When comparing heat recovery products, efficiency numbers can be confusing. Manufacturers may quote temperature effectiveness, sensible recovery efficiency, or total energy recovery. Understanding these metrics helps you make fair comparisons and avoid under- or over-sizing equipment.
Temperature Effectiveness (Sensible)
Sensible effectiveness is the most common metric for air-to-air heat exchangers. It compares the actual temperature change of the supply air to the maximum theoretically possible change.
η = (Tsupply,out − Tsupply,in) / (Texhaust,in − Tsupply,in) × 100 %
Typical values range from 50 % for simple cross-flow plates to 85 % for high-efficiency counter-flow designs.
Total Energy Recovery (Enthalpy)
Total energy recovery includes both temperature and humidity transfer. It is used for enthalpy wheels and membranes, where moisture is moved between air streams. This metric is measured in kW or BTU/h recovered under standard test conditions.
Pressure Drop and Fan Power
A heat exchanger with very high effectiveness may also create high air resistance. Always check pressure drop (Pa or in. w.g.) and calculate the extra fan energy. The net savings are the recovered heat minus the added fan power.
Energy Recovery Ratio (ERR)
ERR compares the recovered energy to the electrical energy consumed by fans and pumps. A ratio above 4 is generally considered economical for continuous operation.
Testing Standards
- AHRI 1060: North American standard for air-to-air heat exchangers
- EN 308: European standard for heat recovery components
- ISO 16494: Test methods for rotary heat exchangers
Selection Checklist
- Ask for effectiveness at design airflow, not maximum airflow.
- Request pressure drop at the same operating point.
- Check whether the rating is for dry or wet conditions.
- Verify the test standard used by the manufacturer.
Frequently Asked Questions
Is higher efficiency always better?
Not necessarily. Very high efficiency can increase pressure drop and capital cost. The best choice balances efficiency, pressure drop, and payback.
Does efficiency change with airflow?
Yes. At lower airflow, effectiveness usually increases, while pressure drop decreases. Always check the performance curve.