6.5 Prescriptive Path
|Climate Zone||Cooling Capacity for which an Economizer Is Required|
|0A, 0B, 1A, 1B||No economizer requirement|
|2A, 2B, 3A, 4A, 5A, 6A, 3B, 3C, 4B, 4C, 5B, 5C, 6B, 7, 8||≥54,000 Btu/h|
|Climate Zone||Efficiency Improvementa|
a. If a unit is rated with an IPLV, IEER, or SEER, then to eliminate the required economizer, the minimum cooling efficiency of the HVAC unit must be increased by the percentage shown. If the HVAC unit is only rated with a full-load metric like EER cooling then these must be increased by the percentage shown.
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 184.108.40.206.3.
|Control Type||Allowed Only in Climate Zone at Listed Set Point||Required High-Limit Set Points (Economizer Off when):|
|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.
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.
|Climate Zone||Water Cooled||Air Cooled|
|Dry Bulb, °F||Wet Bulb, °F||Dry Bulb, °F|
Precooling coils and fluid-to-water heat exchangers used as part of a fluid economizer system shall either have a water-side pressure drop of less than 15 ft 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.
Economizer systems shall be integrated with the mechanical cooling system and be capable of and configured to provide partial cooling even when additional mechanical cooling is required to meet the remainder of the cooling load. Controls shall not false load the mechanical cooling systems by limiting or disabling the economizer or by 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% 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.
- DX units with a rated capacity no less than 65,000 Btu/h that control the capacity of the mechanical cooling directly based on occupied space temperature shall have a minimum of two stages of mechanical cooling capacity.
- All other DX units, including those that control space temperature by modulating the airflow to the space, shall comply with the requirements of Table 220.127.116.11.
|Rating Capacity, Btu/h||Minimum Number of Mechanical Cooling Stages||Minimum Compressor Displacementa|
|≥65,000 and <240,000||3||≤35% of full load|
|≥240,000||4||≤25% full load|
a. 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.