6.5.1.1 Air Economizers
Air economizer systems shall be capable of and configured to modulate outdoor air and return air dampers to provide up to 100% of the design supply air quantity as outdoor air for cooling.
Economizer controls shall be capable of and configured to sequence the dampers with the mechanical cooling equipment and shall not be controlled by only mixed-air temperature.
Exception to 6.5.1.1.2
The use of mixed-air temperature limit control shall be permitted for systems controlled from space temperature (such as single-zone systems).
All air economizers shall be capable of and configured to automatically reduce outdoor air intake to the design minimum outdoor air quantity when outdoor air intake will no longer reduce cooling energy use. High-limit shutoff control types and associated set points for specific climate zones shall be chosen from Table 6.5.1.1.3.
Table 6.5.1.1.3 High-Limit Shutoff Control Settings for Air Economizerb
| Control Type | Allowed Only in Climate Zone at Listed Set Point | Required High-Limit Set Points (Economizer Off when): | |
|---|---|---|---|
| Equation | Description | ||
| Fixed dry-bulb temperature | 0B, 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 | |
| 0A, 1A, 2A, 3A, 4A, | TOA > 65°F | Outdoor air temperature exceeds 65°F | |
| Differential dry-bulb temperature | 0B, 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 temperature | All | hOA > 28 Btu/lba or TOA > 75°F |
Outdoor air enthalpy exceeds 28 Btu/lba 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 |
- At altitudes substantially different than sea level, the fixed enthalpy limit shall be set to the enthalpy value at 75°F and 50% rh. As an example, at approximately 6000 ft elevation, the fixed enthalpy limit is approximately 30.7 Btu/lb.
- Devices with selectable rather than adjustable set points shall be capable of being set to within 2°F and 2 Btu/lb of the set point listed.
Return, exhaust/relief, and outdoor air dampers shall meet the requirements of Section 6.4.3.4.3.
Systems shall provide a means to relieve excess outdoor air during air economizer operation to prevent overpressurizing the building. The relief air outlet shall be located so as to avoid recirculation into the building.
Outdoor air, return air, mixed air, and supply air sensors shall be calibrated within the following accuracies:
- Dry-bulb and wet-bulb temperatures shall be accurate to ±2°F over the range of 40°F to 80°F.
- Enthalpy and the value of a differential enthalpy sensor shall be accurate to ±3 Btu/lb over the range of 20 to 36 Btu/lb.
- Relative humidity shall be accurate to ±5% over the range of 20% to 80% rh.