CRC · California Residential Code
Zone thermostatic controls, off-hour controls, setback and isolation
Each conditioned zone must have its own thermostat; if one thermostat handles both heat and cool it must provide a 5°F dead band. Off‑hour controls (schedules, occupancy sensors, timers, or interlocks) plus setback rules (heating setback 10°F; cooling 5°F; radiant exception 4°F) are required, and large/multi‑floor systems must be divided into isolation areas (≤25,000 ft² and one floor) with devices to shut off supply/outdoor/exhaust air. See § E 503.4.6, § E 503.4.6.3, and § E 503.4.6.3.4 for the full requirements.
Last reviewed: July 6, 2026
What the code requires — in plain English
Each zone must have its own thermostatic control and, when a thermostat controls both heating and cooling, there must be a dead band of at least 5°F (3°C) so heating and cooling aren’t fighting each other. Off‑hour control features (automatic shutdown, setback/restart, or equivalent) are required for applicable systems, and systems that serve zones that do not operate simultaneously must be divided into isolation areas with devices to shut off supply/outdoor/exhaust air to each area. The controlling provisions are § E 503.4.6, § E 503.4.6.3, and § E 503.4.6.3.4.
Most important rule: every zone must be individually thermostatically controlled and, where one thermostat does both heating and cooling, it must provide a dead band ≥ 5°F (3°C) so heating and cooling do not operate simultaneously.
Requirements in detail
Key defined terms (first use bolded)
- Thermostatic controls — the zone-level device that regulates supply of heating and/or cooling in a zone. § E 503.4.6 requires one per zone (a dwelling unit may be treated as one zone).
- Dead band — a temperature range in which both heating and cooling are shut off or reduced to a minimum; must be not less than 5°F (3°C) when one thermostat controls both modes. § E 503.4.6.1.
- Setback controls — off‑hour control that allows the system to restart only as needed to maintain a setback setpoint (different magnitudes for heating vs cooling). § E 503.4.6.3.2.
- Isolation area / isolation device — grouping of zones intended to operate nonsimultaneously; area is served by devices (valves/dampers) that can shut off conditioned/outdoor/exhaust air to that group. § E 503.4.6.3.4.
Decision-relevant table
| Decision dimension | Required value / action | Code Reference |
|---|---|---|
| Individual zone thermostatic control | One thermostatic control per zone (dwelling unit may be one zone) | § E 503.4.6 |
| Dead band (heating + cooling on same thermostat) | ≥ 5°F (3°C) | § E 503.4.6.1 |
| Prevent setpoint overlap (separate heat/cool thermostats) | Interlocks / mechanical stops / DDC software to prevent heating setpoint > cooling setpoint minus proportional band | § E 503.4.6.2 |
| Off-hour automatic shutdown options (pick ≥1) | 7-day programmable schedules (retain program ≥10 hr power loss + manual override ≤2 hr), OR occupancy sensor (shut off after up to 30 min), OR manual timer (≤2 hr), OR security interlock | § E 503.4.6.3.1 |
| Setback magnitudes — heating | Restart to maintain heating setpoint no less than 10°F (6°C) below occupied heating setpoint | § E 503.4.6.3.2 |
| Setback magnitudes — cooling | Restart to maintain cooling setpoint no less than 5°F (3°C) above occupied cooling setpoint (or to prevent high humidity) | § E 503.4.6.3.2 |
| Radiant heating exception | Radiant heating may use 4°F (2°C) heating setback instead of 10°F | § E 503.4.6.3.2 (Exception) |
| Isolation area maximum conditioned floor area | 25,000 ft² (2,322.6 m²) for a single isolation area; isolation area shall not include more than one floor | § E 503.4.6.3.4 |
| Isolation device capabilities | Devices must be able to automatically shut off supply of conditioned air, outdoor air, and exhaust air to the isolation area; each area controlled independently by a device that meets § E 503.4.6.3.1 requirements | § E 503.4.6.3.4 |
| Exceptions to isolation devices | Not required where connected fan system ≤ 5,000 ft³/min; not required when a zone’s exhaust < 10% of exhaust system design airflow; or for zones intended to operate continuously | § E 503.4.6.3.4 (Exceptions) |
| Optimum start (where required) | Systems with setback + DDC must include optimum start algorithm (residential spaces excepted) | § E 503.4.6.3.3 |
Implementation notes
- If heating and cooling are separate pieces of equipment in the same zone (two thermostats), you must provide interlocks or physical stops so heating cannot be commanded above the cooling setpoint (see § E 503.4.6.2) to avoid simultaneous heat/cool operation.
