Measured Outside-Air Percent from Mixed-Air Temperatures
How much outside air a unit is actually pulling in, read from three temperatures.
Example
You enter
- Return-air temperature T_ra (°F) 75
- Mixed-air temperature T_ma (°F) 68
- Outdoor-air temperature T_oa (°F) 40
You get
- Outside-air fraction 20.0% OA
Details, formula, and sources
%OA = 100 (T_ra - T_ma) / (T_ra - T_oa), the field check of the outside-air damper against the design minimum. RA 75, MA 68, OA 40 F -> 20% OA; the damper opening to MA 63 reads 34%. A mixed-air temp outside the return/outdoor band flags a sensor fault; a spread under 10 F warns of low reliability. A field aid.
%OA = 100 (T_ra - T_ma) / (T_ra - T_oa).
The mixed-air temperature balance for the outside-air fraction from ASHRAE / AABC-NEBB field practice, by name.
The mixed-air temperature-balance outside-air percentage is a standard published air-balancing field check.
Estimate. AHJ and licensed professional govern.
Field names used by the API: t_ra_f, t_ma_f, t_oa_f, pct_oa
- Temperature balance %OA = 100 (T_ra - T_ma) / (T_ra - T_oa)ASHRAE / AABC / NEBB
- Well-mixed readings shielded dry-bulb sensors in well-mixed airfield practice
- Low-spread caveat a return-to-outdoor spread under ~10 F is sensitive to sensor errorscope of this tile