Section 6.5.1/Economizer Requirements
ASHRAE 90.1 Section 6.5.1 requires air-side economizers on cooling systems above 54,000 Btu/h in climate zones 2-8.
Air-side economizers are required on individual cooling systems with a cooling capacity of 54,000 Btu/h or greater in climate zones 2 through 8. The economizer must be capable of providing 100 percent outdoor air to the supply when outdoor conditions are favorable. High-limit shutoff controls must be provided to disable the economizer when outdoor conditions exceed the economizer setpoint. Water-side economizers are permitted as an alternative in some climate zones. Exceptions apply to certain system types including dedicated outdoor air systems and systems with energy recovery.
Why this section exists
During mild weather, the outdoor air temperature is lower than the return air temperature from the building. An economizer opens the outdoor air damper to use this free cooling instead of running the mechanical cooling system. In many climates, economizer operation can reduce cooling energy by 20-40% annually. Without the requirement, systems would default to mechanical cooling even when free cooling is available, wasting energy.
What plan reviewers look for
Plan reviewers check the mechanical equipment schedules and control sequences for economizer capability on every cooling system at or above 54,000 Btu/h. They verify the high-limit shutoff type (differential dry-bulb or differential enthalpy, depending on climate zone per Table 6.5.1.1.3). They check that the outdoor air damper is sized for 100% outdoor air when economizing.
Common violations
Related ASHRAE 90.1 requirements
Section 6.4.1 covers equipment efficiency. Section 6.5.2 covers simultaneous heating and cooling limitations. Section 6.5.3 covers air system design and control. IECC Section C403.5 provides equivalent economizer requirements.