AHU Fan Heat and the Air Temperatures Used for Coil Selection
AI-generated editorial illustration; not a verified installation or construction detail.
Identify the air state at the coil boundary
A cooling-coil schedule can meet its stated leaving temperature and still deliver air that is too warm. The missing link is often the supply fan between the coil and the point where supply temperature is specified. Its energy input belongs in the air-side balance.
For a draw-through arrangement, the cooling coil is upstream of the fan. For blow-through, the fan is upstream of the cooling coil. Locate these components on the actual selection drawing, then label mixed-air, coil-entering, coil-leaving and unit-discharge states. A single column called “supply air” is not enough.
Decide which fan power enters the air
Fan heat for design includes the mechanical power delivered to the air and the relevant motor or drive losses. With a motor fully in the airstream, its losses also enter that stream. With a motor outside it, those losses may heat the surrounding plantroom instead. EnergyPlus explicitly models this distinction in its air-system fan reference.
Use absorbed operating power, rather than assuming the motor nameplate rating is the actual load. For a simplified direct-drive calculation, shaft power = volume flow × fan total pressure ÷ fan total efficiency. Motor input = shaft power ÷ motor efficiency. Keep external VFD losses separate unless they actually enter the air.
For the coil-selection balance, heat to air = shaft power + f × (motor input − shaft power), where f is the fraction of motor loss entering the airstream. This is a lumped design model. It is not a promise that two temperature probes immediately beside the fan will measure the entire eventual energy dissipation.
Calculate one fan temperature rise
Consider an illustrative AHU operating at 5.00 actual m³/s at the stated fan inlet. Assume air density of 1.18 kg/m³, a 1,200 Pa fan total-pressure rise, 70% fan total efficiency and 92% motor efficiency. The motor is inside the airstream, f = 1; the VFD is outside. Neglect casing heat exchange and leakage.
Shaft power = 5.00 × 1,200 ÷ 0.70 = 8.571 kW. Motor input = 8.571 ÷ 0.92 = 9.317 kW. Because all motor losses enter the stream in this example, the design fan heat allowance is 9.317 kW.
Mass flow = 1.18 × 5.00 = 5.90 kg/s. Using an assumed constant moist-air specific heat of 1.02 kJ/(kg·K), the air heat-capacity rate is 5.90 × 1.02 = 6.018 kW/K. Fan temperature rise = 9.317 ÷ 6.018 = 1.548 K, approximately 1.55°C.
The EnergyPlus component-sizing reference uses this heat-over-mass-flow-and-specific-heat approach and applies the resulting temperature adjustment according to fan location. The numbers here are original example assumptions, not a rating for a particular AHU.
Draw through means a colder coil leaving temperature
Assume mixed air reaches the coil at 28.00°C and the required air temperature immediately after the fan is 13.00°C. With no other downstream gains, the coil must leave at 13.00 − 1.548 = 11.452°C. Its relevant dry-bulb pair is therefore 28.00°C entering and approximately 11.45°C leaving.
For a sensible-only comparison using the constant properties above, coil sensible duty = 6.018 × (28.00 − 11.452) = 99.59 kW. Selecting that coil to leave at 13.00°C instead would imply 14.55°C after the fan under the same assumptions.
If 13.00°C is required at a remote terminal, also account for the heat gained between the unit and that terminal. Put each gain at its physical location. A duct allowance already included in the system supply state should not be added again at the coil.
Blow through means a warmer coil entering temperature
For an equal-fan-heat comparison, move the fan before the coil while keeping the assumed mass flow and 9.317 kW allowance. Mixed air at 28.00°C becomes 29.548°C entering the coil. With no downstream fan or other gain, the coil leaves at the required 13.00°C.
The sensible duty is 6.018 × (29.548 − 13.00) = 99.59 kW, matching the draw-through balance. The coil selection temperatures are different even though the example duties match. The base sensible change from 28°C to 13°C is 90.27 kW; adding fan heat once gives 99.59 kW.
Carrier discusses the colder leaving-air requirement of draw-through units in its 39M application data. In a real comparison, rerun both selections: moving the fan changes its inlet condition and may change its volume, pressure requirement and absorbed power. Equal fan heat here isolates the accounting issue.
Original constant-property example comparing equal fan heat and mass flow. Actual wet-coil capacity and fan performance must be reselected for each arrangement.
Humidity and total capacity still need a coil selection
The 99.59 kW result is a sensible-only illustration, not a total wet-coil capacity. Select a dehumidifying coil using entering and leaving humidity ratios or other complete psychrometric states, air mass flow, water conditions and manufacturer performance data. A dry-bulb target alone cannot establish moisture removal.
With no condensation, evaporation or moisture addition across the fan, fan heating leaves humidity ratio unchanged. Relative humidity falls as dry-bulb temperature rises, and dew point is approximately unchanged at near-constant pressure. This follows the sensible-heating process described by ASHRAE.
Do not reduce the coil’s moisture-removal requirement because downstream fan heat lowers relative humidity. For blow-through selection, also recalculate entering wet bulb at the warmed dry bulb and unchanged humidity ratio; copying the mixed-air wet bulb would specify the wrong entering state.
Avoid double counting and check the physical arrangement
When a supplier reports coil capacity from the actual coil entering and leaving states, the adjusted temperatures already contain this fan allowance. Do not then add another 9.317 kW to the same example coil duty. Ask whether reported capacity is gross coil capacity or net unit capacity and how fan heat is treated.
Draw-through and blow-through also affect airflow distribution and moisture behavior. Trane’s condensation guidance describes how downstream fan heat and more uniform coil airflow can help limit fogging. That does not make either arrangement universally preferable.
Review coil-face velocity distribution, fan-to-coil spacing, bypass leakage, drain-pan pressure and the manufacturer’s drain-trap requirements alongside the thermal calculation. A correct heat balance cannot establish acceptable drainage or carryover performance by itself.
A compact coil submittal checklist
Mark the states: Show fan location and the exact point where the specified supply temperature applies.
Verify power: Record absorbed fan power, efficiency definitions and which motor, belt or drive losses enter the air.
Check the arithmetic: Reconcile mixed-to-supply duty plus fan heat against the coil-boundary calculation, once.
Complete the psychrometrics: Verify entering wet bulb or humidity ratio and the required leaving moisture content.
Review the envelope: Check part-load fan operation, filter resistance, duct gains and actual chilled-water conditions. The example’s fixed 1.55 K rise should not become a universal operating setpoint.
Related learning resource
For a broader foundation in HVAC calculations and the design sequence, see Nexora’s HVAC Beginner’s Guide. Its public description covers first principles and a small-office design with diagrams and exercises. Use the selected AHU manufacturer’s coil data to close this specific fan-heat review.
Sources and further reading
Engineering education only. Verify project conditions, applicable requirements and manufacturer data; use qualified professional judgment before design, selection, operation or maintenance decisions.
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