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6.5.3.2 Fan Control
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Each cooling system listed in Table 6.5.3.2.1 shall be designed to vary the supply fan airflow as a function of load and shall comply with the following requirements:
- 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% of full speed. At low or minimum speed, the fan system shall draw no more than 40% of the fan power at full fan speed. Low or minimum speed shall be used during periods of low cooling load and ventilation-only operation.
- All 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% of full speed. At minimum speed, the fan system shall draw no more than 30% of the power at full fan speed. Low or minimum speed shall be used during periods of low cooling load and ventilation-only operation.
- Units that include an air economizer to meet the requirements of Section 6.5.1 shall have a minimum of two speeds of fan control during economizer operation.
Exceptions to 6.5.3.2.1
- Modulating fan control is not required for chilled-water and evaporative cooling units with <1 hp fan motors if the units are not used to provide ventilation air and if the indoor fan cycles with the load.
- If the volume of outdoor air required to meet the ventilation requirements of Standard 62.1 at low speed exceeds the air that would be delivered at the speed defined in Section 6.5.3.2.1(a) or 6.5.3.2.1(b) then the minimum speed shall be selected to provide the required ventilation air.
Table 6.5.3.2.1 Fan Airflow Control
| 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 |
Static pressure sensors used to control VAV fans shall be located such that the controller set point is no greater than 1.2 in. of water. If this results in the sensor being located downstream of major duct splits, sensors shall be installed in each major branch to ensure that static pressure can be maintained in each.
Exception to 6.5.3.2.2
Systems complying with Section 6.5.3.2.3.
For multiple-zone VAV systems having a total fan system motor nameplate horsepower exceeding 5 hp with DDC of individual zones reporting to the central control panel, static pressure set point shall be reset based on the zone requiring the most pressure; i.e., the set point is reset lower until one zone damper is nearly wide open. Controls shall provide the following:
Return and relief fans used to meet Section 6.5.1.1.5 shall comply with all of the following:
- Relief air rate shall be controlled to maintain building pressure either directly, or indirectly through differential supply-return airflow tracking. Systems with constant speed or multispeed supply fans shall also be allowed to control the relief system based on outdoor air damper position.
- Fans shall have variable-speed control or other devices that will result in total return/relief fan system demand of no more than 30% of total design power at 50% of total design fan flow.
Exceptions to 6.5.3.2.4
- Return or relief fans with total motor size less than or equal to 0.5 hp.
- Staged relief fans with a minimum of four stages.
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