- Off‑hour control options are alternatives — meeting any one of the listed methods in § E 503.4.6.3.1 complies; the 7‑day programmable option has specific backup/override requirements.
Exceptions & special cases
- HVAC systems intended to operate continuously are not required to have off‑hour shutdown/setback controls. § E 503.4.6.3 (Exceptions).
- Small nonresidential systems with design heating and cooling capacity < 7,000 Btu/h (2.1 kW) that have a readily accessible manual ON/OFF are exempt from some off‑hour requirements. § E 503.4.6.3 (Exceptions).
- Guest rooms/hotel controls have their own special automatic reset rules (unoccupied setpoints and timing) in § E 503.4.6.3.5.
- Radiant heating systems are permitted a smaller heating setback (4°F / 2°C) where configured accordingly. § E 503.4.6.3.2 (Exception).
- Isolation devices are not required for exhaust/outdoor-air connections where the connected fan system is ≤ 5,000 ft³/min or where the single zone exhaust is less than 10% of the system design airflow. § E 503.4.6.3.4 (Exceptions).
Common mistakes
- Installing separate heat and cool thermostats without implementing interlocks or software limits (violates § E 503.4.6.2).
- Using thermostats that force manual changeover (heat/cool) without documenting the exception or obtaining AHJ approval for special occupancies (dead band exception). § E 503.4.6.1.
- Grouping zones into an isolation area that exceeds 25,000 ft² or spans multiple floors — that defeats the isolation-area rule. § E 503.4.6.3.4.
- Choosing an off‑hour strategy but misconfiguring the manual override time or failing to retain schedule programming through expected power interruptions (see the 10‑hour retention / 2‑hour override requirement for the schedule option). § E 503.4.6.3.1.
- Forgetting to provide automatic shutoff of outdoor air and exhaust to the isolation area as required (isolation devices must affect supply, outdoor, and exhaust flows). § E 503.4.6.3.4.
Worked example — concrete scenario
Project: a 3-floor commercial building with conditioned floor area per floor = 20,000 ft², central air system serving multiple zones on each floor.
Zone thermostats: Provide a thermostat in each zone (dwelling-unit rule not applicable here). Each thermostat that controls both heating and cooling must be set up with a dead band of 5°F so heat and cool will not operate simultaneously. § E 503.4.6.1.
Isolation areas: Because each floor is 20,000 ft² (less than 25,000 ft²) and isolation areas may not include more than one floor, you may treat each floor as a separate isolation area. Each floor must have isolation devices (valves/dampers) that can automatically shut off supply conditioned air, outdoor air, and exhaust air to that floor and each isolation area must be controlled independently by a device that meets the automatic‑shutdown requirements of § E 503.4.6.3.1. § E 503.4.6.3.4.
Off‑hour controls: Choose the 7‑day programmable schedule option. The controller must be capable of:
- different schedules for seven day‑types,
- retaining programming/time setting through a power loss of not less than 10 hours, and
- provide an accessible manual override that allows temporary operation up to 2 hours. § E 503.4.6.3.1 (1).
Setback values (example numeric setpoints):
- Occupied heating setpoint = 68°F → heating setback is no less than 58°F (68 − 10°F) per § E 503.4.6.3.2.
- Occupied cooling setpoint = 75°F → cooling setup allowed to 80°F (75 + 5°F) per § E 503.4.6.3.2.
- If one area uses radiant floor heating, its allowed heating setback could be 64°F (68 − 4°F) when the radiant exception is applied. § E 503.4.6.3.2 (Exception).
Prevent setpoint overlap: If a zone uses separate heating and cooling thermostats, interlock them in hardware or DDC logic so the heating setpoint cannot be commanded higher than the cooling setpoint (or higher than cooling minus proportional band) per § E 503.4.6.2.
