Heat Exchanger Efficiency Calculator
Calculate temperature, moisture, and enthalpy transfer efficiency for heat recovery units, air-to-air heat exchangers, and HVAC energy recovery ventilators (ERVs/HRVs).
Use this calculator to evaluate the performance of a heat recovery unit by computing its temperature efficiency, moisture efficiency, or enthalpy efficiency. It also estimates the total heat recovered in kW (metric mode).
Heat Recovery Efficiency Calculator
Efficiency Formulas
Temperature Efficiency (μt):
\[ \mu_t = \frac{t_2 - t_1}{t_3 - t_1} \]
Where: t1 = outside air before, t2 = outside air after, t3 = exhaust air before
Moisture Efficiency (μm):
\[ \mu_m = \frac{x_2 - x_1}{x_3 - x_1} \]
Where: x = moisture content (kg/kg or g/kg of dry air)
Enthalpy Efficiency (μe):
\[ \mu_e = \frac{h_2 - h_1}{h_3 - h_1} \]
Where: h = specific enthalpy (kJ/kg)
Heat Recovered (Q):
\[ Q = \rho \times V \times c_p \times (t_2 - t_1) / 3600 \]
Where: ρ = air density (kg/m³), V = air flow (m³/h), cp ≈ 1.005 kJ/(kg·K). Result in kW.
What is Heat Exchanger Efficiency?
Heat exchanger efficiency (also called effectiveness in some contexts) measures how closely the actual heat transfer approaches the theoretical maximum. It is expressed as the ratio of the actual change in the supply air to the maximum possible change (the difference between the entering exhaust and supply air conditions).
- Temperature efficiency — used for sensible heat recovery (HRVs)
- Moisture efficiency — used for latent/enthalpy wheels (ERVs)
- Enthalpy efficiency — total energy recovery (sensible + latent)
Typical Efficiency Ranges
| Heat Exchanger Type | Typical Efficiency | Application |
|---|---|---|
| Plate (flat) HRV | 50–75% | Residential ventilation |
| Counter-flow plate HRV | 70–90% | High-performance buildings |
| Rotary thermal wheel | 65–85% | Commercial HVAC |
| Run-around coil | 45–65% | Where cross-contamination is a concern |
| Heat pipe | 40–65% | Industrial exhaust |
| Plate-fin (gas-to-gas) | 60–80% | Process industry |
HRV vs ERV: Key Differences
An HRV (Heat Recovery Ventilator) transfers only sensible heat (temperature), while an ERV (Energy Recovery Ventilator) transfers both sensible and latent heat (temperature + moisture). ERVs use enthalpy wheels or membrane cores, making them more suitable for humid climates where moisture control is important.
References & Further Reading
- • ASHRAE Handbook — HVAC Systems and Equipment (2020). American Society of Heating, Refrigerating and Air-Conditioning Engineers. (Heat recovery efficiency standards and testing methods)
- • McQuiston, F.C., Parker, J.D. & Spitler, J.D. (2005). Heating, Ventilating, and Air Conditioning: Analysis and Design (6th ed.). John Wiley & Sons. (HRV/ERV design principles)
- • Holman, J.P. (2010). Heat Transfer (10th ed.). McGraw-Hill. (Heat exchanger effectiveness and NTU method)
- • EN 308:1997. Heat Exchangers — Test Procedures for Establishing the Performance of Air to Air Flue Gas Heat Recovery Devices. European Committee for Standardization.
Frequently Asked Questions
An HRV (Heat Recovery Ventilator) transfers only sensible heat (temperature), while an ERV (Energy Recovery Ventilator) transfers both sensible and latent heat (temperature + moisture). ERVs use enthalpy wheels or membrane cores.
No. Efficiency above 100% indicates measurement error or incorrect data entry. In practice, efficiencies are always below 100% due to real-world heat transfer limitations.
For temperature mode: Q = ρ × V × cp × (t2 − t1) / 3600, where V is air flow in m³/h, ρ is density in kg/m³, cp ≈ 1.005 kJ/(kg·K). This gives Q in kW.
Frost formation, pressure imbalance between streams, bypass leakage, dirty filters, and part-load operation all reduce effective efficiency below the manufacturer's rated value.