Economizer & Outside Air Reference
When an air-side economizer is required by ASHRAE 90.1, what it does, and how it is controlled. An economizer uses cool outdoor air for free cooling through a modulating damper, cutting compressor run time. It is required above 4.5 tons in most climate zones — confirm the edition your jurisdiction has adopted, since the threshold has changed over time.
Economizer requirements
| Topic | Detail |
|---|---|
| When required | Rated cooling capacity ≥ 54,000 BTU/h (4.5 tons) per unit, in climate zones 2A through 8. Exempt in 0A, 0B, 1A, 1B (hottest/most humid). |
| What it does | Provides free cooling by drawing cool outdoor air through a modulating outdoor-air damper — up to 100% outdoor air — to offset or replace mechanical cooling when conditions allow. |
| Minimum outdoor air | The damper’s minimum position always meets the ASHRAE 62.1 ventilation minimum; the economizer modulates above that floor for free cooling and never below it during occupied hours. |
High-limit control types
The economizer needs to know when outdoor air will actually help, and that is the job of the high-limit shutoff control — it disables free cooling and returns the damper to minimum when the outdoor air is too warm or too moist. The control types are:
- Fixed dry-bulb (disable above an outdoor-air temperature setpoint)
- Differential dry-bulb (outdoor-air temp vs. return-air temp)
- Fixed enthalpy (disable above an outdoor-air enthalpy setpoint)
- Differential enthalpy (outdoor-air enthalpy vs. return-air enthalpy)
- Combination fixed enthalpy + fixed dry-bulb
Dry-bulb controls judge outdoor air by temperature alone; enthalpy controls judge it by total heat, counting moisture too — which matters in humid climates, where 70°F outdoor air can carry more heat than warmer, drier return air. The exact setpoints are climate-zone-specific in the 90.1 tables, so they are set per zone rather than fixed. The enthalpy comparison is a direct application of the psychrometric idea that "cooler" air is not always lower-energy air.
Common questions
When is an economizer required by code?
Under ASHRAE 90.1, an air-side economizer is generally required on cooling systems with a rated capacity of 54,000 BTU/h (4.5 tons) or more, in climate zones 2A through 8. The hottest, most humid zones (0A, 0B, 1A, 1B (hottest/most humid)) are exempt regardless of size, and there are additional exemptions for small systems, high-dehumidification loads, and certain heat-recovery cases. The exact threshold is edition-specific — older 90.1 editions used 65,000 BTU/h.
What does an economizer do?
It provides "free cooling." When the outdoor air is cool enough, a modulating outdoor-air damper opens — up to 100% outdoor air — to meet the cooling load with outside air instead of running the compressors. When the outdoor air is too warm or too humid to help, a high-limit control closes the damper back to the minimum ventilation position.
What is the difference between the economizer and the minimum outdoor air?
They share the same damper but do different jobs. The minimum outdoor-air position provides the ASHRAE 62.1 ventilation requirement at all times during occupancy — that floor never changes. The economizer modulates the damper above that minimum for free cooling and never below it, so ventilation is always satisfied whether or not free cooling is available.
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