Related provisions
- § E 503.4.6.1 Dead Band.
- § E 503.4.6.2 Setpoint Overlap Restriction.
- § E 503.4.6.3.1 Automatic Shutdown (off‑hour options and device requirements).
- § E 503.4.6.3.2 Setback Controls (heating and cooling setback magnitudes).
- § E 503.4.6.3.3 Optimum Start Controls (when DDC + setback are used).
- § E 503.4.6.3.4 Zone Isolation (area limits, device functions, exceptions).
- § E 503.4.6.3.5 Guest room HVAC automatic control specifics.
Code references
Grounded in the retrieved California Residential Code — click a citation to read the verbatim passage:
CRC § 90.1 High relevance — show source text
trols capable of and configured to automatically restart and temporarily operate the mechanical cooling system as required to maintain zone temperatures below an adjustable cooling setpoint of not less than 5°F (3°C) above the occupied cooling setpoint or to prevent high space humidity levels.
Exception: Radiant heating systems capable of and configured with a setback heating setpoint at not less than 4°F (2°C) below the occupied heating setpoint. [ASHRAE 90.1:6.4.3.3.2] E 503.4.6.3.3 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: Residential spaces are not required to have optimum start controls. [ASHRAE 90.1:6.4.3.3.3] E 503.4.6.3.4 Zone Isolation. HVAC systems serving zones that are intended to operate or be occupied nonsimultaneously shall be divided into isolation areas. Zones shall be permitted to be grouped into a single isolation area provided it does not exceed 25 000 square feet (2322.6 m [2] ) of conditioned floor area and does not include more than one floor. 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 Section E 503.4.6.3.1. 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: Isolation devices and controls are not required for the following:
(1) Exhaust air and outdoor air connections to isolation zones where the fan system to which they connect is not more than 5000 ft [3] /min (2.3597 m [3] /s).
(2) Exhaust airflow from a single isolation zone of less than 10 percent of the design airflow of the exhaust system to which it
connects.
(3) Zones intended to operate continuously or intended to be inoperative only when all other zones are inoperative. [ASHRAE 90.1:6.4.3.3.4]
2025 CALIFORNIA MECHANICAL CODE 423
), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.
APPENDIX E
E 503.4.6.3.5 Automatic Control of HVAC in Hotel/Motel Guest Rooms. Hotels and motels with more than 50 guest rooms shall be provided with automatic controls for the HVAC equipment serving each guest room capable of and configured according to the requirements in Section E 503.4.6.3.5.1. [ASHRAE 90.1:6.4.3.3.5]
CRC § 2.1 High relevance — show source text
Exceptions:
(1) HVAC systems intended to operate continuously.
(2) HVAC systems not serving residential spaces and having a design heating capacity and cooling capacity less than 7000 Btu/h (2.1 kW) that are equipped with a readily accessible manual ON/OFF controls. [ASHRAE 90.1:6.4.3.3]
E 503.4.6.3.1 Automatic Shutdown. HVAC systems shall be equipped with not less than one of the following:
(1) 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 not less than 10 hours, and include an accessible manual override, or equivalent function, that allows temporary operation of the system for up to 2 hours.
(2) An occupancy sensor that is capable of shutting the system off when no occupant is sensed for a period of up to 30 minutes.
(3) A manually operated timer capable of being adjusted to operate the system for up to 2 hours.
(4) An interlock to a security system that shuts the system off when the security system is activated.
Exceptions:
(1) Systems serving residential occupancies with controls that can start and stop the system under at least two different time schedules per week.
(2) Systems serving non-residential occupancies where heating or cooling capacity is less than 15 000 Btu/h (4.4 kW) with controls that can start and stop the system under not less than 2 different time schedules per week. [ASHRAE 90.1:6.4.3.3.1] E 503.4.6.3.2 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 setpoint of not less than 10°F (6°C) below the occupied heating setpoint. Cooling systems shall be equipped with con
trols capable of and configured to automatically restart and temporarily operate the mechanical cooling system as required to maintain zone temperatures below an adjustable cooling setpoint of not less than 5°F (3°C) above the occupied cooling setpoint or to prevent high space humidity levels.
