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C403.5 Economizers (Prescriptive)
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An air or water economizer shall be provided for the following cooling systems:
- Chilled water systems with a total cooling capacity, less cooling capacity provided with air economizers, as specified in Table C403.5(1).
- Individual fan systems with cooling capacity greater than or equal to 54,000 Btu/h (15.8 kW) in buildings having other than a Group R occupancy.The total supply capacity of all fan cooling units not provided with economizers shall not exceed 20 percent of the total supply capacity of all fan cooling units in the building or 300,000 Btu/h (88 kW), whichever is greater.
- Individual fan systems with cooling capacity greater than or equal to 270,000 Btu/h (79.1 kW) in buildings having a Group R occupancy.The total supply capacity of all fan cooling units not provided with economizers shall not exceed 20 percent of the total supply capacity of all fan cooling units in the building or 1,500,000 Btu/h (440 kW), whichever is greater.
Exceptions: Economizers are not required for the following systems.
- Individual fan systems not served by chilled water for buildings located in Climate Zones 1A and 1B.
- 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 35°F (1.7°C) dew-point temperature to satisfy process needs.
- Systems expected to operate less than 20 hours per week.
- Systems serving supermarket areas with open refrigerated casework.
- Where the cooling efficiency is greater than or equal to the efficiency requirements in Table C403.5(2).
- Systems that include a heat recovery system in accordance with Section C403.9.5.
MINIMUM CHILLED-WATER SYSTEM COOLING CAPACITY FOR DETERMINING ECONOMIZER COOLING REQUIREMENTS
|TOTAL CHILLED-WATER SYSTEM CAPACITY LESS CAPACITY OF COOLING UNITS WITH AIR ECONOMIZERS|
|Local Water-cooled Chilled-water Systems||Air-cooled Chilled-water Systems or District Chilled-Water Systems|
|1A||Economizer not required||Economizer not required|
|1B, 2A, 2B||960,000 Btu/h||1,250,000 Btu/h|
|3A, 3B, 3C, 4A, 4B, 4C||720,000 Btu/h||940,000 Btu/h|
|5A, 5B, 5C, 6A, 6B, 7, 8||1,320,000 Btu/h||1,720,000 Btu/h|
For SI: 1 British thermal unit per hour = 0.2931 W.
C403.5.1 Integrated Economizer Control
Economizer systems shall be integrated with the mechanical cooling system and be configured to provide partial cooling even where additional mechanical cooling is required to provide the remainder of the cooling load. Controls shall not be capable of creating a false load in the mechanical cooling systems by limiting or disabling the economizer or any other means, such as hot gas bypass, except at the lowest stage of mechanical cooling.
Units that include an air economizer shall comply with the following:
- Unit controls shall have the mechanical cooling capacity control interlocked with the air economizer controls such that the outdoor air damper is at the 100-percent open position when mechanical cooling is on and the outdoor air damper does not begin to close to prevent coil freezing due to minimum compressor run time until the leaving air temperature is less than 45°F (7°C).
- Direct expansion (DX) units that control 75,000 Btu/h (22 kW) or greater of rated capacity of the capacity of the mechanical cooling directly based on occupied space temperature shall have not fewer than two stages of mechanical cooling capacity.
- Other DX units, including those that control space temperature by modulating the airflow to the space, shall be in accordance with Table C403.5.1.
DX COOLING STAGE REQUIREMENTS FOR MODULATING AIRFLOW UNITS
|RATING CAPACITY||MINIMUM NUMBER|
OF MECHANICAL COOLING STAGES
|≥ 65,000 Btu/h and < 240,000 Btu/h||3 stages||≤ 35% of full load|
|≥ 240,000 Btu/h||4 stages||≤ 25% full load|
For SI: 1 British thermal unit per hour = 0.2931 W.
- For mechanical cooling stage control that does not use variable compressor displacement, the percent displacement shall be equivalent to the mechanical cooling capacity reduction evaluated at the full load rating conditions for the compressor.
C403.5.2 Economizer Heating System Impact
HVAC system design and economizer controls shall be such that economizer operation does not increase building heating energy use during normal operation.
