HVAC Duct Calculators

Fans and energy

Fan Heat Gain Calculator

Combine shaft power, drive losses, motor location, and airflow to estimate airstream heat gain and temperature rise.

Calculation inputs

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Use actual volume flow at the same fan state as shaft power and air density. Do not enter SCFM without converting its reference state.
Enter mechanical fan shaft BHP, not motor nameplate horsepower or electrical input kW. Zero is allowed for a stopped/no-heat boundary.
Use the motor efficiency on a fractional basis from 0 to 1. The value is not inferred from a nameplate or motor class.
Use 0 for direct drive, or the documented drive loss as a fraction of fan shaft BHP. For example, enter 0.03 for a 3% drive loss.
Enter total air mass per actual fan volume on the same state basis as ACFM. This illustrative value is not a default or an altitude/weather estimate.
Choose whether both the motor and its drive/transmission are in the airstream. If they are split across locations, this two-location balance is not applicable.

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Formula and source basis

Concepts behind this calculation

Calculate fan shaft heat, motor and drive losses, heat added to the airstream and the resulting sensible temperature rise.

Fan shaft power enters the stated heat balance as power transferred to the air. Motor inefficiency and drive loss require additional input power, and the selected location determines where those losses are assigned. When both motor and drive are inside the airstream, their losses join the shaft contribution in the air. When both are outside, those losses are assigned to the equipment space instead.

Air temperature rise depends on heat added to the stream divided by its sensible heat-capacity flow. Actual CFM and total-air density establish mass flow, while this model holds specific heat fixed. The drive loss fraction is referenced to fan shaft BHP and is applied before dividing by motor efficiency. Entering electrical input or nameplate capacity as shaft BHP would misstate that accounting.

How to use this calculator

  1. Enter Fan airflow at actual operating condition and Fan shaft input power for the same operating point.
  2. Supply Motor efficiency as a fraction, Drive loss fraction of fan shaft BHP and Total-air density at the fan condition.
  3. Choose Motor and drive location, then review shaft heat, motor/drive losses, their destinations and the resulting sensible air temperature rise.

Calculation method and applicability

Pmotor,in = BHP × (1 + DL) / ηmotor; Qfan = 2545 × BHP; Qmotor-drive,loss = 2545 × (Pmotor,in − BHP). If motor and drive are in the airstream, Qair = 2545 × Pmotor,in; otherwise Qair = Qfan and Qspace = Qmotor-drive,loss. ΔT = Qair / (60 × ACFM × ρ × cp), with cp = 0.24 Btu/(lbm·°F).

BHP = fan shaft horsepower; DL = drive loss fraction; ηmotor = motor efficiency fraction; thermal Q terms are BTU/h. ACFM is actual flow, ρ is total-air density (lbm/ft³) and ΔT is °F.

Input definitions and units

Fan airflow at actual operating condition (ACFM)
Use actual volume flow at the same fan state as shaft power and air density. Do not enter SCFM without converting its reference state.
Fan shaft input power (BHP)
Enter mechanical fan shaft BHP, not motor nameplate horsepower or electrical input kW. Zero is allowed for a stopped/no-heat boundary.
Motor efficiency (fraction)
Use the motor efficiency on a fractional basis from 0 to 1. The value is not inferred from a nameplate or motor class.
Drive loss fraction of fan shaft BHP (fraction)
Use 0 for direct drive, or the documented drive loss as a fraction of fan shaft BHP. For example, enter 0.03 for a 3% drive loss.
Total-air density at the fan condition (lbm moist air/ft³)
Enter total air mass per actual fan volume on the same state basis as ACFM. This illustrative value is not a default or an altitude/weather estimate.
Motor and drive location
Choose whether both the motor and its drive/transmission are in the airstream. If they are split across locations, this two-location balance is not applicable. Choices: Both inside the airstream; Both outside the airstream.

How to interpret the result

Carry the airstream heat and sensible temperature rise into a matching air-state analysis, and account for equipment-space losses separately. The motor/drive input equivalent is part of this power balance, not a complete building cooling load.

Worked examples

Fan Heat Gain Calculator

Inputs: Published peer scenario: 8,000 ACFM, 10 fan shaft BHP, 90% motor efficiency, direct drive at 100%, fan-state total-air density 0.075 lbm/ft³, with both motor and drive in the airstream.

Calculation: Motor input equivalent = 10×(1+0)/0.90 = 11.111 HP. Motor/drive losses = 2,545×(11.111−10) = 2,827.778 Btu/h. Heat to air = 2,545×11.111 = 28,277.778 Btu/h; air mass flow = 60×8,000×0.075 = 36,000 lbm/h; ΔT = 28,277.778/(36,000×0.24) = 3.272891°F.

Result: 3.273°F temperature rise and 28,277.778 Btu/h to the airstream, matching the peer's rounded 3.3°F and 28,278 Btu/h example at the explicitly entered density.

Assumptions and limits

  • Uses fan shaft BHP as power transferred to the air and converts horsepower at 2,545 Btu/h per HP. Motor efficiency and drive loss fraction are separate. Drive loss is referenced to fan shaft BHP. Motor and drive losses are added to the air only when both are inside it; otherwise they are assigned to the equipment space.
  • Motor and drive are represented as one paired location. Mixed placements, VFD losses, casing heat exchange and non-fan heat sources are not modeled.
  • Uses user-entered total-air density and actual fan ACFM from the same state. No standard-air default, altitude correction, weather lookup, humidity calculation or ACFM/SCFM conversion is applied.
  • Specific heat is fixed at 0.24 Btu/(lbm·°F) for this stated sensible model. Different gas mixtures or materially different specific heat require a different method.
  • The result is fan heat and sensible air-temperature rise only. It is not a full cooling load, fan selection, system operating point, equipment rating, code check or field measurement.

FAQs

Does placing the motor outside remove fan heat from the airstream?

The shaft contribution remains in the air under this model. Moving both motor and drive outside changes the assignment of their losses to the equipment space; it does not remove the shaft-power contribution.

Can I represent a motor outside with its drive inside?

The location selector treats motor and drive as a pair. Split placement requires a separate heat balance so the individual losses are assigned to their actual destinations.

Sources

Related calculators

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Related reading

Duct heat loss, fan heat gain, and outlet temperature Estimate steady sensible duct heat transfer or fan heat gain while keeping their input boundaries and air-property assumptions separate.