Exception: Radiant heating systems capable of and configured with a setback heating setpoint at not less than 4°F (2°C) below the occupied heating setpoint. [ASHRAE 90.1:6.4.3.3.2] E 503.4.6.3.3 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: Residential spaces are not required to have optimum start controls. [ASHRAE 90.1:6.4.3.3.3] E 503.4.6.3.4 Zone Isolation. HVAC systems serving zones that are intended to operate or be occupied nonsimultaneously shall be divided into isolation areas.
CRC § 0.79 High relevance — show source text
(1) the perimeter system includes not less than one thermostatic control zone for each building exposure having walls facing only one orientation for 50 contiguous feet (15 240 mm) or more and
(2) the perimeter system heating and cooling supply is controlled by thermostatic controls located within the zones served by the system.
Exterior walls and semiexterior walls are considered to have different orientations where the exposures they face differ by more than 45 degrees (0.79 rad).
[ASHRAE 90.1:6.4.3.1.1]
E 503.4.6.1 Dead Band. Where used to control both heating and cooling, zone thermostatic controls shall be capable of and configured to provide a temperature range or dead band of not less than 5°F (3°C) within which the supply of heating and cooling energy to the zone is shut off or reduced to a minimum.
Exceptions:
(1) Thermostats that require manual changeover between heating and cooling modes.
(2) Special occupancy or special applications where wide temperature ranges are not acceptable (such as retirement homes, process applications, museums, some areas of hospitals) and are approved by the Authority Having Jurisdiction. [ASHRAE 90.1:6.4.3.1.2]
E 503.4.6.2 Setpoint Overlap Restriction. Where heating and cooling to a zone are controlled by separate zone thermostatic controls located within
422 2025 CALIFORNIA MECHANICAL CODE
), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.
APPENDIX E
the zone, means (such as limit switches, mechanical stops, or, for DDC systems, software programming) shall be provided to prevent the heating setpoint from exceeding the cooling setpoint minus any applicable proportional band. [ASHRAE 90.1:6.4.3.2] E 503.4.6.3 Off-Hour Controls. HVAC systems shall have the off-hour controls required by Section E 503.4.6.3.1 through Section E 503.4.6.3.5.
Exceptions:
(1) HVAC systems intended to operate continuously.
(2) HVAC systems not serving residential spaces and having a design heating capacity and cooling capacity less than 7000 Btu/h (2.1 kW) that are equipped with a readily accessible manual ON/OFF controls. [ASHRAE 90.1:6.4.3.3]
E 503.4.6.3.1 Automatic Shutdown. HVAC systems shall be equipped with not less than one of the following:
(1) 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 not less than 10 hours, and include an accessible manual override, or equivalent function, that allows temporary operation of the system for up to 2 hours.
(2) An occupancy sensor that is capable of shutting the system off when no occupant is sensed for a period of up to 30 minutes.
(3) A manually operated timer capable of being adjusted to operate the system for up to 2 hours.
(4) An interlock to a security system that shuts the system off when the security system is activated.
Exceptions:
CRC § 503.5.6.2. High relevance — show source text
(1) The system serves a single HVAC zone.
(2) The equipment shall comply with the variable flow requirements of Section E 503.5.6.2.
(3) Cooling (where any) shall be provided by a unitary packaged or split-system air conditioner that is either air-cooled or evaporatively cooled, with efficiency that is in accordance with the requirements shown in Table E 503.7.1(1) for air conditioners, Table E 503.7.1(2) for heat pumps, or Table E 503.7.1(4) for packaged terminal and room air conditioners and heat pumps for the applicable equipment category.
(4) The system shall have an air economizer in accordance with Section E 503.5 and Section E 503.4.6.13.
(5) Heating (where any) shall be provided by a unitary packaged or split-system heat pump that is in accordance with the applicable efficiency requirements shown in Table E 503.7.1(2) for heat pumps or Table E 503.7.1(4) for packaged terminal and room air conditioners and heat pumps, a fuel-fired furnace that is in accordance with the applicable efficiency requirements shown in Table E 503.7.1(5) for furnaces, duct furnaces, and unit heaters, an electric resistance heater, or a baseboard system connected to a boiler that is in accordance with the applicable efficiency requirements shown in Table E 503.7.1(6) for boilers.
