Tools · Mixed Air Temperature

Mixed Air Temperature Calculator HVAC

Mix three air streams — pick the weight shape your data is in.
By mass flow when you have CFMs at the panel (outdoor + return + ERV exhaust, economizer + return + makeup). By mass fraction when you have percentages off a schematic. The math is the same mass-weighted blend either way; the tab just picks how you tell the tool what the weights are.

Inputs are seeded with an example — edit them to your numbers.

Input

Stream 1

Stream 2

Stream 3

Specific volume comes off each stream's state; mass flow per stream is CFM ÷ v. Zero a stream's CFM to drop it out of the mix.

Output

Enter values to see the mix.

Mixed-air state

Dry-bulb (°F) —
Wet-bulb (°F) —
Dew point (°F) —
Humidity ratio (gr/lb) —
Rel. humidity (%) —
Enthalpy (Btu/lb) —
Specific volume (ft³/lb) —

Worked example

Summer AHU with energy recovery — three streams meeting in the mixing plenum. Outdoor at 1000 CFM / 95 °F / 75 °F WB, return at 3000 CFM / 75 °F / 50 % RH, ERV exhaust bleed at 500 CFM / 60 °F / 55 °F WB.

  1. For each stream, solve the state (the engine returns specific volume v alongside W and h).
  2. Convert volumetric flow to mass: mi = CFMi ÷ vi. Cold streams are denser; warm streams less so.
  3. Mass-weight the two variables that mix linearly with mass — humidity ratio and enthalpy:
    Wmix = Σ(mi · Wi) ÷ Σmi, hmix = Σ(mi · hi) ÷ Σmi.
  4. Recover dry-bulb by inverting the enthalpy formula h = (0.240 + 0.444·W)·T + 1061·W:
    Tmix = (hmix − 1061·Wmix) ÷ (0.240 + 0.444·Wmix).
  5. Materialize the full state at the site pressure to read out RH, WB, DP, and v.

The output column shows the answer the AHU actually sees — RH and WB are what determine whether the cooling coil works dry or wet at this mix. The formula steps are written in the engine's native IP units (CFM, lb, Btu — the ASHRAE IP formulation); metric inputs and results convert at the display boundary, same math.

Input

Stream 1

Stream 2

Stream 3

Fractions are by mass and must sum to 100 % (±0.1 % tolerance). Use this tab when your data already comes in as percentages — otherwise the mass-flow tab is the one to use.

Output

Enter values to see the mix.

Mixed-air state

Dry-bulb (°F) —
Wet-bulb (°F) —
Dew point (°F) —
Humidity ratio (gr/lb) —
Rel. humidity (%) —
Enthalpy (Btu/lb) —
Specific volume (ft³/lb) —

Worked example

Same three streams expressed as mass fractions — 22 % / 67 % / 11 %.

  1. Skip the volumetric-to-mass step — fractions are already in mass units.
  2. Mass-weight humidity ratio and enthalpy directly: Wmix = 0.22·W1 + 0.67·W2 + 0.11·W3, same for hmix.
  3. Recover dry-bulb the same way and materialize the full state.

A schedule that lists "fraction OA" alongside DBs and WBs lands here cleanly. A panel reading with CFMs and damper positions lands in the mass-flow tab.

Mixed air temperature FAQ

What is the mixed air temperature formula?

Weight each stream's temperature by its mass flow — MAT = Σ(ṁ·T) ÷ Σṁ. Near equal, sea-level density that's close to the airflow-weighted average, MAT ≈ (CFM_oa·T_oa + CFM_ra·T_ra) ÷ (CFM_oa + CFM_ra).

How do you calculate mixed air temperature from percent outdoor air?

MAT = %OA × T_oa + (1 − %OA) × T_ra. With 30% outdoor air at 40 °F and 70% return air at 72 °F, MAT = 0.30 × 40 + 0.70 × 72 = 62.4 °F.

Why weight by mass flow instead of CFM?

Air density changes with temperature and altitude, so two equal CFM streams don't carry equal mass. Weighting by mass flow (or density-corrected CFM) is exact; the plain CFM average is a good approximation only near equal densities.
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