What is ERP and how does it relate to our products?

ERP is a European Union (EU) for Energy-Related Products. Specifically regulation (EU) No 327/2011 sets minimum efficiency standards for Fans driven by electric motors with electrical input powers between 125Watts and 500 kWatts.

The first thing to note is that the “fan efficiency” is for the complete fan assembly, including impeller, drive motor and VSD (if fitted). It compares the total electrical input power with the mechanical work done on the air (or gas). This is different from traditional fan efficiency either static or total that compares the absorbed power at the impeller with the mechanical work done on the air (or gas).

Most industrial fans that fall between the above input power limits are covered by EU 327/2011

What’s not covered?

  • Fans for use in potentially explosive atmospheres. Commonly known as ATEX fans
  • Fans designed for operation with high temperature air above 100OC
  • Where ambient gas temperatures exceed 65OC or fall below -40OC
  • Where the supply voltage to the drive motor >1000 V AC or 1500 V DC
  • Fans for use with toxic, corrosive or flammable gasses or subject to abrasive substances
  • Fans where the work per unit mass exceeds 25 kJ/kg. What this means is that for a fan handling standard atmospheric air with a density of 1.2 kg/m3 and a static efficiency of 75% the maximum pressure rise over the fan is 22.5 kPa.
  • Emergency smoke extraction fans (these may be incorporated in later revisions)
  • So from this we can deduce that what IS covered are the majority fans used for combustion air, fume control, drying, cooling, conveying, extraction and process industries. Note this is not an exhaustive list. Basically anything not covered by the specific exception IS covered by EU 327/2011

Do Woodcock and Wilson fans comply?

Yes it is a legal requirement that they do comply for any fan to be put into use within the EU – each fan when selected, and prior to quoting, is checked using our own in house fan tests along with published motor efficiencies and drive losses to calculate the overall efficiency and compare it with the target efficiency published in EU 327/2011. Currently we use the requirements set out for 1st  January 2015 onwards. Note that compliance with EU 327/2011 is based on calculation, as we manufacture bespoke one off units individual fan tests are not possible. Compliance is calculated at quotation stage.

What assumptions do we use in calculating our efficiency?

All fans are supplied as a complete assembly. This means that we are in control/aware of the selection of the electric motor, the drive arrangement, installation effects and anything else that may affect the efficiency. Even if for example the motor is free issued we can use the correct motor efficiency supplied by the customer, or in the absence of this, then the minimum required motor efficiency eg IE2 or IE3

Centrifugal fans are calculated using measurement category A,C with efficiency based on “static”. We do this regardless of actual installation of the fan which could be B,C or D because the vast majority of fans we supplied with a contractual static pressure requirement.

Axial fans are calculated using measurement category A,C with efficiency based on “static”. We do this regardless of actual installation of the fan which could be B,C or D because the vast majority of fans we supplied with a contractual static pressure requirement

As specified in EU 327/2011 efficiency calculations are based on “best efficiency point”. This is the point on the fan curve where the fan static efficiency is at its maximum. It is not necessarily the fan operating point, though generally we aim to select fans at or near this point.

If the specific motor efficiency is not available motor efficiency can be estimated using clause 3.2 of Annex II of  EU327/2011

In absence of specific information regarding vee belt drives, if fitted, these will be calculated using clause 3.2 of Annex II of  EU327/2011 “low efficiency drive”. Please note that “low efficiency drive” is a term used in EU327/2011, vee belt drive manufacturers attest that modern vee belt drives are a highly efficient means of power transmission.

In absence of specific information regarding variable speed drives, if fitted, an efficiency of 98% will be applied along with a “part load compensation factor” as defined in  clause 3.2 of Annex II of  EU327/2011.

Kps compressibility coefficient of fan static gas power taken as 1 as this is already calculated in the fan testing carried out to BS EN 5801. Each fan range is tested in accordance with this standard to establish a baseline fan performance. This is then corrected using the fan laws to derive performance for different sized fans running at different speeds and operating at different densities.

What information do we provide?

The regulation sets out the information to be supplied in clause 3 annex I of EU327/2011, however as set out above as we manufacture bespoke units it is not practical to publish, even on our website, efficiency data for all the possible fan units we may supply. For example a given fan could be supplied with a dozen different motors, be direct or belt driven.

This is why this document explains our philosophy for bespoke units.

Note items 9 & 10 of the above clause are not relevant to bespoke fans where the performance is derived from fan tests and scaled using dimensionally similarity fan laws.


Woodcock & Wilson 600mm dia BFN Single Inlet Single Width (SISW) fan running at 2000 rpm.

Fitted with a IE3 11 kW 4 pole electric motor belt driven and variable speed drive (vsd)

Reference to the fan curve figure 1 shows the best efficiency point to be 2.81 m3/sec at 2.08 kPa static pressure. Absorbed power (of the impeller) is 7.71 kW. Static efficiency at best efficiency point 75.8%.

Step 1 Work out “target efficiency” using table 2

“Centrifugal backward curved fan with housing” measurement category A,C Efficiency category “static”

Electrical input power will be less than 10 kW (note this is not the motor power it is the actual power drawn by the fan despite motor rating)


N = Efficiency grade from table 2 (right hand column) 61

P = electrical power input (note at this stage before the calculation is done this can only be estimated whereas for a verification test it would have to be measured) In this case assume 11 kW the motor rating this gives a slightly higher value for the target efficiency.


Step 2 work out the fans actual efficiency using clause 3.1 Annex II


P_us=q × p_sf×k_sp P_us=2.81 × 2080×1.0=5844.8

Pe cannot be measured therefore is calculated as follows

P_e=impeller power×1/?_m ×1/?_t ×1/?_vsd ×?1/C?_c


?_m=motor efficiency=0.914 IE3 11 kW 4 pole
?_t=transmission drive efficiency=0.96 (clause 3.2 annexII)
?_vsd=variable speed drive efficicney if fitted=0.98 (estimate)
C_c=Part load compensation factor= 1.04

P_e=98035.5 Watts

So actual overall efficiency becomes


Efficiency would be declared as 64.7 %

Step 3 compare calculated efficiency with target efficiency

64.7 > 61.43

As the overall efficiency is > than target efficiency fan is compliant.


Motor efficiency taken at full load

Drive loss is based on full load motor power (11 kW) ie Pa = 11 kW

VSD power Taken as motor full load power ie Ped = 11 Kw