What Is a VFD in HVAC? Uses, Savings, Selection Guide

What is a VFD in HVAC?

A variable frequency drive (VFD) is installed between the electrical supply and a motor (typically induction motors in HVAC equipment). By changing the frequency of the electrical power delivered to the motor, the VFD changes motor speed (RPM). In HVAC, VFDs are most commonly used on variable-torque loads such as centrifugal fans and centrifugal pumps, where speed control is an efficient way to match capacity to real-time demand.

What a VFD does in practical terms

• Slows down or speeds up a fan/pump motor based on sensors (pressure, flow, temperature, CO₂, etc.).
• Replaces “wasting” control methods (throttling valves, inlet vanes, bypass loops) with efficient speed control.
• Adds soft-start/soft-stop behavior, reducing mechanical stress and inrush current.

Why VFDs save energy in HVAC (the affinity laws)

For centrifugal fans and pumps, the affinity laws describe how performance changes with speed. The key relationship for energy is that power varies roughly with the cube of speed. That means small reductions in speed can produce large reductions in power.

• Flow ∝ Speed
• Pressure/Head ∝ Speed²
• Power ∝ Speed³

A widely used rule of thumb is: a 10% reduction in speed can reduce power by about 30% on variable-torque loads under typical conditions. At 50% speed, idealized fan/pump power is about 12.5% (one-eighth) of full-load power.

These are estimates; real savings depend on the system curve, control strategy, and operating hours. Still, the physics explains why VFDs are often a top-tier HVAC retrofit when loads vary through the day.

Common HVAC applications for VFDs

VFDs deliver the best return where demand varies and equipment can safely run at reduced speed for long periods.

Fans

• AHU supply fans (static pressure reset, VAV systems)
• Return/exhaust fans (building pressure control)
• Cooling tower fans (condenser water temperature control)

Pumps

• Chilled water pumps (differential pressure control, two-way valves)
• Condenser water pumps (flow optimization, tower integration)
• Hot water pumps (reset strategies tied to outdoor air temperature)

Note: VFDs are also used in some compressor applications, but compressor control is equipment- and manufacturer-specific. The most straightforward HVAC wins are typically fans and pumps.

VFD control strategies that work (and what to avoid)

Savings are created by the control sequence, not by the VFD alone. The most effective sequences reduce speed as much as possible while maintaining comfort and stability.

Best-practice strategies

• Static pressure reset for VAV supply fans (reset based on “most open damper” or critical zone demand)
• Differential pressure reset for variable-flow hydronic loops (reset based on valve position at remote coils)
• Cooling tower fan speed control to maintain condenser water setpoint with minimum fan energy
• Night setback and optimum start/stop coordinated with VFD minimum speeds

Common pitfalls

• Maintaining an unnecessarily high static or differential pressure setpoint all day (the fan/pump never slows down)
• Using bypass loops that force constant flow (undermines the value of variable speed)
• Setting minimum speed too high “for safety,” eliminating meaningful part-load operation
• Control loops tuned poorly, causing hunting, noise complaints, or trips

VFD vs. other HVAC capacity control methods

If your system currently controls flow by “creating resistance” (throttling), a VFD typically reduces energy because it lowers speed instead of wasting pressure.

How to size and select a VFD for HVAC equipment

Proper VFD selection is largely an electrical and environmental exercise: match the drive to the motor, the load type, the supply, and the installation conditions.

Selection checklist

• Motor nameplate: HP/kW, voltage, full-load amps (FLA), base frequency, service factor
• Load type: variable torque (fans/pumps) vs constant torque (some conveyors) — HVAC fans/pumps are usually variable torque
• Supply: 480V/208V, 3-phase, available fault current, grounding, harmonic considerations
• Environment: electrical room vs rooftop; temperature, dust, moisture; enclosure rating and cooling requirements
• Controls: BAS integration (BACnet/Modbus), analog inputs, PID capability, safety interlocks
• Motor protection: overload, phase loss, under/over-voltage, thermal inputs

In HVAC retrofits, a common sizing approach is selecting a VFD with an output current rating at or above the motor FLA (considering service factor and site conditions). For long motor leads, older motors, or sensitive environments, include appropriate filtering (such as output reactors or dv/dt filters) per manufacturer guidance.