(6) The system shall comply with the exhaust air energy recovery requirements in accordance with Section E 503.5.10.1.2.
(7) The system shall be controlled by a manual changeover or dual setpoint thermostat.
(8) Where a heat pump equipped with auxiliary internal electric resistance heaters is installed, controls shall be provided that prevent supplemental heater operation where the heating load is capable of being met by the heat pump alone during both steady-state operation and setback recovery. Supplemental heater operation shall be permitted during outdoor coil defrost cycles. The heat pump shall be controlled in accordance with one of the following:
(a) A digital or electronic thermostat designed for heat pump use that energizes auxiliary heat where the heat pump has insufficient capacity
2025 CALIFORNIA MECHANICAL CODE 419
), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.
APPENDIX E
to maintain setpoint or to warm up the space at a sufficient rate.
(b) A multistage space thermostat and an outdoor air thermostat wired to energize auxiliary heat on the last stage of the space thermostat and where outdoor air temperature is less than 40°F (4°C).
Exceptions: Heat pumps that comply with the following:
(1) Have a minimum efficiency regulated by NAECA.
(2) In accordance with the requirements shown in Table E 503.7.1(2).
(3) Include all usage of internal electric resistance heating.
(9) The system controls shall not permit reheat or other form of simultaneous heating and cooling for humidity control.
(10)Systems serving spaces other than residential spaces, that do not require continuous operation with a cooling or heating capacity more than 7000 Btu/h (2.1 kW) shall comply with Section E 503.4.6.3.1 and Section E 503.4.6.3.2.
CRC § 503.1.2 High relevance — show source text
E 503.1.2 Additions to Existing Buildings. Mechanical equipment and systems serving the heating, cooling, ventilating, or refrigeration needs of additions to existing buildings shall be in accordance with the requirements of this section as described in Section E 503.2.
Exception: Where HVACR to an addition is provided by existing HVACR systems and equipment, such existing systems and equipment shall not be required to be in accordance with this appendix. A new system or equipment installed shall be in accordance with specific requirements applicable to those systems and equipment.
[ASHRAE 90.1:6.1.1.2]
E 503.1.3 Alterations to Heating, Ventilating, Air- Conditioning, and Refrigeration in Existing Buildings. New HVACR equipment as a direct replacement of existing HVACR equipment shall comply with the following sections as applicable for the equipment being replaced:
(1) Section E 503.3 “Simplified Approach Option for HVAC Systems”
(2) Section E 503.4 “Equipment Efficiencies, Verification, and Labeling Requirements”
(3) Section E 503.4.6 “Zone Thermostatic Controls”
(4) Section E 503.4.6.2 “Set-point Overlap Restrictions”
(5) Section E 503.4.6.3 “Off-Hour Controls” except for Section E 503.4.6.3.4, “Zone Isolation”
(6) Section E 503.4.6.4 “Ventilation System Controls”
(7) Section E 503.4.6.8 “Freeze Protection and Snow/Ice Melting Systems”
(8) Section E 503.4.6.9 “Ventilation Controls for HighOccupancy Areas” only for single-zone equipment
(9) Section E 503.4.6.11 “Heated or Cooled Vestibules”
(10)Section E 503.4.8 “Walk-In Coolers and Walk-In Freezers”
(11)Section E 503.5.1 “Air Economizers” for units located outdoors
(12)Section E 503.5.3 “Integrated Economizer Control”
(13)Section E 503.5.4 “Economizer Heating System Impact”
(14)Section E 503.5.6.1.2 “Fan Efficiency”
(15)Section E 503.5.6.2 “Supply Fan Airflow Control”
(16)Section E 503.5.6.5 “Fractional Horsepower (Kilowatt) Fan Motors”
(17)Section E 503.5.7 “Boiler Turndown”
(18)Section E 503.5.7.3 “Chiller and Boiler Isolation”
(19)Section E 503.5.8.1 “Fan Speed Control.” [ASHRAE 90.1:6.1.1.3.1]
E 503.1.3.1 New Cooling Systems. New cooling systems installed to serve previously uncooled spaces shall be in accordance with this section as described in Section E 503.2. [ASHRAE 90.1:6.1.1.3.2]
418 2025 CALIFORNIA MECHANICAL CODE
), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.
