EC fans: retrofit savings for AHUs and fan coils
Updated 23 October 2026 · SEO Dons Editorial
What is an EC fan, and how does it differ from a belt-driven fan?
An EC (electronically commutated) fan is a direct-drive fan with a brushless motor. It replaces the older arrangement of an AC induction motor driving a centrifugal fan through a belt and pulley. CIBSE Journal says direct drives are becoming more frequent as electronic speed controls improve and EC motors become more available. Removing the belt takes out inefficiencies of 2% to more than 10%, depending on maintenance, and is likely to lessen vibration and maintenance.
| Point | Belt-driven AC fan | Direct-drive EC fan |
|---|---|---|
| Drive losses | Belt inefficiency of 2% to more than 10%, depending on maintenance | No belt, so that loss is removed |
| Speed control | Variable speed needs a separate electronic drive | Motors make it simple to control the fan speed |
| Upkeep | Belts, pulleys and extra bearings | Fewer bearings and cleaning requirements |
| Efficiency | Depends on fan type, motor and drive | CIBSE Journal says direct-drive EC motors can approach 90% in a total fan efficiency example |
Many EC retrofits use plug fans, which are unhoused centrifugal impellers placed directly in the unit's casing. CIBSE Journal says their initial cost is likely to be lower than housed centrifugal fans and they can offer space advantages, at the price of lost operating efficiency. The air handling unit page covers the refurbish-or-replace decision around this.
What do the rules require of a replacement fan?
Regulation 327/2011 sets ecodesign requirements for fans driven by motors with an electric input power between 125 W and 500 kW, including fans integrated in other energy-related products. It has two minimum efficiency tiers, the first from 1 January 2013 and the second from 1 January 2015, and its exemption for replacements is narrow: fans placed on the market before 1 January 2015 as a replacement for identical fans in products placed on the market before 1 January 2013. A fan chosen for a retrofit today is not that case.
Annex I also tells you what to ask for. The maker must display the measurement category (A to D), whether efficiency is static or total, the efficiency grade, and whether the efficiency calculation assumed a variable speed drive and if so whether it is integrated in the fan or must be installed with it. It must also give the motor power, flow rate and pressure at the optimum efficiency point. Comparing two fans on a brochure percentage without matching those details is not a like-for-like test.
Two further points. The ventilation unit regulation, 1253/2014, requires every unit except a dual use unit to have a multi-speed or variable speed drive. And the Energy Technology List is narrower than many assume: its commercial fans criteria exclude fans within air handling units and fan coil units, though it does list a motor with a matched variable speed drive as a converter-fed motor, the alternative to going EC. Its criteria do cover ducted fans for supply and extract, such as in-line duct fans; see commercial extractor fans. Tax relief questions are for an adviser, and the capital allowances guide sets out the basics.
What could a retrofit save? A worked example
No official or recognised body publishes a general percentage saving for EC retrofits, so none is claimed here. The table is illustrative arithmetic on assumed inputs: fan input power is airflow times pressure divided by overall fan efficiency, the relationship CIBSE Journal uses. Prices are left out because you know your own tariff.
Assumptions: a supply fan moving 6 cubic metres per second against 700 pascals, which is 4,200 watts of air power, running 3,500 hours a year (14 hours, 5 days, 50 weeks). Overall efficiency is assumed at 55% for the existing belt-driven set and 68% for an EC array, a cautious figure beside CIBSE's 90% remark. In the third row the building needs only 70% of the airflow for 2,500 of those hours, and duct pressure follows the square of flow, so air power follows the cube.
| Scenario | Input power at design flow | Electricity a year |
|---|---|---|
| Existing belt-driven fan at design flow all hours | 7,636 W | 26,727 kWh |
| EC array at design flow all hours (efficiency gain only) | 6,176 W | 21,618 kWh, 5,110 kWh less (19%) |
| EC array at 70% flow for 2,500 hours, design flow for 1,000 | 2,119 W at 70% flow | 11,473 kWh, 15,254 kWh less (57%) |
Read the third row carefully. Most of that saving comes from running slower, which a belt-driven fan on a variable speed drive could also do. EC adds the efficiency gain and simpler control. Replace every assumption with measured airflow, pressure and motor power. Simple payback in years is the installed cost divided by annual kilowatt hours saved multiplied by your tariff.
When does a retrofit pay back?
