E 503.5.6.2 Fan Airflow Control
- DX and chilled-water cooling units that control the capacity of the mechanical cooling directly based on space temperature shall have a minimum of two stages of fan control. Low or minimum speed shall not exceed 66 percent of full speed. At low or minimum speed, the fan system shall draw not more than 40 percent of the fan power at fall fan speed. Low or minimum speed shall be used during periods of low cooling load and ventilation-only operation.
- Other units, including DX cooling units and chilled water units that control the space temperature by modulating the airflow to the space, shall have modulating fan control. Minimum speed shall not exceed 50 percent of fall speed. At minimum speed, the fan system shall draw not more than 30 percent of the power at fall fan speed. Low or minimum speed shall be used during periods of low cooling load and ventilation-only operation.
- Units that include an air-side economizer to comply with Section E 503.5 through Section E 503.5.4.1 shall have not less than of two speeds of fan control during economizer operation.
- Modulating fan control shall not be required for chilled-water and evaporative cooling units with less than l hp (0.7 kW) fan motors where the units are not used to provide ventilation air and the indoor fan cycles with the load.
- Where the volume of outdoor air required to comply with the ventilation requirements of Chapter 4 or ASHRAE 62.1 at low speed exceeds the air that would be delivered at the speed defined in Section E 503.5.6.2(1), or Section E 503.5.6.2(2), then the minimum speed shall be selected to provide the required ventilation air. [ASHRAE 90.1:184.108.40.206.1]
TABLE E 503.5.6.2 FAN AIRFLOW CONTROL
[ASHRAE 90.1:TABLE 220.127.116.11.1]
|COOLING SYSTEM TYPE||FAN MOTOR SIZE, (hp)||MECHANICAL COOLING CAPACITY, (Btu/h)|
|DX cooling||Any||≥65 000|
|Chilled-water and evaporative cooling||≥1/4||Any|
For SI units: 1000 British thermal units per hour = 0.293 kW, 1 horsepower = 0.746 kW, 1 cubic foot per minute = 0.00047 m3/s
Exception: Systems that are in accordance with Section E 503.5.6.2.2. [ASHRAE 90.1:18.104.22.168.2]
- Monitor zone damper positions or other indicator of need for static pressure.
- Automatically detect those zones that are capable of excessively driving the reset logic and generate an alarm to the system operator.
- Readily allow operator removal of zones from the reset algorithm. [ASHRAE 90.1:22.214.171.124.3]