APPENDIX E
CRC § 503.7.1 High relevance — show source text
(8) Where a heat pump equipped with auxiliary internal electric resistance heaters is installed, controls shall be provided that prevent supplemental heater operation where the heating load is capable of being met by the heat pump alone during both steady-state operation and setback recovery. Supplemental heater operation shall be permitted during outdoor coil defrost cycles. The heat pump shall be controlled in accordance with one of the following:
(a) A digital or electronic thermostat designed for heat pump use that energizes auxiliary heat where the heat pump has insufficient capacity
2025 CALIFORNIA MECHANICAL CODE 419
), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.
APPENDIX E
to maintain setpoint or to warm up the space at a sufficient rate.
(b) A multistage space thermostat and an outdoor air thermostat wired to energize auxiliary heat on the last stage of the space thermostat and where outdoor air temperature is less than 40°F (4°C).
Exceptions: Heat pumps that comply with the following:
(1) Have a minimum efficiency regulated by NAECA.
(2) In accordance with the requirements shown in Table E 503.7.1(2).
(3) Include all usage of internal electric resistance heating.
(9) The system controls shall not permit reheat or other form of simultaneous heating and cooling for humidity control.
(10)Systems serving spaces other than residential spaces, that do not require continuous operation with a cooling or heating capacity more than 7000 Btu/h (2.1 kW) shall comply with Section E 503.4.6.3.1 and Section E 503.4.6.3.2.
(11) Systems serving residential spaces other than hotel/motel guest rooms shall comply with Section E 503.4.6.3.1 and Section E 503.4.6.3.2 except for electric resistance heaters rated at 5000 Btu/h (1.5 kW) or less with a readily accessible manual control that lowers the set point or turns the unit off.
(12) Systems serving hotel/motel guest rooms shall comply with Section E 503.4.6.3.5.
(13) Except for piping within manufacturer’s units, HVAC piping shall be insulated in accordance with Table E 503.7.3(1) and Table E 503.7.3(2). Insulation exposed to weather shall be suitable for outdoor service, e.g., protected by aluminum, sheet metal, painted canvas, or plastic cover. Cellular foam insulation shall be protected as above or painted with a coating that is water retardant and provides shielding from solar radiation.
(14) Ductwork and plenums shall be insulated in accordance with Table E 503.7.2 and shall be sealed in
accordance with Section E 503.4.7.2.
(15) Construction documents shall require a ducted system to be air balanced in accordance with industryaccepted procedures.
(16) Outdoor air intake and exhaust systems shall comply with Section E 503.4.6.4 through Section E 503.4.6.5.
(17) Where separate heating and cooling equipment serves the same temperature zone, thermostats shall be interlocked to prevent simultaneous heating and cooling.
(18) Systems with a design supply air capacity more than 10 000 ft [3] /min (4.7195 m [3] /s) shall have optimum start controls.
CRC § 503.4.6.3.1. High relevance — show source text
Each isolation area shall be controlled independently by a device meeting the requirements of Section E 503.4.6.3.1. 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: Isolation devices and controls are not required for the following:
(1) Exhaust air and outdoor air connections to isolation zones where the fan system to which they connect is not more than 5000 ft [3] /min (2.3597 m [3] /s).
(2) Exhaust airflow from a single isolation zone of less than 10 percent of the design airflow of the exhaust system to which it
connects.
(3) Zones intended to operate continuously or intended to be inoperative only when all other zones are inoperative. [ASHRAE 90.1:6.4.3.3.4]
2025 CALIFORNIA MECHANICAL CODE 423
), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.