The arithmetic favours fans that run long hours, work against high pressure, spend time at part load and are belt-driven with doubtful maintenance. It is weaker where the fan runs few hours, already has a drive, or sits in a plenum where a replacement set would lose efficiency. Ideal cube-law savings need system pressure to fall with flow, so a control that holds a high duct pressure setpoint defeats them, as the VAV systems guide explains. Loaded filters add resistance too; see the filter grades guide.
Fan coils differ. CIBSE Journal says modern fan coil units typically use EC DC motors, significantly more efficient than older motors, particularly at reduced speeds, but that fan speed turndown is typically limited to no lower than 70% of design speed to keep air distribution effective. At 70% speed the ideal power is 34% of design (0.7 cubed). See the fan coil units guide.
What does a retrofit involve?
- Survey. Measure airflow, pressure, motor power and running hours, and inspect belts, bearings and the plenum.
- Selection. Pick the fan from the duty point and the Annex I data, not the nameplate power. CIBSE Journal notes manufacturers should identify preferred safe working ranges, and that fan curves include areas of instability.
- Physical fit. Frames, inlet cones, guards, vibration isolation and the position in the plenum.
- Electrical and controls. Supplies, isolation, and the control signal from the building management system.
- Commissioning. Measure the result against the selection and record it, as in the commissioning and handover guide.
What are the risks?
- Wrong duty selection. A fan picked on nameplate power can sit outside its working range. Insist on the duty point.
- Harmonics. ebm-papst, a fan maker, says devices with a poor power factor generate harmonics and reactive power that strain the grid, and says it integrates active power factor correction into its EC centrifugal fans. Ask every supplier for harmonic and power factor data per model, and have a competent electrician assess the combined effect of many fans. ENA's Engineering Recommendation G5 sets harmonic voltage distortion planning levels for UK networks; check the current issue.
- Controls. A fan that can modulate saves nothing if the control never asks it to. The AHU maintenance checklist includes control checks.
Readers weighing a retrofit against replacing an air handling unit can send an enquiry through the site's form. Equipment prices for new units are on the air handling unit cost guide.
Sources
- Module 35: Fans for ducted ventilation systems, CIBSE Journal, December 2011, checked 22 October 2026
- Module 49: Saving energy through simple HVAC heat recovery, CIBSE Journal, February 2013, checked 22 October 2026
- Module 252: Design considerations for fan coil units with low-temperature heat sources, CIBSE Journal, August 2025, checked 22 October 2026
- Module 143: Variable air volume (VAV) air conditioning matures, adapts and flourishes, CIBSE Journal, March 2019, checked 22 October 2026
- Regulation 327/2011, Article 1, legislation.gov.uk, checked 22 October 2026
- Regulation 327/2011, Annex I, legislation.gov.uk, checked 22 October 2026
- Regulation 1253/2014, Annex III, legislation.gov.uk, checked 22 October 2026
- Commercial Fans, Energy Technology List, DESNZ, checked 22 October 2026
- Motors, drives and fans, Energy Technology List, DESNZ, checked 22 October 2026
- Active PFC: the power behind efficient fan performance, ebm-papst, 21 July 2025, checked 22 October 2026 (manufacturer's statement about its own products)
- Engineering Recommendation G5 Issue 5, Energy Networks Association document catalogue (ena-eng.org), June 2020, checked 22 October 2026
Common questions
Does an EC fan need a separate variable speed drive?
It depends on the model. Regulation 327/2011 requires the maker to state whether the efficiency figure assumed a variable speed drive and, if so, whether it is integrated in the fan or must be installed with it. Where a drive must be added, the fan's marking has to say so.
Do replacement fans have to meet ecodesign rules?
Regulation 327/2011 covers fans placed on the market, including those integrated in other products. Its replacement exemption is limited to identical fans placed on the market before 1 January 2015 for products placed on the market before 1 January 2013, so a new array chosen today is not within it.
Can I rely on a manufacturer's percentage saving?
Treat it as a starting point only. No official body publishes a general retrofit saving, and the result depends on your airflow, pressure, running hours and the fan you have now. Ask for the calculation inputs and the Annex I data for the fan, then check it against measurements.
How do fan coils differ from air handling units?
CIBSE Journal says modern fan coils typically already use EC motors, and that fan speed turndown is typically limited to no lower than 70% of design speed, so that floor limits how far the fan can be slowed.
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