ASHRAE-90.1-2022- - Page 091
Extracted Content
Exceptions to 6.5.2.1:
- Zones for which the volume of air that is reheated, recooled, or mixed is less than the larger of the following: a. For systems without DDC, 30% of the zone design peak supply . b. For systems with DDC, the minimum primary airflow rate required to meet the Simplified
Procedure ventilation requirements of ASHRAE Standard 62.1 for the zone, permitted to be the average airflow rate as allowed by ASHRAE Standard 62.1. c. Any higher rate that can be demonstrated to the satisfaction of the authority having jurisdiction
to reduce overall system annual energy use by offsetting reheat / recool energy losses through a reduction in outdoor air intake for the system. d. The airflow rate required to comply with applicable codes or accreditation standards, such as
pressure relationships or minimum air change rates. 2. Zones with DDC that comply with all of the following: a. The airflow rate in dead band between heating and cooling does not exceed the larger of the
following: i. The minimum primary airflow rate required to meet the Simplified Procedure ventilation requirements of ASHRAE Standard 62.1 for the zone, permitted to be the average airflow rate as allowed by ASHRAE Standard 62.1. ii. Any higher rate that can be demonstrated, to the satisfaction of the authority having juris-
diction, to reduce overall system annual energy use by offsetting reheat / recool energy losses through a reduction in outdoor air intake. iii. The airflow rate required to comply with applicable codes or accreditation standards, such
as pressure relationships or minimum air change rates. b. The airflow rate that is reheated, recooled, or mixed shall be less than 50% of the zone design
peak supply rate. c. The first stage of heating consists of modulating the zone supply air temperature set point up to
a maximum set point while the airflow is maintained at the dead band flow rate. d. The second stage of heating consists of modulating the airflow rate from the dead band flow rate
up to the heating maximum flow rate. 3. Laboratory exhaust systems that comply with Section 6.5.7.3. 4. Zones where at least 75% of the energy for reheating or for providing warm air in mixing systems is provided from site-recovered energy (including condenser heat) or on-site renewable energy .
Informative Note: ASHRAE Guideline 36 includes detailed sequences of control for minimizing simultaneous heating and cooling for most types of VAV system zone air terminal units including the supply air temperature reheat limit.
6.5.2.1.1 Supply Air Temperature Reheat Limit. Where reheating is permitted by other parts of this standard, zones that have both supply and return/exhaust air openings greater than 6 ft above the floor shall not supply heating air more than 20°F above the space temperature set point .
Exceptions to 6.5.2.1.1:
- Laboratory exhaust systems that comply with Section 6.5.7.3.
- During preoccupancy building warm-up and setback .
6.5.2.2 Hydronic System Controls. The heating of fluids in hydronic systems that have been previously mechanically cooled, and the cooling of fluids that have been previously mechanically heated, shall be limited in accordance with Sections 6.5.2.2.1 through 6.5.2.2.3.
6.5.2.2.1 Three-Pipe System. Hydronic systems that use a common return system for both hot water and chilled water shall not be used.
6.5.2.2.2 Two-Pipe Changeover System. Systems that use a common distribution system to supply both heated and chilled water are acceptable, provided all of the following are met:
a. The system is designed to allow a dead band between changeover from one mode to the other of at least
15°F outdoor air temperature. b. The system is designed to operate, and is provided with controls that will allow operation, in one mode
for at least four hours before changing over to the other mode. c. Reset controls are provided that allow heating and cooling supply temperatures at the changeover point
to be no more than 30°F apart.
ANSI/ASHRAE/IES Standard 90.1-2022 (I-P) 89