EN 308 Purpose
EN 308, Test procedures for establishing the performance of air to air heat exchangers in thermal energy recovery systems, is the European counterpart to ASHRAE Standard 84. It sets the laboratory method for measuring how much heat (and, where applicable, moisture) an air-to-air exchanger actually transfers between supply and exhaust airstreams.
What the Procedure Measures
The standard defines test rigs and calculations for:
- Sensible effectiveness from dry-bulb temperature changes.
- Latent effectiveness from humidity-ratio changes (for enthalpy devices).
- Total effectiveness combining heat and moisture.
- Pressure drop on both sides.
- Leakage and bypass between the two airstreams.
Test Configurations
| Exchanger type | EN 308 rig focus |
|---|---|
| Fixed plate | Steady cross/counter-flow, no rotation |
| Rotary wheel | Segmented carry-over and purge loss |
| Heat pipe | Orientation and freeze behaviour |
| Run-around coil | Pump and fluid loop losses |
Rating Conditions
Like ASHRAE 84, EN 308 prescribes both a winter and a summer condition so that heating and cooling performance are both reported. Airflows, inlet temperatures, and humidity are fixed per the procedure; effectiveness is computed at the rated flow, not at a favourable partial load.
Effectiveness Formula
For a sensible exchanger the sensible effectiveness is:
εs = (To,in − Ts,out) / (To,in − Te,in)
using inlet and outlet dry-bulb temperatures of the outdoor and exhaust streams at rated flow. Latent effectiveness substitutes humidity ratio. The lower value of the two streams governs when flow rates differ.
Why EN 308 Matters to Specifiers
- It makes "effectiveness" a defined, comparable quantity across European suppliers.
- It exposes leakage that pure efficiency claims hide — important for labs, hospitals, kitchens.
- It underpins CE marking and EN 13053 AHU ratings that embed the exchanger.
From Lab Rating to Field Reality
EN 308 describes the laboratory test; the rest of 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.
Practical Use and Configuration Choice
Request EN 308 (or equivalent ASHRAE 84 / AHRI 1060) tested effectiveness at your design airflow, not a generic figure. Real performance drops with face velocity and fouling, so choose the rating point near your operating condition and plan maintenance that preserves the tested pressure drop and leakage class. Match the EN 308 rig to the device you actually buy: a fixed plate is tested as a steady cross- or counter-flow element, so read its report at the same flow split you will run; a rotary wheel is tested with its purge and carry-over included, which is why a wheel's certified number already accounts for the supply air it loses each rotation — a useful honesty that a plain "80% wheel" claim rarely offers; heat pipes are tested in their operating orientation, because tipping them changes performance; and run-around coils are tested with their pump and fluid losses included. When comparing bids, confirm each was tested to the configuration that matches its installation; otherwise you are comparing a plate tested one way against a wheel tested another, and the numbers are not equivalent even if the format looks the same. Enter the certified effectiveness and pressure drop into your EN 16798 or AHRI-based energy model at the design flow.
Worked Examples
Wheel carry-over. A rotary enthalpy wheel spends part of each rotation in the exhaust stream and part in the supply stream. A small "purge" sector returns a slice of supply air to exhaust to limit carry-over of supply (conditioned) air back outdoors. Under EN 308 the test measures this carry-over; a wheel with 2% carry-over loses 2% of its supply to exhaust each cycle — invisible in a brochure, but part of the certified number. Selecting on EN 308 data lets you trade purge loss against effectiveness knowingly.
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
Is EN 308 the same as ASHRAE 84? Functionally yes — both are air-to-air exchanger test methods; EN 308 is the European standard, ASHRAE 84 the US one.
Why report both summer and winter? Performance differs by season; one number would mislead selection.
What about run-around coils? EN 308 covers them too, including pump and fluid-loop losses that plate/wheel tests do not have.
How do I use the result? Enter the certified effectiveness and pressure drop into your EN 16798 or AHRI-based energy model at the design flow.
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.
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.
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.