Outdoor Air Mix
Mixed dry-bulb and humidity ratio from RA/OA states and OA fraction.
Example
You enter
- Return air temp (°F) 75
- Return RH (%) 50
- Outdoor air temp (°F) 95
- Outdoor RH (%) 60
- OA fraction (0-1) 0.2
You get
- Mixed dry-bulb 79 F
- Oa fraction 0.2
Checked against published worked examples from: ASHRAE Handbook (Fundamentals).
Details, formula, and sources
Formula or tableMixed dry bulb = f × T_OA + (1 − f) × T_RA; mixed humidity ratio mass-weighted the same way (f = OA fraction).
Source and editionASHRAE 62.1-2022 (Ventilation for Acceptable Indoor Air Quality).
Free accessFree read-only access to ASHRAE standards at ashrae.org (Technical Resources, read-only standards).
What governsEstimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.
API field namesreturn_T_F, return_RH_percent, outdoor_T_F, outdoor_RH_percent, oa_fraction, mixed_T_F
- OA fraction user-entered; ASHRAE 62.1 sets the minimumASHRAE 62.1-2022