Case Study: Energy Savings Achieved Using Danfoss VLT Drives in HVAC Systems
Modern HVAC systems consume a significant portion of a building’s total electrical energy. When air-handling units, pumps, and chillers operate at full speed all day, energy waste is inevitable. This case study explores how a UK commercial building achieved 27% energy savings and improved temperature control after upgrading to Danfoss VLT® variable frequency drives (VFDs) across its HVAC plant.
We’ll review the background, engineering approach, installation process, and measurable results — plus provide free engineering tools such as our VFD Sizing Calculator and Cable Sizing Calculator so you can model your own energy-saving upgrade.
Background: Rising Energy Costs and Inefficient Fan Control
The 12-storey commercial building in Birmingham was equipped with conventional HVAC fan and pump systems controlled by star-delta starters. Each air-handling unit (AHU) ran continuously at full speed, regardless of occupancy levels or air-flow requirements. Maintenance logs showed:
- ⚙️ Fan belts and bearings replaced every 6–8 months due to hard starts
- ⚙️ Frequent complaints about temperature fluctuations between zones
- ⚙️ Excessive electrical demand peaks at morning start-up
Energy audits estimated the HVAC plant accounted for nearly 55% of total building energy use. Management set a target of at least 20% reduction in annual HVAC electricity consumption through intelligent motor control.
Project Objective
- 🏢 Retrofit VFDs to all AHU and chilled-water pump motors
- ⚡ Achieve ≥20% reduction in kWh consumption
- 🔧 Minimise disruption during installation
- 📈 Improve occupant comfort through better airflow modulation
Solution: Danfoss VLT Drives for Intelligent HVAC Control
Following a site survey, engineers specified the Danfoss VLT® HVAC Drive FC-102 for 11 kW and larger motors, and the compact VLT® Micro Drive FC-051 for smaller fan and pump applications.
Why Danfoss VLT®?
- ✔️ Optimised for HVAC — built-in PID controller and flow/pressure set-point control
- ✔️ Automatic Energy Optimisation (AEO) reduces losses at partial loads
- ✔️ Sleep mode and automatic restart functions for unoccupied hours
- ✔️ Integrated RFI filter ensuring EMC compliance in building networks
- ✔️ Robust design with conformal coating for humid plant rooms
The VLT FC-102 drives communicate directly with the Building Management System (BMS) via BACnet MS/TP, eliminating the need for separate controllers or analog interfaces.
System Design and Sizing
The engineering consultant used the VFD Sizing Calculator to determine the correct drive ratings for each motor. Loads included:
- 4 × 11 kW AHU supply fans
- 2 × 7.5 kW return fans
- 3 × 15 kW chilled-water pumps
- 2 × 5.5 kW hot-water pumps
Each drive was sized with a 10% overload margin for peak flow periods. Cable runs were verified using the VFD Cable Sizing Calculator to maintain voltage drop below 3%. Braking resistors for fan coast-down control were calculated with the Braking Resistor Calculator.
Installation Process
The upgrade was completed over two weekends to avoid disruption to tenants. Old starters were removed and replaced with VFD panels in existing MCC enclosures. Drives were pre-configured off-site and integrated into the BMS network for real-time feedback of motor speed, power, and faults.
Commissioning Highlights
- ✅ Auto-tuning completed for all fans and pumps
- ✅ PID loops tuned to maintain constant duct static pressure
- ✅ Sleep/wake logic configured based on CO₂ and occupancy sensors
- ✅ EMC verification and ground bonding tested per EN 61800-3
Performance Results After 6 Months
| Metric | Before Upgrade | After VLT Upgrade |
|---|---|---|
| Average Fan Speed | 100% | 70 – 80% |
| Monthly Energy Use | 118 MWh | 86 MWh (−27%) |
| Peak Current Draw | 680 A | 470 A (−31%) |
| Temperature Complaints | 17 /month | 5 /month |
The system now operates at variable speed according to demand, with fans running at lower speeds during mild conditions. Morning startup peaks have been virtually eliminated thanks to the drives’ programmable soft-start ramps.
