ASHRAE-90.1-2022- - Page 079
Extracted Content
6.4.3.3 Off-Hour Controls. HVAC systems shall have the off-hour controls required by Sections 6.4.3.3.1 through 6.4.3.3.5. Exceptions to 6.4.3.3:
- HVAC systems intended to operate continuously.
- HVAC systems not serving residential spaces and having a design heating capacity and cooling capacity less than 7000 Btu/h that are equipped with readily accessible manual on/off controls .
6.4.3.3.1 Automatic Shutdown. HVAC systems shall be equipped with at least one of the following:
a. Controls that can start and stop the system under different time schedules for seven different day types
per week, are capable of retaining programming and time setting during loss of power for a period of at least ten hours, and include an accessible manual override or equivalent function that allows temporary operation of the system for up to two hours. b. An occupancy sensor that is capable of shutting the system off when no occupant is sensed for a period
of up to 30 minutes. c. A manually operated timer capable of being adjusted to operate the system for up to two hours. d. An interlock to a security system that shuts the system off when the security system is activated.
Exceptions to 6.4.3.3.1:
- Systems serving residential occupancies with controls that can start and stop the system under at least two different time schedules per week.
- Systems serving non- residential occupancies where heating or cooling capacity is less than 15,000 Btu/h, with controls that can start and stop the system under at least two different time schedules per week.
6.4.3.3.2 Setback Controls. Heating systems shall be equipped with controls capable of and configured to automatically restart and temporarily operate the system as required to maintain zone temperatures above an adjustable heating set point at least 10°F below the occupied heating set point . Cooling systems shall be equipped with controls capable of and configured to automatically restart and temporarily operate the mechanical cooling system at the lowest practical fan speed as required to maintain zone temperatures below an adjustable cooling set point at least 5°F above the occupied cooling set point or to prevent maximum space humidity levels as required by Standard 62.1.
Exception to 6.4.3.3.2: Radiant heating systems capable of and configured with a setback heating set
point at least 4°F below the occupied heating set point .
6.4.3.3.3 Optimum Start Controls. Individual heating and cooling systems with setback controls and DDC shall have optimum start controls . The control algorithm shall, as a minimum, be a function of the difference between space temperature and occupied set point, the outdoor temperature, and the amount of time prior to scheduled occupancy. Mass radiant floor slab systems shall incorporate floor temperature into the optimum start algorithm.
Exception to 6.4.3.3.3 : Residential spaces are not required to have optimum start controls .
6.4.3.3.4 Zone Isolation. HVAC systems serving zones that are intended to operate or be occupied nonsimultaneously shall be divided into isolation areas. Zones may be grouped into a single isolation area provided it does not exceed 25,000 ft [2] of conditioned floor area nor include more than one story . Each isolation area shall be equipped with isolation devices capable of and configured to automatically shut off the supply of conditioned air and outdoor air to and exhaust air from the area. Each isolation area shall be controlled independently by a device meeting the requirements of Sections 6.4.3.3.1. For central systems and plants, controls and devices shall be provided to allow stable system and equipment operation for any length of time while serving only the smallest isolation area served by the system or plant.
Exceptions to 6.4.3.3.4: Isolation devices and controls are not required for
- exhaust air and outdoor air connections to isolation zones when the fan system to which they connect is 5000 cfm and smaller;
- exhaust airflow from a single isolation zone of less than 10% of the design airflow of the exhaust system to which it connects; or
- zones intended to operate continuously or intended to be inoperative only when all other zones are inoperative.
Informative Note: ASHRAE Guideline 36 includes detailed sequences of control for zone isolation using logical groups of zone air terminal units serving each isolation area.
ANSI/ASHRAE/IES Standard 90.1-2022 (I-P) 77