Exception: Economizers on variable air volume (VAV) systems that cause zone level heating to increase because of a reduction in supply air temperature.
C403.5.3 Air Economizers
C403.5.3.1 Design Capacity
C403.5.3.2 Control Signal
Economizer controls and dampers shall be configured to sequence the dampers 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).
C403.5.3.3 High-Limit Shutoff
Air economizers shall be configured to automatically reduce outdoor air intake to the design minimum outdoor air quantity when outdoor air intake will not reduce cooling energy usage. High-limit shutoff control types for specific climates shall be chosen from Table C403.5.3.3. High-limit shutoff control settings for these control types shall be those specified in Table C403.5.3.3.
HIGH-LIMIT SHUTOFF CONTROL SETTING FOR AIR ECONOMIZERSb
|DEVICE TYPE||CLIMATE ZONE||REQUIRED HIGH LIMIT |
(ECONOMIZER OFF WHEN):
|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|
|All||hOA > 28 Btu/lba or |
TOA > 75°F
|Outdoor air enthalpy exceeds|
28 Btu/lb of dry aira orOutdoor 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 orOutdoor 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.
C403.5.3.4 Relief of Excess Outdoor Air
Systems shall be capable of relieving excess outdoor air during air economizer operation to prevent overpressurizing the building. The relief air outlet shall be located to avoid recirculation into the building.
C403.5.3.5 Economizer Dampers
C403.5.4 Water-Side Economizers
C403.5.4.1 Design Capacity
Water economizer systems shall be configured to cool supply air by indirect evaporation and providing up to 100 percent of the expected system cooling load at outdoor air temperatures of not greater than 50°F (10°C) dry bulb/45°F (7°C) wet bulb.
- Systems primarily serving computer rooms in which 100 percent of the expected system cooling load at 40°F (4°C) dry bulb/35°F (1.7°C) wet bulb is met with evaporative water economizers.
- Systems primarily serving computer rooms with dry cooler water economizers that satisfy 100 percent of the expected system cooling load at 35°F (1.7°C) dry bulb.
- Systems where dehumidification requirements cannot be met using outdoor air temperatures of 50°F (10°C) dry bulb/45°F (7°C) wet bulb and where 100 percent of the expected system cooling load at 45°F (7°C) dry bulb/40°F (4°C) wet bulb is met with evaporative water economizers.
C403.5.4.2 Maximum Pressure Drop
Precooling coils and water-to-water heat exchangers used as part of a water economizer system shall either have a water-side pressure drop of less than 15 feet (45 kPa) of water 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.
C403.5.5 Economizer Fault Detection and Diagnostics (Mandatory)
Air-cooled unitary direct-expansion units listed in Tables C403.3.2(1) through C403.3.2(3) and variable refrigerant flow (VRF) units that are equipped with an economizer in accordance with Sections C403.5 through C403.5.4 shall include a fault detection and diagnostics system complying with the following:
- The following temperature sensors shall be permanently installed to monitor system operation:
- 1.1. Outside air.
- 1.2. Supply air.
- 1.3. Return air.
- Temperature sensors shall have an accuracy of ±2°F (1.1°C) over the range of 40°F to 80°F (4°C to 26.7°C).
- Refrigerant pressure sensors, where used, shall have an accuracy of ±3 percent of full scale.
- The unit controller shall be configured to provide system status by indicating the following:
- 4.1. Free cooling available.
- 4.2. Economizer enabled.
- 4.3. Compressor enabled.
- 4.4. Heating enabled.
- 4.5. Mixed air low limit cycle active.
- 4.6. The current value of each sensor.
- The unit controller shall be capable of manually initiating each operating mode so that the operation of compressors, economizers, fans and the heating system can be independently tested and verified.
- The unit shall be configured to report faults to a fault management application available for access by day-to-day operating or service personnel, or annunciated locally on zone thermostats.
- The fault detection and diagnostics system shall be configured to detect the following faults:
- 7.1. Air temperature sensor failure/fault.
- 7.2. Not economizing when the unit should be economizing.
- 7.3. Economizing when the unit should not be economizing.
- 7.4. Damper not modulating.
- 7.5. Excess outdoor air.
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