The entered economizer predicate is To≤Tlim (true), ho≤hlim (true), and Δh>0 (true); therefore the entered enable conditions are all met. Outdoor air has 9.952 Btu/lb of enthalpy advantage relative to return air.
Compare outdoor dry bulb and enthalpy with the entered high limits.
Require all three explicit predicates: outdoor temperature at or below its limit, outdoor enthalpy at or below its limit, and positive return-minus-outdoor enthalpy advantage.
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 — %
P
Barometric pressure — psia or kPa
Tlim
Entered outdoor-air high limit — °F or °C
hlim
Entered outdoor-air enthalpy limit — Btu/lb or kJ/kg
Δh
Return-minus-outdoor enthalpy advantage — Btu/lb
ho
Outdoor-air enthalpy — Btu/lb
hr
Return-air enthalpy — Btu/lb
MT
Outdoor temperature margin to entered limit — °F
Mh
Outdoor enthalpy margin to entered limit — Btu/lb
Applications
How this calculation is used
Building design
Screen dry-bulb and enthalpy control thresholds.
Construction
Check sensors, sequences, and high-limit settings.
Commissioning
Compare field outdoor and return states with the entered enable limits.
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.
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.
