confirm high/low tubing, port cleanliness, transducer range, zero, engineering-unit scaling, and configured inlet size.

Low differential pressure deserves special care. A stable displayed CFM value can be wrong if the pressure signal is near the transducer's resolution, tubing is leaking, the pickup is installed backward, or software imposes a minimum. Sensor straight-run needs and inlet conditions are manufacturer-specific; an elbow or branch immediately at the inlet can bias the pickup.

Independent field comparison

Establish a stable authorized test state and measure flow with a method that captures the same boundary. A suitable downstream duct traverse or a complete terminal-outlet sum may be used depending on access and uncertainty. One hood reading does not represent the box if multiple outlets are omitted, and a room-side fan-powered discharge measurement includes induced air that is not primary flow.

Record command, damper position, differential pressure, BAS airflow, independent airflow, fan state, duct static pressure, and system mode at the same time. If the values disagree, check the measurement methods and control configuration before changing a factor.

If a controller truly uses Q = K times the square root of differential pressure, a first linear factor correction at the same stable signal is:

Knew = Kold x Qmeasured / QBAS

For example, Kold = 450, QBAS = 920 CFM, and Qmeasured = 780 CFM gives a provisional Knew of about 382. Enter or recommend that value only through the authorized manufacturer/controller procedure. Some applications store pickup gain, area, separate heating/cooling values, or a flow multiplier instead of one K. Changing the wrong parameter can create a correct display at one point and a wrong display everywhere else.

Maximum, minimum, and heating checks

Verify the operating points required by the project and controller procedure. Maximum flow checks whether the box can reach design without excessive inlet pressure or a saturated damper command. Minimum flow checks ventilation control and whether the sensor remains usable at low signal. Heating checks may involve a separate primary-flow target, terminal fan, reheat valve, or discharge limit.

A one-point correction does not prove linearity. After any authorized adjustment, recheck at least the other required points and return to the first point if the controller or system has shifted. If minimum flow is below the reliable sensor range, report the limitation and use the approved remedy; do not invent a K factor that hides it.

Fan-powered terminal checks

For a parallel box, distinguish primary airflow from induced plenum airflow and total discharge. Verify fan command, backdraft or isolation behavior where provided, rotation, speed, current, and discharge response. For a series box, verify the terminal fan and mixing path before using discharge flow to evaluate primary-air calibration.

Downstream outlet balancing is a separate task. First establish the terminal's primary or total discharge condition called for by the procedure, then proportion the outlets. Recheck the box total because outlet damper changes alter downstream resistance.

Diagnostic logic and record

When BAS flow is high but the independent result is low, inspect tubing reversal or leakage, pickup orientation, inlet disturbance, zero, transducer scaling, K or area configuration, omitted outlets, and system drift. When the damper is open but flow remains low, check available inlet pressure and branch restriction before raising central duct pressure.

The final record identifies box type, design maximum/minimum/heating values, tested mode, independent method, BAS value, differential pressure, factor before and after, damper position, terminal-fan state, inlet pressure where required, downstream total, and unresolved limitation. Release every temporary override and verify the normal sequence.