6.5.1.3 Integrated Economizer Control
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 6.5.1.3.
Table 6.5.1.3 DX Cooling Stage Requirements for Modulating Airflow Units
| 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.