ASHRAE-90.1-2022- - Page 095
Extracted Content
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. c. 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.
6.5.3.2.2 VAV Static Pressure Sensor Location. 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.
6.5.3.2.3 VAV Set-Point Reset. For multiple-zone VAV systems having a total fan system motor name- plate 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:
a. Monitor zone damper positions or other indicator of need for static pressure. b. Automatically detect those zones that may be excessively driving the reset logic and generate an alarm to
the system operator. c. Readily allow operator removal of zones from the reset algorithm.
Informative Note: ASHRAE Guideline 36 includes detailed sequences of control for resetting static pressure set point based on VAV system zone damper position and other indicators of zone demand, including automatically detecting and alarming zones that are excessively driving reset logic and readily excluding those zones from reset logic.
6.5.3.2.4 Return and Relief Fan Control. Return and relief fans used to meet Section 6.5.1.1.5 shall comply with all of the following:
a. Relief air rate shall be controlled to maintain building pressure either directly, or indirectly through dif ferential 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. b. 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.
Informative Note: ASHRAE Guideline 36 includes detailed sequences of control for controlling build- ing pressure using economizer relief systems such as relief dampers, relief fans, and return fans.
6.5.3.3 Multiple-Zone VAV System Ventilation Optimization Control. Multiple-zone VAV systems with DDC of individual zone boxes reporting to a central control panel shall include means to automatically reduce outdoor air intake flow below design rates in response to changes in system ventilation efficiency as defined by ASHRAE Standard 62.1, Normative Appendix A.
Exceptions to 6.5.3.3:
- VAV systems with zonal transfer fans that recirculate air from other zones without directly mixing it with outdoor air, dual-duct dual-fan VAV systems, and VAV systems with fan-powered terminal units.
- Systems where total design exhaust airflow is more than 70% of total design outdoor air intake flow requirements.
Informative Note: ASHRAE Guideline 36 includes detailed sequences of control for dynamically adjusting minimum ventilation rate set points based on ASHRAE Standard 62.1, Normative Appendix A.
ANSI/ASHRAE/IES Standard 90.1-2022 (I-P) 93