Variable flow rate (VFD pumps/fans)

Variable flow rate (VFR) involves continuously adjusting the speed of pumps and fans using a variable frequency drive (VFD) to provide only the flow rate and pressure required at any given time. Rather than "pushing hard and then throttling," the motor speed is modulated: the electrical power then drops almost proportionally to the cube of the speed (affinity law) — resulting in energy savings, reduced noise, cooler return water (for heating), and more consistent comfort. Variable flow rate works particularly well with two-way valves, balanced systems, and a building management system (BMS) that intelligently controls pressure and pressure differential (ΔP).

Variable flow rate: principle and implementation

The VFD adapts the motor frequency to follow a setpoint (network pressure difference, static pressure, flow rate, CO₂ for air). Some key points:

- Pumps : controlled by a maintained ΔP (sensor at the base of the column or critical point), or even a ΔP “reset” (setpoint lowered when the valves open only slightly). Significant gains are achieved when the network uses 2-way valves and is balanced.

- Ventilation : control based on static pressure in duct with reset according to the opening of the terminal flaps (VAV boxes) or the air quality (CO₂/PM).

- Hydraulics : primary/secondary decoupling if the generators require a minimum flow rate (pressure-reducing tank, manifold). Target ΔT monitored to avoid excessively hot return.

- Electrical : EMI/harmonic filters according to power, suitable motor protection, carefully designed acceleration ramps and PID for stable modulation.

- GTB : useful alarms (sensor loss, min/max range), time ranges, peak clipping scenarios.

Advantages, limitations and points to consider regarding variable flow rate

Interests

• Energy: at 80% speed, power can drop ~-50%; at 60%, ~-80% (order of magnitude).
• Comfort: fine regulation of flow/pressure, reduced noise, stable temperatures.
• Operation: less wear, bypasses avoided, ΔT better maintained (heat pump/boiler efficiency ↑).
• Flexibility: adaptation to occupancy (CO₂, presence) and seasons.

Boundaries

• Unbalanced networks → instabilities (noise, poorly served areas).
• Generators imposing a minimum flow rate: decoupling is necessary.
• Poorly placed sensors = constant pumping (pump oscillates).
• Harmonics/EMC at high power levels if filtering is absent.

Points to consider

• Choice of setpoint: ΔP at the critical point (or reset depending on valve position) rather than at the technical room.
• Valve authority: Kvs adapted, ΔP limitation in closing.
• PID: opening steps, integration time and ramp to avoid pumping/oscillation.
• Minimum speeds: provide a floor (avoid blade stalling, cavitation).
• Parallelism: on multiple pumps, rotation of priorities and clear start/stop laws.
• Filtration/ducts: in ventilation, measure ΔP filters so that the setpoint reflects the actual state of the network.

Anecdote — “The calm wind of Nanterre”

In Nanterre, office buildings were complaining of whistling noises in the afternoon and inconsistent cooling. The site was operating at a fixed speed with throttling dampers. The changes included switching to VFD (Variable Frequency Drive), relocating the duct pressure sensor to the critical point, implementing a setpoint reset based on the opening of the terminal boxes, and adjusting the PID (Picture-in-Picture) system. The result: a 38% reduction in fan energy consumption for the season, a significant decrease in noise, and consistent temperatures. The operations manager summarized: "We stopped pushing the air against walls."