Measured Outside-Air Percent from Mixed-Air Temperatures
%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.
Formula and source
%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.
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