How to Size an Air-to-Air Heat Exchanger (Calculation Guide)
How to Size an Air-to-Air Heat Exchanger
Sizing an air-to-air heat exchanger is mostly arithmetic: match the airflow, find the heat to recover, and pick a core that delivers it at the effectiveness you need. Here is the working method.
1. Know Your Airflow
Start with the supply and exhaust volume flow rates, usually in m3/h or CFM. They should be near-equal so the exchanger stays balanced. If you only know the room, estimate from air-change rate times room volume.
2. Find the Temperature Difference
Note the outdoor supply temperature and the warm exhaust temperature. The bigger the gap, the more heat is available — and the more valuable recovery becomes (cold climates especially).
3. Estimate Recovered Heat
Use the ventilation heat formula:
Q = 0.33 × V × ΔT
where Q is heat recovered in watts, V is airflow in m3/h, and ΔT is the temperature difference in °C (the 0.33 factor is air's volumetric heat capacity in Wh/(m3·°C)). Example: 2,000 m3/h with a 20°C difference recovers about 0.33 × 2,000 × 20 ≈ 13,200 W — over 13 kW of free heating.
4. Apply Effectiveness
Multiply Q by the core effectiveness (say 0.80) to get actual recovery. Higher effectiveness recovers more but needs a larger core, so it is a cost/size trade-off.
5. Check Pressure Drop
Every core adds resistance. Confirm the fan can deliver the airflow at the core's pressure drop; oversized cores can starve the system. Balance effectiveness against fan power.
6. Pick the Core Type
For strict no-mix duties use a plate; for very large volumes or humidity recovery use a rotary wheel. Material follows temperature and gas chemistry.
FAQ
What airflow do I need?
Match supply and exhaust, sized from your ventilation requirement (air changes or process need). Near-equal flows keep the core balanced.
Is the formula exact?
It is the sensible-heat estimate; for humid climates add latent recovery (use an ERV/enthalpy core). Our selection guide has the full method.
Conclusion
Size by airflow × temperature difference × effectiveness, then check fan pressure drop. Do it once and the savings are locked in — start with our selection guide.