Using VSDs to Reduce HVAC Fan Energy Use by 30%

If you are managing a commercial building in Australia, you know that HVAC consumption typically accounts for 40–50% of your total electricity bill. Within that plant room, supply and extract fans are often the biggest culprits of energy waste.

The solution isn't just buying newer fans; it is controlling the ones you have intelligently.

At Controls Traders, we have supplied building automation components for over 40 years. One of the most effective upgrades we see for immediate ROI is the installation of Variable Speed Drives (VSDs). This guide explains the physics behind the savings and how to apply them to your facility.

In short, Variable Speed Drives reduce HVAC fan energy use because they:

1. Match fan speed to real-time airflow demand instead of running at full speed.
2. Exploit the fan affinity laws, where small speed reductions deliver large energy savings.
3. Enable intelligent control via BMS signals, pressure sensors, and CO₂-based demand control.
 

1. Introduction to VSDs

A Variable Speed Drive (VSD)—also known as a Variable Frequency Drive (VFD)—is an electronic device that controls the speed of an AC induction motor by varying the frequency and voltage supplied to it.

Think of a VSD as a "dimmer switch" for your heavy industrial motors. Without a VSD, your AHU (Air Handling Unit) fan is either OFF or running at 100% full speed. With a VSD, that same fan can run at 40%, 60%, or 95%—matching the exact airflow required by the building at that moment.

2. Why HVAC Fans Waste Energy at Full Speed

Most HVAC systems in Australia are designed for "Design Day" conditions—the hottest days of the year (e.g., a 40°C day in Adelaide or Western Sydney).

However, these peak conditions occur for only a fraction of the year. For the remaining 95% of the time, the building operates at partial load. If your supply fans are running at full speed during mild weather, you are pushing more air than necessary.

In older systems without VSDs, this excess air is often choked back using mechanical dampers or inlet guide vanes. This is the energy equivalent of driving your car with your foot flat on the accelerator and controlling your speed by riding the brakes. It is inefficient and mechanically stressful.

3. How VSDs Optimise Supply and Extract Fans

A VSD replaces the need for mechanical throttling. By receiving a signal from your Building Management System (BMS) or a standalone controller, the VSD slows the motor itself down.

• Static Pressure Control: As VAV (Variable Air Volume) boxes in the office close, duct pressure rises. A pressure sensor detects this and tells the VSD to slow the fan down to maintain a setpoint.
• CO₂ Demand Control: If a meeting room is empty, [CO₂ Sensors] detect low occupancy. The BMS signals the VSD to reduce fresh air intake, saving energy on both fan power and the conditioning of outside air.

4. Energy Savings Explained (The Affinity Laws)

The financial magic of VSDs lies in the Fan Affinity Laws.

While flow is proportional to speed, power is proportional to the cube of the speed. This is known as the "Cube Law." $$Power \propto Speed^3$$

This means a small reduction in fan speed results in a massive reduction in energy consumption.

5. Example: Reducing Fan Speed by 20%

Let’s look at the math for a standard supply fan running at 80% speed (a 20% reduction):

• Flow: $80%$ speed = $80%$ airflow.
• Power: $0.80 \times 0.80 \times 0.80 = 0.512$ (or $51.2%$).

The Result: By slowing your fan down by just 20%, you reduce its electricity consumption by roughly 50%. Even a modest reduction of 10% speed saves nearly 30% in energy. This is why VSDs offer such a rapid payback period.

6. Where VSDs Are Typically Installed

VSDs are versatile and can be applied to almost any rotating equipment in the plant room:

• AHU Supply & Return Fans: To match airflow to VAV demand.
• Chilled Water Pumps: To vary water flow through chillers and coils (Variable Primary Flow).
• Cooling Tower Fans: To ramp fans up/down based on condenser water return temperature, rather than cycling them on/off.
• Car Park Exhaust: To run fans at low speed for ventilation and ramp to high speed only when CO levels rise.

7. Common Issues and Installation Notes

While VSDs are powerful, they require correct installation:

• Harmonics: VSDs can introduce electrical noise (harmonics) back into the building's power supply. Ensure you select drives with built-in filters or line reactors.
• Motor Cooling: A standard motor relies on its internal fan for cooling. If you run it at very low speeds (e.g., <20Hz) for long periods, it may overheat.
• Cable Length: Long cable runs between the VSD and the motor can cause voltage spikes. You may need specific screened cables.

8. Economic Benefits and Payback

Beyond the electricity bill, VSDs reduce mechanical wear. By "soft starting" the motor (ramping up slowly), you eliminate the high-torque shock of "Direct On Line" (DOL) starting, which extends the life of belts, bearings, and couplings.

For most commercial buildings, the ROI on a VSD retrofit is typically under 2 years, making it one of the most attractive CapEx projects for facility managers.

Summary

If your HVAC fans are running at constant speed while your building load varies, you are paying for energy you don't use. Implementing Variable Speed Drives allows you to harness the Cube Law, turning a 20% speed reduction into a 50% energy saving.

At Controls Traders, we warehouse a range of drives and controls suitable for the Australian market, ready for fast delivery.

Ready to upgrade your plant room? Read the full guide on our website for installation specs and recommended models. Browse our range of Variable Speed Drives and Test Instruments to get started.