Example: estimating savings and payback with real numbers

The simplest business case uses baseline kW, operating hours, expected speed reduction profile, and electricity rate. The example below is illustrative and should be refined with trend data (kW, speed, static pressure/DP, valve positions) from your building.

Illustrative fan example

• Motor: 30 HP supply fan (approximately 22.4 kW mechanical at full load)
• Operating hours: 4,000 hours/year
• Average speed after optimization: 80% (0.8 per-unit) for most occupied hours
• Electricity rate: $0.18/kWh

If power scales roughly with the cube of speed, average power at 80% speed is about 0.8³ = 0.512, meaning about a 48.8% reduction relative to full-speed power for that portion of runtime. If full-speed electrical demand were 25 kW and you truly average ~51% of that after VFD control, annual energy would be:

• Before: 25 kW × 4,000 h = 100,000 kWh
• After: 25 kW × 0.512 × 4,000 h ≈ 51,200 kWh
• Estimated savings: ~48,800 kWh/year
• Estimated cost savings: ~48,800 × $0.18 ≈ $8,784/year

If a turnkey VFD retrofit (drive, install, programming, commissioning) cost $12,000, simple payback would be about 1.4 years. Real projects should also include maintenance impacts, potential demand-charge reduction, and any utility incentives.

Commissioning checklist for stable performance

Commissioning ensures the VFD actually runs at reduced speed without causing comfort, noise, or reliability issues.

Key commissioning items

• Confirm motor rotation and verify actual airflow/flow at several speeds.
• Set minimum and maximum speeds based on equipment limits (coil freeze risk, minimum ventilation, minimum pump flow, tower basin control).
• Tune PID loops to eliminate hunting (confirm sensor location and stability).
• Implement setpoint reset logic (static pressure/DP reset) and validate it with trend logs.
• Verify safety interlocks: smoke control sequences, freezestats, proof switches, HOA logic, fire alarm integration.
• Check electrical quality: grounding, shielding, and any required reactors/filters.

Maintenance and troubleshooting basics

VFDs are reliable when installed correctly, but they do add electronics that require basic preventive maintenance.

Preventive maintenance

• Keep enclosures clean; maintain proper cooling airflow and room temperature.
• Inspect fans, filters, and heat sinks; replace clogged filters on schedule.
• Periodically check terminals for torque and signs of overheating.
• Back up parameters (drive configuration) after commissioning changes.

Frequent issues and likely causes

• Nuisance trips: aggressive acceleration/deceleration ramps, unstable PID, poor power quality, or inadequate cooling.
• Noise/whine: carrier frequency settings, motor condition, or mechanical resonance at certain speeds.
• Low savings: setpoints not reset, minimum speed too high, or system not truly variable (bypass/constant-flow conditions).

Direct conclusion: when a VFD is worth it in HVAC

A VFD is most valuable in HVAC when you have variable demand, long run-hours, and centrifugal fans or pumps that can operate safely at reduced speed. If your current system controls capacity by throttling or dampers and your load varies daily or seasonally, a VFD retrofit paired with proper setpoint reset can deliver substantial, measurable energy reduction while improving controllability and equipment life.

References (for the key energy relationships)

• U.S. Department of Energy, motor tip sheets and reports discussing affinity-law impacts (including 10% speed reduction ≈ 30% power reduction): Motor Energy Savings Potential Report
• U.S. Department of Energy tip sheet example (half speed ≈ one-eighth power for fans): Adjustable Speed Drive Part-Load Efficiency
• ASHRAE Journal guidance on estimating VFD power draw from speed and clarifying exponents: To Affinity… and Beyond!
• Eaton white paper summarizing affinity laws for pumps (flow, head, power relationships): Variable frequency drives: energy savings for pumping applications