Laboratory exhaust energy

Fume Hood Exhaust Load Calculator

Estimate thermal conditioning load from documented exhaust airflow and a matched indoor-to-outdoor moist-air enthalpy difference.

Created · Updated

Airflow to thermal load

Use absolute enthalpy difference for one design or operating condition. The sign of heating or cooling is handled outside this magnitude calculation.

Enter values and calculate.

Effective enthalpy differenceBtu/lb dry air
Thermal loadkW thermal
Converted thermal loadBTU/h
Annual thermal energykWh thermal/year

How to use this calculator

  1. Establish the exhaust airflow scenario with measured values or the Multi-Fume-Hood Exhaust Planner.
  2. Use matched indoor and outdoor air states to obtain absolute moist-air enthalpy difference and density.
  3. Enter only a documented effective recovery value; enter zero when none is supported.
  4. Review peak thermal load separately from annual thermal energy.

Calculation method

US: load (BTU/h) = CFM × 60 × density (lb/ft³) × effective Δh (Btu/lb)
SI: load (kW) = m³/h × density (kg/m³) × effective Δh (kJ/kg) ÷ 3600
Effective Δh = |Δh| × (1 − recovery fraction)

Worked example

At 1,000 CFM, 0.075 lb/ft³, a 10 Btu/lb enthalpy difference, no recovery, and 2,000 hours/year, the thermal load is 45,000 BTU/h or 13.19 kW. Annual thermal energy is about 26,376 kWh. These are thermal values, not electrical consumption.

Result interpretation

The load represents the energy-rate magnitude required to move the replacement air from one documented state to another under the inputs. Actual plant input depends on heating/cooling system efficiency, part-load behavior, reheat, humidification, fan power, controls, heat recovery limitations, and simultaneous building loads.

Frequently asked questions

Where do I get the enthalpy difference?

Calculate the two moist-air states with the Psychrometric Calculator, then use the absolute difference with a density appropriate to the airflow basis.

Is 0.075 lb/ft³ always correct?

No. It is only the example input. Air density varies with temperature, moisture, pressure, and elevation.

Does the result size heating or cooling equipment?

No. It isolates one exhaust-conditioning component and excludes the rest of the load and equipment selection process.

Related resources

Sources