Energy Recovery Wheel Efficiency Calculator

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Overview

An Energy Recovery Wheel Efficiency Calculator estimates the sensible, latent, and total effectiveness of a rotary energy recovery device (enthalpy wheel or heat wheel) based on measured or design airflow conditions. The calculator uses the standard ASHRAE effectiveness definitions: sensible effectiveness is the ratio of actual sensible heat transfer to the maximum possible sensible heat transfer, and latent effectiveness is the ratio of actual moisture transfer to the maximum possible moisture transfer.

Energy recovery wheels are widely used in HVAC systems to precondition outdoor ventilation air by transferring heat and moisture between the exhaust and supply airstreams. ASHRAE Standard 90.1 and ASHRAE Handbook describe energy recovery effectiveness in terms of these ratios.

Enter the outdoor air conditions (temperature and humidity ratio), the supply air leaving conditions after the wheel, and the return air conditions entering the exhaust side. The calculator determines sensible, latent, and total effectiveness, plus an estimate of the energy recovered per hour.

How to Use This Calculator

  1. Enter supply airflow rate — in CFM or m³/h.

  2. Enter outdoor air temperature (entering supply) — in °F or °C.

  3. Enter supply air leaving temperature — in °F or °C.

  4. Enter return air temperature (entering exhaust) — in °F or °C.

  5. Enter outdoor air humidity ratio — in gr/lb or g/kg.

  6. Enter supply air leaving humidity ratio — in gr/lb or g/kg.

  7. Enter return air humidity ratio — in gr/lb or g/kg.

  8. Click "Calculate" — get sensible effectiveness, latent effectiveness, total effectiveness, and energy recovered.

Compare the measured effectiveness against the manufacturer's AHRI 1060 rating; verify airflow balance and measure at the wheel face for valid commissioning results.

Inputs & Outputs

Inputs

  • •Supply Airflow Rate (m³/h / CFM)
  • •Outdoor Air Temperature (Entering Supply) (°C / °F)
  • •Supply Air Leaving Temperature (°C / °F)
  • •Return Air Temperature (Entering Exhaust) (°C / °F)
  • •Outdoor Air Humidity Ratio (Entering Supply) (g/kg / gr/lb)
  • •Supply Air Leaving Humidity Ratio (g/kg / gr/lb)
  • •Return Air Humidity Ratio (Entering Exhaust) (g/kg / gr/lb)

Outputs

  • •Sensible Effectiveness (%)
  • •Latent Effectiveness (%)
  • •Total Effectiveness (Approximate) (%)
  • •Sensible Energy Recovered (W / BTU/hr)
  • •Latent Energy Recovered (W / BTU/hr)

Formula

Calculator Formula

This calculator uses the standard ASHRAE effectiveness definitions for energy recovery devices.

Step 1: Sensible Effectiveness

ε_s = |T_oa − T_sa| / |T_oa − T_ra| × 100

Where:

  • T_oa = outdoor air temperature entering the supply side
  • T_sa = supply air temperature leaving the wheel
  • T_ra = return air temperature entering the exhaust side

Step 2: Latent Effectiveness

ε_L = |W_oa − W_sa| / |W_oa − W_ra| × 100

Where:

  • W_oa = outdoor air humidity ratio entering the supply side
  • W_sa = supply air humidity ratio leaving the wheel
  • W_ra = return air humidity ratio entering the exhaust side

Step 3: Total Effectiveness (Approximate)

ε_total = (ε_s + ε_L) / 2

This is a simplified average. True total effectiveness requires enthalpy-based calculation.

Step 4: Sensible Energy Recovered

Imperial:

Q_s = CFM × 1.08 × |T_oa − T_sa|   (BTU/hr)

Metric:

Q_s = m³/h × 0.3353 × |T_oa − T_sa|   (W)

The metric factor 0.3353 = ρ × Cp / 3600 = 1.2 × 1006 / 3600.

Step 5: Latent Energy Recovered

Imperial:

Q_L = CFM × 0.68 × |W_oa − W_sa|   (BTU/hr)

Metric:

Q_L = m³/h × 0.8337 × |ΔW|   (W)

The metric factor 0.8337 = ρ × h_fg / 3600 / 1000 = 1.2 × 2501000 / 3600 / 1000 (with ΔW in g/kg).

