♻️ HVAC

Free Online Energy Recovery Wheel Calculator

Calculate sensible and total energy recovered by a heat recovery wheel (HRW/ERV) from airflow, effectiveness and conditions. ASHRAE 90.1 basis. Free online tool, no sign-up.

📐 Standard: ASHRAE 90.1
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Energy Recovery Wheel Calculator
Reference: ASHRAE 90.1
♻️ HVAC
Calculate sensible and total energy recovered by a heat recovery wheel (HRW/ERV) from airflow, effectiveness and conditions. ASHRAE 90.1 basis. Free online tool, no sign-up.
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ℹ About This Calculator

An energy recovery wheel (HRW/ERV) transfers heat - and, if enthalpy-coated, moisture - between the exhaust and fresh-air streams, cutting the load on the cooling or heating coil. This calculator estimates the sensible and total energy recovered from the airflow, the wheel effectiveness and the indoor/outdoor conditions, per ASHRAE 90.1.

Recovery is the wheel effectiveness (typically 65-80%) multiplied by the maximum possible transfer between the two streams. An enthalpy wheel recovers both sensible heat and latent (moisture) energy, so it saves most in hot-humid climates by pre-drying the fresh air; a sensible-only wheel recovers temperature alone. ASHRAE 90.1 and the ASHRAE 90.1 mandate recovery above certain outdoor-air fractions.

Energy Recovery Formula

ASHRAE 90.1

Q_sensible = 1.21 × Q × ε × ΔT
Q_total = 1.2 × Q × ε × Δh Q = airflow, L/s
ε = wheel effectiveness (0.65-0.80)
ΔT = outdoor-to-indoor temp difference, °C
Δh = enthalpy difference, kJ/kg (enthalpy wheel)

Worked Example

Example: A wheel on 1000 L/s of fresh air at 75% effectiveness, with a 15 °C difference between 40 °C outdoor and 25 °C indoor air, recovers sensible energy ≈ 1.21 × 1000 × 0.75 × 15 ≈ 13.6 kW. An enthalpy wheel recovering the latent difference too can lift the total recovery well above 20 kW in humid conditions - a large cut in coil load.

Recovery Device Effectiveness (ASHRAE 90.1)

Typical effectiveness and characteristics of common recovery devices:

DeviceEffectivenessRecovers
Enthalpy (desiccant) wheel70 – 80%Sensible + latent
Sensible (heat) wheel65 – 80%Sensible only
Plate heat exchanger50 – 75%Sensible (no cross-mix)
Run-around coil45 – 65%Sensible, streams apart

When Recovery Pays

Recovery saves most where the outdoor air is far from the indoor condition and the outdoor-air fraction is high - hot-humid or cold climates, and spaces like hospitals, labs and assembly halls with lots of ventilation. Enthalpy wheels shine in humid climates by cutting the latent (dehumidification) load, which a sensible-only device cannot touch.

Indicative values - use the certified wheel performance at your airflow and conditions.

Frequently Asked Questions

How do I calculate energy recovered by a heat recovery wheel? +
Multiply the airflow, the wheel effectiveness and the difference between the two air streams. For sensible recovery use the temperature difference (Q ≈ 1.21 × L/s × ε × ΔT in watts); for an enthalpy wheel use the enthalpy difference to include latent recovery.
What is the effectiveness of an energy recovery wheel? +
Enthalpy and sensible wheels typically run 65-80% effective; plate exchangers 50-75%; run-around coils 45-65%. Effectiveness is the fraction of the maximum possible transfer that the device actually achieves.
Sensible vs enthalpy wheel - which should I use? +
A sensible wheel recovers temperature only; an enthalpy (desiccant-coated) wheel also transfers moisture. In hot-humid climates the enthalpy wheel saves far more by pre-drying the fresh air; in dry climates a sensible wheel may be enough.
When is heat recovery required by code? +
ASHRAE 90.1 and US ASHRAE 90.1 require energy recovery when the supply airflow and outdoor-air fraction exceed defined thresholds - broadly, systems with large amounts of ventilation air. This calculator helps quantify the saving.
How much energy can a recovery wheel save? +
It commonly recovers 65-80% of the difference between the exhaust and fresh-air streams, which can cut the fresh-air coil load by a similar fraction. In extreme climates with high ventilation, this is a very large saving in cooling or heating capacity.
What is cross-contamination in an energy wheel? +
A small carry-over of exhaust air into the supply as the wheel rotates. A purge sector minimises it. Where any cross-mixing is unacceptable (e.g. some hospital areas), a plate exchanger or run-around coil that keeps the streams fully separate is used instead.
Does an energy recovery wheel add fan power? +
Yes - the wheel adds air-side pressure drop, so the fans use a little more power. The recovered heating/cooling energy normally far outweighs this, but it must be included when checking the net saving and the specific fan power.
Do energy wheels work in winter? +
Yes - in cold climates they recover heat from the warm exhaust to pre-warm incoming fresh air, and an enthalpy wheel also returns moisture, reducing winter dryness. Frost control may be needed at very low outdoor temperatures.

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⚠️ Disclaimer: For preliminary engineering design only. Verify all results with a licensed engineer before use. Full disclaimer →