Calculation sequence
Step-by-step method
- Calculate outdoor- and return-air humidity ratio and enthalpy.
- Mass-average humidity ratio and enthalpy using outdoor-air fraction.
- Solve dry-bulb temperature from mixed enthalpy and humidity ratio.
MechanicalAdvancedPsychrometrics
Mixed-Air Conditions Calculator
Calculate mixed-air dry-bulb temperature, humidity ratio, and enthalpy from outdoor and return moist-air states.
Why it matters: Mass- and energy-based mixing is more complete than dry-bulb interpolation alone.
Calculated result
Engineering evaluation
Mixed-air dry-bulb temperature (Tm)65.08 °F
- Mixed-air humidity ratio (Wm)
- 0.007 kg/kg dry air
- Mixed-air enthalpy (hm)
- 23.1 Btu/lb
- Outdoor-air fraction (Xoa)
- 25 %
Engineering interpretation
The ideal mixed state is 65.08 °F, humidity ratio 0.00686074, and enthalpy 23.1 Btu/lb.
Important limitations
- Assumes dry-air mass fraction and adiabatic mixing. Leakage, fan heat, casing gain, moisture sources, stratification, and sensor bias require separate review.
Engineering knowledge center
Understand the calculation—not just the answer
Governing relationships
Wo = f(To,RHo,P)
Wr = f(Tr,RHr,P)
Wm = Xoa·Wo+(1−Xoa)Wr
hm = Xoa·ho+(1−Xoa)hr
Tm = f(hm,Wm)
Current substitution
Xoa = 25%; Wm = 0.00686074; hm = 23.1; Tm = 65.08 °F.
Equation legend
Inputs and calculated quantities
- To
- Outdoor-air dry-bulb temperature — °F or °C
- RHo
- Outdoor-air relative humidity — %
- Tr
- Return-air dry-bulb temperature — °F or °C
- RHr
- Return-air relative humidity — %
- Xoa
- Outdoor-air percentage — %
- P
- Barometric pressure — psia or kPa
- Tm
- Mixed-air dry-bulb temperature — °F
- Wm
- Mixed-air humidity ratio — kg/kg dry air
- hm
- Mixed-air enthalpy — Btu/lb
- Xoa
- Outdoor-air fraction — %
Applications
How this calculation is used
Building design
Screen mixed-air conditions at an outdoor-air fraction.
Construction
Check damper, sensor, and mixing-box expectations.
Commissioning
Compare measured mixed air with the calculated conservation result.
Quality controls
Assumptions and limitations
Assumptions
- Entered values represent one stable operating condition and use consistent measurement boundaries.
- Fluid and air properties are representative of the evaluated condition.
Limitations
- This is a preliminary evaluation and field-verification tool, not a final sizing or code-compliance determination.
- Manufacturer data, adopted codes, project criteria, and professional engineering review remain controlling.
Field use
Verification procedure
- Confirm measurement boundaries, operating mode, and instrument calibration.
- Stabilize the system and record simultaneous readings.
- Compare the calculation with project requirements and manufacturer data before disposition.
Common engineering mistakes
- Mixing values from different operating conditions.
- Using nominal rather than measured flow, temperature, power, or fluid properties.
Technical basis
References and source standards
- ASHRAE Handbook—FundamentalsPsychrometrics, heat transfer, load relationships, and HVAC calculation context; verify the licensed current edition during professional review.
- NIST Guide for the Use of the International System of UnitsFederal unit-system and conversion reference used for SI/I-P presentation.
Engineering calculation disclaimer
These engineering tools are provided for educational, preliminary evaluation, field verification, and commissioning support. Results depend on the accuracy of user-entered information and the assumptions stated for each calculation. Every colored status and comparison is a screening indicator only; project criteria, contract requirements, applicable codes, manufacturer instructions, measurement uncertainty, and authorized engineering judgment govern. The tools do not replace project-specific engineering analysis or the judgment of a licensed professional engineer.
Read the full use and limitation statement