Fixed Decision Path

This page follows one exact path:

Airflow + Entering/Leaving Conditions → Sensible & Latent Effectiveness → Energy Recovered

That is the fixed model used on this page.

Calculator Variables

Variable Meaning Units
supplyAirflow Supply airflow rate CFM / m³/h
outdoorTemp / T_oa Outdoor air temperature entering supply °F / °C
outdoorHR / W_oa Outdoor air humidity ratio entering supply gr/lb / g/kg
supplyLeavingTemp / T_sa Supply air leaving temperature °F / °C
supplyLeavingHR / W_sa Supply air leaving humidity ratio gr/lb / g/kg
returnTemp / T_ra Return air temperature entering exhaust °F / °C
returnHR / W_ra Return air humidity ratio entering exhaust gr/lb / g/kg
sensibleEffectiveness / ε_s Sensible effectiveness (output) %
latentEffectiveness / ε_L Latent effectiveness (output) %
totalEffectiveness / ε_total Total effectiveness approximate (output) %
sensibleRecovered / Q_s Sensible energy recovered (output) BTU/hr / W
latentRecovered / Q_L Latent energy recovered (output) BTU/hr / W

What is an Energy Recovery Wheel

An energy recovery wheel (also called a rotary heat exchanger, enthalpy wheel, or heat wheel) is a rotating device that transfers heat and/or moisture between two airstreams — typically the outdoor supply air and the building exhaust air. The wheel rotates slowly between the two airstreams, absorbing energy from one side and releasing it to the other.

Enthalpy wheels use desiccant coatings to transfer both sensible heat and latent moisture. Sensible-only heat wheels transfer temperature only. Both types reduce the energy required to condition outdoor ventilation air.

Effectiveness Definitions

ASHRAE defines energy recovery effectiveness as the ratio of actual energy transfer to the maximum possible energy transfer. Sensible effectiveness uses temperature, latent effectiveness uses humidity ratio, and total effectiveness uses enthalpy.

For this calculator, total effectiveness is approximated as the average of sensible and latent effectiveness. This is a practical simplification — true total effectiveness requires full psychrometric enthalpy calculations.

Sensible vs Enthalpy Wheels

Sensible-only heat wheels transfer temperature but not moisture. They are appropriate when moisture transfer is undesirable or when cross-contamination concerns prevent the use of desiccant-coated wheels.

Enthalpy wheels transfer both heat and moisture, providing greater total energy recovery in humid climates. The desiccant coating absorbs moisture from the humid airstream and releases it to the drier airstream.

Practical Tips

When measuring effectiveness in the field, ensure airflow is balanced between supply and exhaust sides. Measure temperatures and humidity ratios at the wheel face, not downstream of other coils or mixing points. Wheel rotation speed affects effectiveness — most wheels operate at 10–20 RPM.

This calculator is a screening tool for effectiveness estimation. Final energy recovery system design should include pressure drop analysis, cross-contamination assessment, frost control strategy, and annual energy savings calculation per ASHRAE and AHRI standards.

Key Facts

  • Energy recovery wheels can transfer both sensible heat and latent moisture between airstreams, unlike many other recovery devices.
  • ASHRAE Standard 90.1 requires energy recovery in many climate zones when outdoor air requirements exceed specified thresholds.
  • Typical enthalpy wheels achieve 50–80% total effectiveness depending on wheel construction and operating conditions.
  • Sensible-only wheels (heat wheels) transfer heat but not moisture, while enthalpy wheels use desiccant coatings to transfer both.
  • Wheel rotation speed affects effectiveness — too slow reduces transfer, too fast increases carryover and pressure drop.
  • Cross-contamination (exhaust air leaking into supply) is a design consideration, especially in healthcare and laboratory applications.

Applications

  • HVAC energy recovery system design and verification.
  • Commissioning checks for energy recovery wheels.
  • Energy modeling input validation.
  • Comparing manufacturer performance claims against field measurements.
  • ASHRAE 90.1 energy recovery compliance screening.
  • Ventilation system energy analysis.
  • Indoor air quality system design with energy recovery.
  • Retrofit evaluation for adding energy recovery to existing systems.

