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Evgenii Konkin
Evgenii Konkin

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Mixed Air Conditions: The Calculation Step That Undersizes Cooling Coils by 20%

#hvac #engineering #energy #tutorial

When an AHU brings in outdoor air for ventilation, the air entering the cooling coil is not return air. It is a mixture of return air and outdoor air. The properties of this mix determine the actual coil load.

The mixing formulas

Mixed air properties are mass-weighted averages of the two streams:

T_mix = (CFM_return × T_return + CFM_outdoor × T_outdoor) / CFM_total

W_mix = (CFM_return × W_return + CFM_outdoor × W_outdoor) / CFM_total

h_mix = (CFM_return × h_return + CFM_outdoor × h_outdoor) / CFM_total
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Where T = dry-bulb temperature, W = humidity ratio (grains/lb), h = enthalpy (BTU/lb).

Example: 30% outdoor air in Atlanta 

Return air (from conditioned space):
  T = 75F, RH = 50%
  W = 65 grains/lb
  h = 28.1 BTU/lb

Outdoor air (ASHRAE design day):
  T = 95F, WB = 78F
  W = 118 grains/lb
  h = 38.5 BTU/lb

Mix ratio: 70% return / 30% outdoor
Total: 5,000 CFM (3,500 return + 1,500 outdoor)
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The mixed air result 

T_mix = 0.70 × 75 + 0.30 × 95 = 81.0F
W_mix = 0.70 × 65 + 0.30 × 118 = 80.9 grains/lb
h_mix = 0.70 × 28.1 + 0.30 × 38.5 = 31.3 BTU/lb
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The coil sees 81F air at 81 grains/lb — not 75F at 65 grains/lb.

The capacity impact 

Coil load if sized for return air only:
  Q = 4.5 × 5,000 × (28.1 - 22.7) = 121,500 BTU/hr = 10.1 tons

Coil load at actual mixed air condition:
  Q = 4.5 × 5,000 × (31.3 - 22.7) = 193,500 BTU/hr = 16.1 tons

Difference: 72,000 BTU/hr = 6 tons undersized
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The 22.7 BTU/lb is the enthalpy of 55F supply air at 95% RH (coil leaving condition).

Where outdoor air fraction matters most 

OA Fraction    T_mix    W_mix      h_mix     Extra Load vs Return
─────────────────────────────────────────────────────────────────
0% (recirculate) 75.0F   65 gr/lb   28.1      baseline
10%              77.0F   70 gr/lb   29.1      +3%
20%              79.0F   76 gr/lb   30.2      +8%
30%              81.0F   81 gr/lb   31.3      +11%
50%              85.0F   92 gr/lb   33.3      +19%
100% (all OA)    95.0F  118 gr/lb   38.5      +37%
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Hospitals and labs running 100% outdoor air have coil loads 37% higher than the same space with recirculated air.

The psychrometric shortcut

On a psychrometric chart, the mixed air point lies on a straight line between the two state points. The position along the line corresponds to the mixing ratio.

30% outdoor air → point is 30% of the way from return to outdoor
50% outdoor air → point is exactly halfway
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This visual method lets you read off all mixed properties — temperature, humidity ratio, enthalpy, wet-bulb — without calculating each one separately.

The rule

Never size a cooling coil for return air conditions when the system includes outdoor air. Calculate the mixed air state point first, then size the coil from mixed air to supply air.

The difference is small in dry climates with low outdoor air fractions. It is enormous in humid climates with high ventilation rates.

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