Economizer Enthalpy Changeover
Differential-enthalpy: enable free cooling when outdoor enthalpy < return (accounts for humidity); fixed-dry-bulb: enable below a setpoint. OA 24 vs RA 28 Btu/lb -> enable (4 margin); a humid OA 32 vs 28 locks out even if cool; fixed 70 F vs 65 F setpoint locks out. The ASHRAE 90.1 high-limit governs.
Formula and source
differential enthalpy: enable if h_outdoor < h_return; fixed dry-bulb: enable if t_outdoor < setpoint; margin = the difference to the limit.
The air-side economizer high-limit (changeover) control, differential enthalpy and fixed dry-bulb, from ASHRAE 90.1 6.5.1.1.3, by name.
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