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C403.3.3 Air Economizers
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Air economizers shall comply with Sections C403.3.3.1 through C403.3.3.5.
Air economizer systems shall be capable of modulatingoutdoor air and return air dampers to provide up to 100 percent of the design supply air quantity as outdoor air for cooling.
Economizer dampers shall be capable of being sequenced with the mechanical cooling equipment and shall not be controlled by only mixed-air temperature.
Exception: The use of mixed-air temperature limit control shall be permitted for systems controlled from space temperature (such as single-zone systems).
Air economizers shall be capable of automatically reducing outdoor air intake to the design minimum outdoor air quantity when outdoor air intake will no longer reduce cooling energy usage. High-limit shutoff control types for specific climates shall be chosen from Table C403.3.3.3. High-limit shutoff control settings for these control types shall be those specified in Table C403.3.3.3.
HIGH-LIMIT SHUTOFF CONTROL SETTING FOR AIR ECONOMIZERSb
| DEVICE TYPE | CLIMATE ZONE | REQUIRED HIGH LIMIT (ECONOMIZER OFF WHEN): | |
| Equation | Description | ||
| Fixed dry bulb | 1B, 2B, 3B, 3C, 4B, 4C, 5B, 5C, 6B, 7, 8 | TOA > 75°F | Outdoor air temperature exceeds 75°F |
| 5A, 6A | TOA > 70°F | Outdoor air temperature exceeds 70°F | |
| 1A, 2A, 3A, 4A | TOA > 65°F | Outdoor air temperature exceeds 65°F | |
| Differential dry bulb | 1B, 2B, 3B, 3C, 4B, 4C, 5A, 5B, 5C, 6A, 6B, 7, 8 | TOA > TRA | Outdoor air temperature exceeds return air temperature |
| Fixed enthalpy with fixed dry-bulb temperatures | All | hOA > 28 Btu/lba or TOA > 75°F | Outdoor air enthalpy exceeds 28 Btu/lb of dry aira or Outdoor air temperature exceeds 75°F |
| Differential enthalpy with fixed dry-bulb temperature | All | hOA > hRA or TOA > 75°F | Outdoor air enthalpy exceeds return air enthalpy or Outdoor air temperature exceeds 75°F |
For SI: 1 foot = 305 mm, °C = (°F - 32)/1.8, 1 Btu/lb = 2.33 kJ/kg.
- At altitudes substantially different than sea level, the fixed enthalpy limit shall be set to the enthalpy value at 75°F and 50-percent relative humidity. As an example, at approximately 6,000 feet elevation, the fixed enthalpy limit is approximately 30.7 Btu/lb.
- Devices with selectable setpoints shall be capable of being set to within 2°F and 2 Btu/lb of the setpoint listed.
Systems shall be capable of relieving excessoutdoor air during air economizer operation to prevent overpressurizing the building. The relief air outlet shall be located to avoid recirculation into the building.
Return, exhaust/relief and outdoor air dampers used in economizers shall comply with Section C403.2.4.3.
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