Fans run more hours than almost any other HVAC equipment. In many commercial buildings, supply, return and exhaust fans operate 12 to 24 hours a day, every day, and fan energy can be one of the largest electrical end uses in the building. Because fan power rises steeply with airflow, modest reductions in airflow or pressure can produce surprisingly large savings.
This guide explains where fan energy goes and the measures that reduce it, roughly in order of how much they usually matter.
What determines fan power
The power a fan draws from the electrical system is:
Fan input power (W) = airflow (m³/s) x total pressure (Pa) / overall efficiency
Overall efficiency combines the fan, the drive (belt or direct) and the motor, plus any variable frequency drive losses. In imperial units, brake horsepower equals airflow in cfm times total pressure in inches of water, divided by 6,356 times fan efficiency.
Three levers follow directly:
- Move less air.
- Move it against less pressure.
- Move it more efficiently.
The fan laws show why the first lever is so powerful. For a given system, power varies roughly with the cube of fan speed. Reducing airflow by 20 percent can, in theory, reduce fan power by about half. In real VAV systems with a fixed static pressure setpoint the savings are smaller, which is why pressure control matters as much as airflow. See fan laws and how to apply them for the details and limitations.
Move less air
Right-size design airflow
Many systems were designed with generous safety factors on cooling loads, which carry straight through to airflow. Re-examining actual loads after lighting retrofits, occupancy changes or envelope improvements often shows that design airflow can be reduced.
Reduce minimum airflows
VAV boxes with high minimum airflow settings keep the fan running at higher flow than necessary and cause reheat. ASHRAE Guideline 36 sequences use lower minimums tied to ventilation requirements. Lowering minimums requires checking ventilation and air distribution, but is among the most cost-effective retrofit measures.
Ventilation and schedules
- Demand-controlled ventilation reduces outdoor air during partial occupancy.
- Accurate schedules stop fans running when the building is empty.
- Exhaust fans for washrooms, garages and storage can often be scheduled or sensor controlled.
- Garage exhaust controlled by CO and NO2 sensors typically runs far fewer hours than continuous operation.
Reduce pressure
Static pressure reset
Most VAV systems control supply fan speed to hold a fixed duct static pressure setpoint. That setpoint is set for design conditions, so at part load the fan works harder than necessary and VAV dampers throttle away the excess. Resetting the setpoint based on damper positions, using trim and respond logic, lets the fan run at the lowest pressure that still satisfies the most demanding zone. See static pressure reset with trim and respond.
Find and remove pressure drops
Common sources of avoidable pressure drop:
| Source | Typical fix |
|---|---|
| Dirty filters left too long | Change based on pressure drop, not only on a calendar |
| Filters with high initial resistance | Select deeper filters with more media area at the same MERV rating |
| Dirty coils | Clean coils; restores both airflow and heat transfer |
| Poor fan inlet and outlet conditions (system effect) | Straighten inlet ducts, add turning vanes, replace abrupt transitions |
| Undersized or crushed ductwork, tight elbows | Replace critical sections identified during balancing |
| Balancing dampers left heavily throttled | Rebalance and reduce fan speed instead |
| Unused sound attenuators, abandoned coils | Remove where no longer needed |
Our guide to duct static pressure covers diagnosis in more depth.
Balance, then slow down
In constant volume systems, fans are often throttled by dampers to reach design airflow. A proper air balance followed by a sheave change or VFD setting at the reduced speed delivers the same airflow with less pressure and power.
Move air more efficiently
Fan selection
Fans have a best efficiency point. A fan selected far to the left or right of it can waste a significant share of its input power. AMCA Standard 208 defines the Fan Energy Index (FEI), which compares a fan's power at a given duty point with a reference fan. Specifying a minimum FEI at the design duty point is a better way to secure efficiency than specifying peak fan efficiency alone.
Fan arrays
Replacing a single large plenum or housed fan with an array of smaller direct-drive fans can improve efficiency, provide redundancy and free up space. Benefits depend on the original fan and the inlet conditions. See fan arrays and fan wall technology.
Drives and motors
- Belt drives lose energy through slip and wear. Direct drive or synchronous belts reduce losses.
- Premium efficiency motors are required by federal regulations for most new motors; electronically commutated (EC) motors are efficient in smaller fans and fan coils.
- VFDs on fans that were throttled by dampers or inlet vanes save substantial energy, provided controls actually reduce speed.
Procedure for a fan energy review
- Inventory fans over a chosen size threshold: airflow, static pressure, motor power, operating hours and control method.
- Trend fan speed, duct static pressure and VAV damper positions for at least a week.
- Measure actual airflow and pressure on the largest fans; compare with design.
- Identify measures from the categories above and estimate savings with measured data.
- Implement low-cost controls measures first: schedules, resets, minimum flows.
- Rebalance after any change that alters airflow distribution.
- Verify savings with trended data before and after.
Frequently asked questions
Will reducing airflow cause comfort problems?
It can if done without checking loads and ventilation. Changes should be based on measured data and followed by balancing and a review of space temperatures and CO2 levels.
Is a VFD always worth installing?
VFDs pay off on fans that run long hours at varying load or that are currently throttled. On a properly balanced constant volume fan that must run at one speed, the benefit is mostly the ability to fine-tune speed.
How much can static pressure reset save?
It depends on how far the fixed setpoint is above what the system actually needs and how many hours the system runs at part load. Trending damper positions before implementing reset shows the potential.
If you want measured airflow and pressure data as the basis for these decisions, air balancing is the place to start.
Key takeaways
- Fan power depends on airflow, pressure and overall efficiency, and airflow has the strongest effect.
- Lower minimum airflows, schedules and demand-controlled ventilation reduce air moved.
- Static pressure reset and removing avoidable pressure drops cut the work each unit of air requires.
- Select fans near their best efficiency point using FEI, and consider direct drive and fan arrays.
- Base changes on measured data, rebalance afterward and verify savings with trends.
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