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606.5.1 Economizer Systems
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Each cooling system that has a fan shall include either an air economizer complying with Section 606.5.1.1 or a water economizer complying with Section 606.5.1.2.
Exception:
Economizers are not required for the following.
- Individual fan-cooling units with a supply capacity less than the minimum listed in Table 606.5.1(1).
- In Group I-2 occupancies, hospitals, and Group B occupancies, ambulatory care facilities, where more than 75 percent of the air designed to be supplied by the system is to spaces that are required to be humidified above a 35°F (1.7°C) dew-point temperature to comply with applicable codes or accreditation standards. In other occupancies, where more than 25 percent of the air designed to be supplied by the system is to spaces that are designed to be humidified above a 35°F (1.7°C) dew-point temperature to satisfy process needs.
- Systems that include a condenser heat recovery system that is designed to utilize 60 percent of the peak heat rejection load at design conditions and there is a documented need for that rejected heat for either service hot water or space heating during peak heat rejection design conditions.
- Systems that serve spaces estimated as having a sensible cooling load at design conditions, excluding transmission and infiltration loads, of less than or equal to transmission and infiltration losses at the temperature and relative humidity design conditions in accordance with Section 6.1 of ASHRAE 55.
- Where the use of outdoor air for cooling will affect supermarket open refrigerated casework systems.
- Where the cooling efficiency is equal to, or greater than, the efficiency improvement requirements in Table 606.5.1(2).
| CLIMATE ZONES | ECONOMIZER REQUIREMENT |
| 1A, 1B | No requirement |
| 2A, 2B, 3A, 3B, 3C, 4A, 4B, 4C, 5A, 5B, 5C, 6A, 6B, 7, 8 | Economizers on all cooling systems having a capacity ≥ 33,000 Btu/ha |
For SI: 1 British thermal unit per hour = 0.293 W.
- The total capacity of all systems without economizers shall not exceed 480,000 Btu/h per building or 20 percent of the building's air economizer capacity, whichever is greater.
| CLIMATE ZONES | COOLING EQUIPMENT EFFICIENCY IMPROVEMENT (%)a |
| 2A | 17 |
| 2B | 21 |
| 3A | 27 |
| 3B | 32 |
| 4A | 42 |
| 4B | 49 |
IPLV = Integrated part load value, IEER = Integrated energy-efficiency ratio, SEER = Seasonal energy-efficiency rating, EER = Energy-efficiency ratio, COP = Coefficient of performance
- Where a unit is rated with an IPLV, IEER or SEER, the minimum values for these metrics shall be increased by the percentage listed in the table in order to eliminate the required air or water economizer. Where a unit is rated only with a full load metric such as EER or COP cooling, these metrics shall be increased by the percentage shown.
606.5.1.1 Air Economizers
Air economizers shall be designed in accordance with Sections 606.5.1.1.1 through 606.5.1.1.4.
606.5.1.1.1 Design Capacity
Air economizer systems shall be capable of modulating outdoor air and return air dampers to provide up to 100 percent of the design supply air quantity as outdoor air for cooling.
606.5.1.1.2 Control Signal
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.
606.5.1.1.3 High-Limit Shutoff
Air economizers shall be capable of automatically reducing outdoor air intake to the design minimum outdoor air quantity when the outdoor air intake will not reduce cooling energy usage. High-limit shutoff control types for specific climates shall be chosen from Table 606.5.1.1.3(1). High-limit shutoff control settings for the Table 606.5.1.1.3(1) control types shall be as specified in Table 606.5.1.1.3(2).
| CLIMATE ZONES | ALLOWED CONTROL TYPES | PROHIBITED CONTROL TYPES |
| 1B, 2B, 3B, 3C, 4B, 4C, 5B, 5C, 6B, 7, 8 | Fixed dry bulb | Fixed enthalpy |
| Differential dry bulb | ||
| Electronic enthalpya | ||
| Differential enthalpy | ||
| Dew-point and dry-bulb temperatures | ||
| 1A, 2A, 3A, 4A | Fixed enthalpy | Fixed dry bulb Differential dry bulb |
| Electronic enthalpya | ||
| Differential enthalpy | ||
| Dew-point and dry-bulb temperatures | ||
| All other climates zones | Fixed dry bulb | — |
| Differential dry bulb | ||
| Fixed enthalpy | ||
| Electronic enthalpya | ||
| Differential enthalpy | ||
| Dew-point and dry-bulb temperatures |
- Electronic enthalpy controllers are devices that use a combination of humidity and dry-bulb temperature in their switching algorithm.
