When people say "heat exchanger efficiency" they usually mean effectiveness (ε): the share of recoverable heat actually transferred. A unit rated at 80% effectiveness returns 80% of the energy it theoretically could. Real installations rarely hit the brochure number, so it pays to know what moves it.
What drives effectiveness
- Flow arrangement: counterflow plates reach ~80%–90%; crossflow ~60%–70%.
- Airflow balance: recovery is highest when supply and exhaust flows are nearly equal. Big imbalances waste capacity.
- Temperature difference: larger ΔT gives more driving potential, but very cold exhaust triggers frost.
- Face velocity: slower air = more contact time = higher recovery, at the cost of larger size and fan power.
- Fouling: dust on plates or wheel blocks heat transfer and raises pressure drop.
- Carryover (rotary): seal leakage and moisture migration reduce net recovery.
Frost: the cold-climate killer
Below about −5°C exhaust, condensate on the cold side freezes and blocks channels. Solutions: preheat the supply, partially by-pass exhaust around the core, or use a wheel that carries heat to melt frost. Ignoring it silently tanks effectiveness and can damage the unit.
How to improve real-world recovery
| Lever | Effect |
|---|---|
| Balance supply/exhaust airflows | +5 to +15 pts effectiveness |
| Counterflow instead of crossflow core | +10 to +20 pts |
| Scheduled cleaning / filtration | Stops fouling losses |
| Frost control in winter | Keeps winter recovery high |
| Right-size, don't oversize | Best face velocity & fan cost |
For the calculation side, see our sizing & selection guide.
Underperforming existing unit?
EN159 can audit a duty and supply a higher-effectiveness plate or rotary core, including frost-protected versions for cold climates.
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