Economizer Enthalpy Changeover

Differential-enthalpy: enable free cooling when outdoor enthalpy < return (accounts for humidity); fixed-dry-bulb.

Run the calculator

Example

You enter

You get

No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.

Details, formula, and sources

ScopeEnable 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 90.1-2013+ combined mode also locks out a dry 80 F day whose enthalpy is below the return (75 F limit). The ASHRAE 90.1 high-limit governs.

Formula or tabledifferential enthalpy: enable unless h_outdoor > h_return; fixed dry-bulb: enable unless t_outdoor > setpoint; differential enthalpy + fixed dry-bulb (90.1-2013+): enable unless h_outdoor > h_return or t_outdoor > the entered fixed dry-bulb limit (75 F under 90.1-2013+ Table 6.5.1.1.3); margin = the difference to the binding limit.

Source and editionThe air-side economizer high-limit (changeover) control, differential enthalpy and fixed dry-bulb, from ASHRAE 90.1 6.5.1.1.3, by name.

Free accessThe differential-enthalpy and fixed-dry-bulb high-limit logic is the published ASHRAE 90.1 economizer control requirement.

What governsEstimate. AHJ and licensed professional govern.

API field namesmode, h_outdoor, h_return, t_outdoor_f, setpoint_f, margin

Related tools