Example Calculation

Example Calculation

Given (Imperial — Summer Cooling):

  • Supply Airflow = 3,000 CFM
  • Outdoor Air Temperature = 95°F
  • Outdoor Humidity Ratio = 100 gr/lb
  • Supply Leaving Temperature = 82°F
  • Supply Leaving Humidity Ratio = 70 gr/lb
  • Return Air Temperature = 75°F
  • Return Humidity Ratio = 65 gr/lb

Step 1: Sensible Effectiveness

ε_s = |95 − 82| / |95 − 75| × 100
ε_s = 13 / 20 × 100
ε_s = 65%

Step 2: Latent Effectiveness

ε_L = |100 − 70| / |100 − 65| × 100
ε_L = 30 / 35 × 100
ε_L ≈ 85.7%

Step 3: Total Effectiveness

ε_total = (65 + 85.7) / 2 ≈ 75.4%

Step 4: Sensible Energy Recovered

Q_s = 3,000 × 1.08 × |95 − 82|
Q_s = 3,000 × 1.08 × 13
Q_s = 42,120 BTU/hr

Step 5: Latent Energy Recovered

Q_L = 3,000 × 0.68 × |100 − 70|
Q_L = 3,000 × 0.68 × 30
Q_L = 61,200 BTU/hr

That is the fixed worked-example path for this page.

Standards & References

  • ASHRAE Standard 90.1 — requires energy recovery when outdoor air exceeds specified thresholds in applicable climate zones
  • ASHRAE Handbook — HVAC Systems and Equipment — defines energy recovery effectiveness and describes rotary wheel performance characteristics (air-to-air energy recovery; rotary wheel effectiveness)
  • AHRI Standard 1060 — rating standard for air-to-air energy recovery ventilation equipment
  • ASHRAE Standard 84 — method of testing air-to-air heat/energy exchangers

Limitations

  • This calculator is a screening tool, not a full energy recovery system design package.
  • It does not calculate: pressure drop, cross-contamination/carryover, frost control, wheel sizing, or annual energy savings.
  • Total effectiveness is approximated as the average of sensible and latent — true total effectiveness requires enthalpy-based calculation.
  • The energy recovered estimates use standard air density assumptions and may not be accurate at extreme temperatures or altitudes.
  • Actual wheel performance varies with face velocity, rotation speed, seal condition, and airflow balance.

Common Mistakes to Avoid

  • Confusing sensible effectiveness with total effectiveness — they are different quantities and should not be used interchangeably.
  • Using outdoor dry-bulb temperature without verifying the humidity ratio, which affects latent effectiveness calculations.
  • Ignoring airflow imbalance between supply and exhaust sides, which reduces actual effectiveness below rated values.
  • Assuming manufacturer-rated effectiveness applies at all operating conditions — effectiveness varies with face velocity, temperature, and humidity.
  • Forgetting that energy recovery wheels have pressure drop that must be accounted for in fan sizing.

Frequently Asked Questions

What does this Energy Recovery Wheel Efficiency Calculator calculate?
It calculates sensible effectiveness, latent effectiveness, total effectiveness (approximate), and estimated energy recovered from the entering and leaving air conditions on both sides of the wheel.
What is the difference between sensible and latent effectiveness?
Sensible effectiveness measures how much temperature change is achieved relative to the maximum possible. Latent effectiveness measures how much moisture transfer is achieved relative to the maximum possible.
What is a good effectiveness value for an energy recovery wheel?
Typical enthalpy wheels achieve 50–80% total effectiveness. Sensible-only wheels may achieve 60–85% sensible effectiveness. Values depend on wheel type, face velocity, and operating conditions.
Does this calculator work for sensible-only heat wheels?
Yes. For a sensible-only wheel, the latent effectiveness will be zero or near zero because no moisture transfer occurs. The sensible effectiveness result is still valid.
Why is total effectiveness shown as approximate?
Because true total effectiveness requires enthalpy-based calculation using the full psychrometric properties of the air. The average of sensible and latent effectiveness is a simplified approximation.
Does imperial or metric mode change the logic?
No. The effectiveness percentages use the same ratio formulas. Only the energy recovered values use different unit factors.
Can effectiveness exceed 100%?
In normal operation, no. If the calculator shows effectiveness above 100%, it usually indicates measurement error or physically inconsistent input values.
What inputs can make the result invalid?
If the outdoor and return temperatures are equal, sensible effectiveness is undefined. Similarly, if outdoor and return humidity ratios are equal, latent effectiveness is undefined. The calculator returns zero in these cases.

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