Section 6.5.3.1/Economizer Requirements
ASHRAE 90.1 Section 6.5.3 requires air-side or water-side economizers on cooling systems above specified capacity thresholds, based on climate zone and system type.
Cooling systems with a total cooling capacity greater than the threshold in Table 6.5.3.1 (54,000 Btu/h for most systems) must include either an air-side economizer or a water-side economizer. Air-side economizers use outdoor air for free cooling when the outdoor temperature or enthalpy is below the return air condition. The economizer must be capable of providing 100% outdoor air to the system, with controls that modulate the outdoor air damper based on outdoor conditions. High-limit shutoff controls must prevent economizer operation when outdoor conditions exceed the setpoint (fixed dry-bulb, differential dry-bulb, fixed enthalpy, or differential enthalpy, depending on climate zone per Table 6.5.3.1.1). Water-side economizers use a cooling tower or fluid cooler to produce chilled water directly when outdoor wet-bulb conditions permit, bypassing the chiller. Exceptions include systems in climate zones 1a, 1b, and 2a (hot/humid), systems with energy recovery per Section 6.5.6, and systems serving spaces with special humidity requirements. The IECC references these requirements for prescriptive compliance.
Why this section exists
Economizers provide free cooling by using outdoor air (or outdoor wet-bulb temperatures) to offset mechanical cooling during mild weather. In temperate and cold climates, economizer hours can represent 30 to 60% of annual cooling hours, dramatically reducing compressor energy use. The requirement is waived in hot/humid climates where outdoor air rarely provides a cooling benefit and the humidity control challenges outweigh the energy savings. The capacity threshold (54,000 Btu/h) targets systems large enough that the damper and control cost is justified by energy savings, exempting small packaged equipment.
What plan reviewers look for
Plan reviewers check the HVAC equipment schedule for cooling capacity and verify an economizer is specified for systems above the threshold. They check the control sequence for economizer operation, including the high-limit shutoff type and setpoint. They verify the outdoor air damper can open to 100% for full economizer mode. For water-side economizers, they verify the cooling tower or fluid cooler is sized for economizer operation and the piping arrangement allows chiller bypass. They check the climate zone against Table 6.5.3.1 for applicable exceptions.