Exhaust hood VFD control starts with a pressure limit, not a speed command. With all 20 canopy hoods open, this system produced 1.625 in. w.g. static pressure. That number describes one operating state only. Closing dampers changes system resistance, shifts airflow among the remaining hoods, and can drive their flow above the balanced value unless fan speed decreases.
The number that matters is the static pressure at the selected control point when the required hood airflow has been confirmed by test and balance. Motor current and thermal load still matter: the commanded speed must remain inside the fan, motor, and VFD operating limits throughout the response to damper changes.
Pressure and airflow mechanism
A VFD changes fan speed, while the duct system determines the resulting pressure and airflow. When a hood damper closes, system resistance rises. At unchanged speed, the operating point moves and the open hoods can receive more airflow. The pressure controller detects the change and reduces the speed command until pressure returns to setpoint.
Static pressure control regulates pressure at one location; it does not measure each hood's CFM. Equal or repeatable hood flow therefore depends on duct geometry, balancing-damper positions, branch interaction, and the relationship between the sensor location and the hydraulically most difficult hood.
| Quantity | Value or limit | Where to read or establish it |
|---|---|---|
| Number of hoods | 20 total; 10 per branch | Duct layout and field inspection |
| All-open system static pressure | 1.625 in. w.g. | Measured system condition stated for all hoods operating |
| Possible initial operating target | About 1.6 in. w.g. | Use only if measured at the final sensor location and verified against required hood flow |
| Example critical-hood target | 0.4 in. w.c. | Illustrative only; replace with the pressure measured after test and balance |
| Required hood airflow | Project-specific CFM | Hood schedule, design documents, and balancing report |
| Speed and current limits | Equipment-specific | Fan data, motor nameplate, VFD configuration, and measured motor current |
Sensor-location comparison
| Approach | Control behavior | Advantages | Limitations |
|---|---|---|---|
| Sensor about two-thirds of the duct length from the fan | Regulates pressure at a representative point within the common system | Common starting point for a distributed duct network; responds to overall resistance changes | May not protect the hood with the greatest pressure loss. Branch changes can alter the relationship between sensed pressure and individual hood CFM. |
| Sensor at the farthest or critical hood, with that path kept open | Reduces fan speed when other dampers close and pressure rises at the critical hood | Directly protects the most difficult flow path and gives the controller a clear minimum-service point | The selected hood must truly be critical. Removing or locking open its shutoff damper changes operating practice and must be acceptable for the process. |
The two-thirds location is suitable when the branches are reasonably symmetrical and field measurements show that all hoods remain within their allowable airflow range. The farthest-hood approach is preferable when one path clearly has the largest loss or when maintaining that hood's flow is the controlling requirement.
Recommended control architecture
Use the farthest or hydraulically weakest hood as the initial critical-point candidate. Confirm the choice by measuring hood flow with all dampers open; physical distance alone does not prove which path has the greatest pressure loss. Place the static-pressure pickup in a straight, representative section serving that hood, away from elbows, transitions, damper turbulence, and direct velocity impact.
Keep the critical path continuously available to the controller. If its damper must close, provide another method for determining that no airflow is requested; otherwise the pressure loop can accelerate the fan while attempting to recover pressure from an isolated sensing point.
Separate balancing from on/off operation. A lockable balancing position establishes each hood's design resistance. A shutoff function opens to that repeatable position or closes fully. If one manual blade performs both functions, add mechanical stops or durable position marks; arbitrary reopening positions destroy the balance.
Balancing and setpoint procedure
- Open all 20 hood shutoff dampers and place every balancing damper at its initial commissioned position.
- Run the fan at the speed required to deliver the specified CFM at each hood. Measure every hood rather than inferring flow from common-duct pressure.
- Adjust individual balancing dampers so the two branches and their ten hoods distribute airflow acceptably. Recheck earlier hoods after each significant adjustment because the dampers interact.
- Measure static pressure at the permanent sensor location after airflow is correct. Record this value as the initial pressure setpoint. Use 1.625 in. w.g. or approximately 1.6 in. w.g. only when that value represents this exact location and balanced condition.
- Close selected noncritical hoods and confirm that the VFD slows while the critical hood remains within its required airflow range.
- Repeat representative combinations on both branches. Correct branch imbalance with balancing adjustments or sensor relocation rather than masking it with a higher pressure setpoint.
The 0.4 in. w.c. value is an example of a possible critical-hood setpoint, not a transferable design value. Establish the actual setting from the final balanced airflow and permanent pickup location.
VFD control sequence
- Enable the exhaust fan when at least one permitted hood requires exhaust. If no hood-status signals exist, define a manual system enable and keep the critical path open whenever the fan runs.
- Start the fan at a commissioned low command that produces stable sensor feedback without stalling the fan or losing motor cooling.
- Compare measured static pressure with the commissioned setpoint. Increase speed below setpoint and decrease speed above setpoint.
- Apply acceleration, deceleration, minimum-speed, and maximum-speed limits obtained from the fan, motor, and VFD documentation. This is heat, not logic: excessive motor current or operation outside the allowed cooling range requires a hardware or configuration correction.
- On sensor failure or an implausible pressure reading, generate an alarm and use the project's defined safe fallback speed or shutdown state. Select that state from the exhaust hazard assessment rather than from energy-saving preference.
- Stop the fan when the system is disabled and the process permits exhaust removal to cease.
Tune the pressure loop after the dampers and sensor tubing are final. A loop that hunts as manual dampers move needs slower correction, stable sensing, or both. A controller that remains at maximum speed while pressure stays low indicates insufficient fan capacity, a disconnected pickup, an open access panel, unexpected leakage, or an isolated sensor location.
Verification and recurring pitfalls
Verify control with airflow measurements, pressure trends, speed command, and motor current. Test all-open operation, each branch heavily loaded, representative partial-load combinations, and the minimum intended number of open hoods. Allow the pressure and speed to settle after each damper change before recording results.
| Observed symptom | Likely cause | Field check |
|---|---|---|
| Open hoods gain excessive CFM as other hoods close | Setpoint too high, poor sensor location, or slow loop response | Trend pressure and speed during a damper closure; compare individual hood CFM before and after |
| One branch is consistently weak | Branch resistance or balancing mismatch | Measure hood CFM on both branches at the same operating state |
| Pressure is stable but hood flows vary widely | Static control is working, but terminal balancing is not | Inspect damper stops and rebalance every hood |
| Speed oscillates | Turbulent pickup location, aggressive tuning, or unstable manual damper movement | Inspect pickup placement and trend pressure without moving dampers |
| Fan stays at a speed limit | Unreachable setpoint or inadequate operating range | Compare actual pressure, speed command, motor current, and verified fan capability |
Frequently asked questions
How do I choose the static-pressure sensor location for an exhaust hood VFD?
Test the farthest and weakest-flow hoods with all 20 hoods open. Use the critical hood location when it reliably represents the limiting path; use the two-thirds duct location only after measurements show that it maintains acceptable airflow across both branches.
How do I set the exhaust fan static-pressure setpoint?
Balance every hood to its required CFM, then record pressure at the permanent sensor location. The measured 1.625 in. w.g. can support an initial setting near 1.6 in. w.g. only if it was taken at that location under the balanced all-open condition.
How do I know when to stop adjusting the VFD and escalate?
Stop when the fan remains at a speed or current limit, the controller cannot reach setpoint, hood airflow remains unacceptable after balancing, or the safe response to sensor failure is undefined. Escalate to the fan, VFD, or controls manufacturer's official support channel with the balancing report, duct arrangement, pressure trend, speed command, motor current, and equipment identification.