Operator and Facilities Feedback
“The Danfoss VLT drives gave us precise airflow control without sacrificing comfort. The energy savings were immediate — our monthly electricity bill dropped by nearly 25%. Maintenance staff love the built-in diagnostics, and we haven’t replaced a single belt or bearing since the retrofit.” – Facilities Manager, Birmingham Office Tower
Integration with Building Management System
The FC-102 drives connect directly to the BMS via BACnet. Real-time feedback includes motor speed, temperature, power draw, and alarm status. The drives also provide trending data for predictive maintenance, allowing service teams to plan bearing lubrication and filter changes based on hours-run rather than time schedules.
In addition, the BMS logic automatically places drives into sleep mode during off-hours, saving approximately 12 MWh per year without affecting comfort levels.
Energy Analysis
The engineering consultant conducted a pre- and post-installation energy analysis using the drive’s built-in kWh counters. The breakdown of energy savings was:
- 💨 17% from variable airflow control on supply fans
- 💧 8% from pressure-based pump control
- 🔋 2% from reduced inrush and harmonic losses
Total verified savings equated to ≈ 32 MWh per quarter — equal to £11,000 annual reduction at £0.35/kWh.
Maintenance and Reliability Gains
- 🛠 Mean time between failures (MTBF) increased by 40%
- 🧰 Belt replacements reduced from twice to once per year
- 🔧 Drive fault codes and alarms accessible via BMS
- 🌡 Improved motor temperature regulation through reduced start stress
Using the Motor Starter Selection Calculator, engineers confirmed that replacing the legacy DOL starters with VFDs reduced average inrush current from 600% to less than 120% of nominal — a key factor behind reduced mechanical wear.
Environmental Impact
The building’s carbon emissions dropped by roughly 52 tonnes CO₂ per year, aligning with the company’s ESG targets. The project also improved indoor air quality and comfort stability, resulting in fewer occupant complaints and more consistent BMS trend data.
Lessons Learned
- 🔹 Always size drives with headroom for duct pressure and pump head changes using the VFD Sizing Calculator.
- 🔹 Proper cable screening and grounding are critical for noise-free BACnet networks — verify with the Cable Sizing Calculator.
- 🔹 Take advantage of Danfoss’s Auto Energy Optimisation (AEO) feature — it fine-tunes voltage to load for additional efficiency.
- 🔹 Train operators on alarm codes and parameter backup procedures.
Financial Overview
- 💰 Total Project Cost: £72,000 (equipment + installation)
- 💡 Annual Energy Savings: £27,000
- 🛠 Maintenance Savings: £3,500
- ⏱ Payback Period: 2.3 years
- 📈 ROI (5 years): 184%
Future Upgrades
The success of this retrofit has prompted the building’s management to evaluate further applications for Danfoss drives on chilled-water secondary pumps and cooling-tower fans. The engineering team also plans to trial VLT AutomationDrive FC-302 units for advanced harmonic filtering and digital twin simulation via Modbus TCP.
Conclusion
This case study demonstrates the tangible energy and maintenance benefits achievable with Danfoss VLT drives in HVAC systems. By combining variable speed control, intelligent optimisation, and seamless BMS communication, the facility exceeded its original energy-saving target by 7 percentage points within the first year.
If you operate large fans, pumps, or chillers, upgrading to VFD control can yield rapid payback while enhancing comfort and reliability. Start your design using our free tools:
- ⚙️ VFD Sizing Calculator
- ⚙️ Cable Sizing Calculator
- ⚙️ Braking Resistor Calculator
- ⚙️ Motor Starter Selection Calculator
Browse our full range of Danfoss VLT drives, including the VLT Micro Drive FC-051 (132F0005) and the VLT HVAC Drive FC-102 — all available for next-day UK delivery with a 2-year warranty.
At Drive Outlet Megastore, we deliver OEM automation products at unbeatable prices. Contact our team today for a same-day quotation or technical guidance on your next HVAC efficiency upgrade.
