A single exchanger usually suffices, but extreme climates or very low supply-air temperature targets can justify staging two recovery devices.
Comparison
| Single-stage | Two-stage | |
|---|---|---|
| Typical effectiveness | 70-90% | Up to ~95% combined |
| Supply temp achievable | Moderate | Very low |
| Complexity | Low | Higher (controls, defrost) |
| Cost | Lower | Higher |
When to stage
Consider two-stage (e.g., plate + heat pumpassisted or plate + wheel) only when the heating design temperature is severe or when indoor conditions demand very low supply temps.
When single-stage is enough
Most commercial buildings meet code with one well-sized exchanger.