Step 1 - Gather Duty

Write down supply and exhaust airflow, indoor and outdoor design temperatures, and the allowable supply-air temperature.

Step 2 - Pick Effectiveness

Choose a target (say 70 percent) and confirm the technology (plate for sensible, membrane for latent).

Step 3 - Estimate Face Area

Effectiveness rises with face area and falls with face velocity. A larger core at lower velocity gives more recovery but costs more space.

Step 4 - Check Pressure Drop

Each core adds static pressure. Confirm the existing fan can deliver the combined airflow at the new total pressure, or upsize the fan.

Step 5 - Verify

Confirm against an EN 308 or AHRI 1060 rating at your actual operating point, not the headline number.