Outdoor air economizers are typically activated based on a comparison of which condition to indoor conditions?

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The key idea is that outdoor air economizers save energy by bringing in outdoor air only when it has a lower total energy content than the indoor air, as measured by enthalpy. Enthalpy combines both heat and moisture, so lower outdoor enthalpy means the outdoor air is cooler and drier, reducing the cooling (and dehumidification) load when it is mixed with return air.

When outdoor enthalpy is lower than indoor enthalpy, introducing outdoor air effectively provides free cooling, lowering the need for mechanical cooling. If outdoor enthalpy is higher, bringing in outdoor air would add energy to the system, so the economizer should remain closed.

So the best condition is outdoor enthalpy being lower than indoor enthalpy, enabling outdoor air to be used for cooling. Humidity or temperature alone aren’t reliable indicators, and outdoor air isn’t always cooler; the decision is driven by enthalpy comparison.