| DEVICE TYPE | CLIMATE ZONE | REQUIRED HIGH LIMIT (Economizer off when) | |
| Equation | Description of equation | ||
| Fixed dry bulb | 1B, 2B, 3B, 3C, 4B, 4C, 5B, 5C, 6B, 7, 8 | TOA > 75°F | Outdoor air temperature (TOA) is greater than 75°F |
| 5A, 6A, 7A | TOA > 70°F | Outdoor air temperature (TOA) is greater than 70°F | |
| All other zones | TOA > 65°F | Outdoor air temperature (TOA) is greater than 65°F | |
| Differential dry bulb | 1B, 2B, 3B, 3C, 4B, 4C, 5A, 5B, 5C, 6A, 6B, 7, 8 | TOA > TRA | Outdoor air temperature (TOA) is greater than return air temperature (TRA) |
| Fixed enthalpy | All | hOA > 28 Btu/lba | Outdoor air enthalpy (hOA) is greater than 28 Btu/lb of dry aira |
| Electronic enthalpy | All | (TOA/RHOA) > A | Outdoor air temperature (TOA) divided by RHOA is greater than the "A" setpoint curveb |
| Differential enthalpy | All | hOA > hRA | Outdoor air enthalpy (hOA) is greater than return air enthalpy (hRA) |
| Dew-point and dry bulb temperatures | All | DPOA > 55°F or TOA > 75°F | Outside dew point (DPOA) is greater than 55°F or Outdoor air dry bulb (TOA) is greater than 75°F |
For SI: °C = [(°F) - 32]/1.8, 1 foot = 304.8 mm, 1 British thermal unit per pound = 2326 J/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 6000 feet elevation the fixed enthalpy limit is approximately 30.7 Btu/lb.
- Setpoint "A" corresponds to a curve on the psychometric chart that goes through a point at approximately 75°F and 40-percent relative humidity and is nearly parallel to dry-bulb lines at low humidity levels and nearly parallel to enthalpy lines at high-humidity levels.
606.5.1.1.4 Relief of Excess Outdoor Air
Systems shall provide a means to relieve excess outdoor air during air economizer operation to prevent over-pressurizing of the building. The relief air outlets shall be located to avoid recirculation of the relief of air into the building.
606.5.1.2 Water Economizer Systems for HVAC Equipment
Economizer systems for heating, ventilating and air-conditioning (HVAC) equipment shall be designed in accordance with Sections 606.5.1.2.1 through 606.5.1.2.4.
606.5.1.2.1 Design Capacity
Water economizer systems shall be capable of cooling supply air by indirect evaporation and providing up to 100 percent of the expected system cooling load at outdoor air temperatures of 50°F (10°C) dry bulb/45°F (7.2°C) wet bulb and below.
Exception: Systems in which a water economizer is used and where dehumidification requirements cannot be met using outdoor air temperatures of 50°F (10°C) dry bulb/45°F (7.2°C) wet bulb, shall satisfy 100 percent of the expected system cooling load at 45°F (7.2°C) dry bulb/40°F (4.4°C) wet bulb.
606.5.1.2.2 Maximum Pressure Drop
Precooling coils and water-to-water heat exchangers used as part of a water economizer system shall have a water-side pressure drop of less than 15 feet of water column (44 835 Pa) including the control valve or a secondary loop shall be created so that the coil or heat exchanger pressure drop is not seen by the circulating pumps when the system is in the normal cooling noneconomizer mode.
606.5.1.2.3 Integrated Economizer Control
Economizer systems shall be integrated with the mechanical cooling system and shall be capable of providing partial cooling whether or not additional mechanical cooling is required to meet the remainder of the cooling load.
606.5.1.2.4 Economizer Heating System Impact
Heating, ventilating and air-conditioning (HVAC) system design and economizer controls shall be so that economizer operation does not increase the building heating energy use during normal operation.
Exception: Economizers on variable air volume (VAV) systems that cause zone level heating to increase because of reduction in supply air temperature.
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