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.
Practical Use
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.
Worked Example: 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.
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.
Choosing the Right Test Configuration
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.