From Lab Rating to Field Reality
EN 308 describes the laboratory test; this article focuses on turning that test into a recovery device that actually performs once installed. The gap between the certified number and the field number is usually created by installation and operation, not by the hardware.
Common Field Losses
| Loss source | Effect on effectiveness |
|---|---|
| Casing / partition leakage | Supply air short-circuits to exhaust |
| Bypass dampers leaking | Unconditioned air bypasses the core |
| Wrong face velocity | Effectiveness drops off the rating point |
| Fouling | Pressure drop rises, flow falls |
| Frost protection cycling | Core periodically taken offline |
Installing to Preserve the EN 308 Number
- Seal the partition between supply and exhaust inside the AHU to the rated leakage class.
- Specify tight bypass dampers with proof of closure; a leaking summer-bypass wastes recovered cooling.
- Match flow to the certified rating; oversizing the fan lowers face velocity and can change (usually lowers) effectiveness while raising fan energy.
- Provide access for core removal and washing so fouling does not quietly erode performance.
Commissioning and Verification
- Measure inlet/outlet temperatures on both streams at operating flow.
- Compute field effectiveness and compare to the EN 308 value at the same condition.
- Check pressures to confirm the fan sees the rated drop.
- Document the result for the commissioning binder and future re-verification.
Maintaining Performance
A plate core washed annually, or a wheel with balanced purge and belt tension, holds its rated effectiveness for years. Neglected, the same device can lose 10–20% of recovery within a season in dusty or humid service.
Seasonal Behaviour
Real buildings run at part load most of the time. Choose a device whose effectiveness stays high across the modulation range, and set frost protection to the least-disruptive strategy for your climate, so the core is online as much of the year as possible.
Bottom Line
The EN 308 test earns the rating; installation, commissioning, and maintenance keep it. Treat the field number as the one that pays the energy bill — and verify it.
Worked Example: Commissioning Catch
On one install, the measured field effectiveness was 52% against a 70% EN 308 rating. Investigation found a summer-bypass damper leaking 15% and a partition sealed to only class L3. Tightening the damper and re-sealing the partition to L1 raised field effectiveness to 68% — a 30% recovery improvement with no new hardware. The example shows that the field number, not the catalogue number, pays the bill.
Frequently Asked Questions
How often should I re-verify? At commissioning and then annually, or after any core service.
What is the biggest hidden loss? Leaking bypass dampers and poor partition sealing.
Does fouling really matter? Yes — a fouled core loses effectiveness and gains pressure drop within a season in dust or humidity.
Can frost protection hurt performance? Yes, if it offlines the core too often; choose the least-disruptive strategy for the climate.
Final note: the certified rating earns the specification, but commissioning and maintenance keep it. Verify the field effectiveness annually so the recovered energy you paid for is actually delivered rather than quietly lost to leakage and fouling.
A Commissioning Checklist That Holds
Turn the field lesson into a repeatable checklist. Before start-up, confirm the partition between streams is sealed to the rated class and that bypass dampers show proof of closure. At balance, measure inlet and outlet dry-bulb (and wet-bulb where moisture matters) on both streams at the operating flow, compute effectiveness, and compare it to the EN 308 value at that condition; a gap of more than a few points means something is leaking or bypassing. Record the pressure drop and confirm it matches the fan curve. Set a wash or inspection interval based on the exhaust dust or humidity load, and put the next verification date in the maintenance system. Done this way, commissioning is not a one-day event but the first step of a loop that keeps the rated recovery delivered for the life of the equipment, which is what the energy saving depends on.