Heat recovery comes in two broad forms. Air-to-air exchangers move heat directly between two air streams. Air-to-water devices (like run-around coils or exhaust water heaters) transfer heat into a fluid loop that can serve distant or mixed loads.
Side-by-Side
| Factor | Air-to-Air | Air-to-Water |
|---|---|---|
| Streams | Two air ducts | Air + water/glycol loop |
| Distance | Short (cores co-located) | Long (piping carries heat) |
| Cross-contamination | None (plate/pipe) | None (closed loop) |
| Reuse options | Pre-heat supply air | Air, water heating, process |
| Complexity | Low | Higher (pump, expansion tank) |
| Cost | Lower | Higher |
When Air-to-Water Wins
- Exhaust and the heat sink are on different floors or buildings.
- You want to heat domestic water or a process, not just air.
- Multiple exhaust points feed one loop.
When Air-to-Air Wins
- Supply and exhaust are side by side.
- You only need pre-conditioned make-up air.
- Budget and simplicity are priorities.
Hybrid arrangements are common in practice. A building may use air-to-air recovery for the bulk of the ventilation load and an air-to-water loop to serve a distant domestic-water or process load that the air stream cannot reach. Controls should coordinate the two so the water loop does not fight the air core; a simple priority rule keyed to outside-air temperature works well. In a laboratory or mixed-use building, for example, the air-to-air core trims ventilation load while the loop pre-heats service water, and the combined effect shortens payback versus either alone. Keep the loop insulated and the pump sized to design flow to protect efficiency. Our heat recovery solutions cover both air-to-air and air-to-water forms.
Frequently Asked Questions
Can I combine both?
Yes. Air-to-air handles the main recovery; an air-to-water loop serves remote or water loads.
Which is more efficient?
Air-to-air is usually a few points higher because there is no pump and fewer transfer steps.