APPENDIX E
E 503.4.6.3.5 Automatic Control of HVAC in Hotel/Motel Guest Rooms. Hotels and motels with more than 50 guest rooms shall be provided with automatic controls for the HVAC equipment serving each guest room capable of and configured according to the requirements in Section E 503.4.6.3.5.1. [ASHRAE 90.1:6.4.3.3.5]
E 503.4.6.3.5.1 Guest Room HVAC Set-Point Control. Within 30 minutes of all occupants leaving the guest room, HVAC set points shall be automatically raised by not less than 4°F (2°C) from the occupant set point in the cooling mode and automatically lowered by at least 4°F (2°C) from the occupant set point in the heating mode. When the guest room is unrented and unoccupied, HVAC set points shall be automatically reset to 80°F (27°C) or higher in the cooling mode and to 60°F (16°C) or lower in the heating mode. Unrented and unoccupied guest rooms shall be determined by either of the following:
(1) The guest room has been continuously unoccupied for up to 16 hours.
(2) A networked guest room control system indicates the guest room is unrented and the guest room is unoccupied for no more than 30 minutes.
Exceptions:
(1) A networked guest room control system shall be permitted to return the thermostat set points to their default occupied set points 60 minutes prior to the time the room is scheduled to be occupied.
(2) Cooling for humidity control shall be permitted during unoccupied periods.
E 503.4.6.4 Stair and Elevator Shaft Vent Dampers. Where stair and elevator shafts have vents, they shall be equipped with motorized dampers that are capable of and configured to automatically close during normal building operation and are interlocked to only open as required by fire and smoke detection systems, or by thermostatic control systems.
CRC § 120.2 High relevance — show source text
- All zones are also served by an interior cooling system; and
- The perimeter system is designed solely to offset envelope heat losses or gains; and
- The perimeter system has at least one thermostatic control for each building orientation of 50 feet or more; and
- The perimeter system is controlled by at least one thermostat located in one of the zones served by the system.
(b) Criteria for zonal thermostatic controls. The individual thermostatic controls required by Section 120.2(a) shall meet the following requirements as applicable:
Where used to control comfort heating, the thermostatic controls shall be capable of being set, locally or remotely, down to 55°F or lower.
Where used to control comfort cooling, the thermostatic controls shall be capable of being set, locally or remotely, up to 85°F or higher.
Where used to control both comfort heating and comfort cooling, the thermostatic controls shall meet Items 1 and 2 and shall be capable of providing a temperature range or dead band of at least 5°F within which the supply of heating and cooling energy to the zone is shut off or reduced to a minimum.
72 2025 CALIFORNIA ENERGY CODE
on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.
NONRESIDENTIAL, HOTEL/MOTEL OCCUPANCIES, AND COVERED PROCESSES—MANDATORY REQUIREMENTS
Exception 1 to Section 120.2(b)3: Systems with thermostats that require manual changeover between heating and cooling modes. Exception 2 to Section 120.2(b)3: Systems serving healthcare facilities. 4. Thermostatic controls for all single zone, air conditioners and heat pumps shall comply with the requirements of Sections 110.2(c) and 110.12(a) and, if equipped with DDC to the Zone level, with the Automatic Demand Shed Controls of Section 110.12(b). Exception 1 to Section 120.2(b)4: Systems serving non-covered process loads that must have constant temperatures to prevent degradation of materials, a process, plants or animals. Exception 2 to Section 120.2(b)4: Package terminal air conditioners, package terminal heat pumps, room air conditioners and room air conditioner heat pumps. Exception 3 to Section 120.2(b)4: Systems serving healthcare facilities.
(c) Hotel/motel guest room thermostats.
- Hotel/motel guest room thermostats shall: A. Have numeric temperature setpoints in °F and °C; and B. Have setpoint stops, which are accessible only to authorized personnel, such that guest room occupants cannot adjust the setpoint more than ±5°F (±3°C); and C. Meet the requirements of Section 110.2(c).
Exception to Section 120.2(c)1: Thermostats that are integrated into the room heating and cooling equipment.
(d) Heat pump controls. All heat pumps with supplementary electric resistance heaters shall be installed with controls that comply with Section 110.2(b).
(e) Shut-off and reset controls for space-conditioning systems. Each space-conditioning system shall be installed with controls that comply with the following:
- The control shall be capable of automatically shutting off the system during periods of nonuse and shall have: A. An automatic time switch control device complying with Section 110.9 with an accessible manual override that allows operation of the system for up to 4 hours; or
B. An occupancy sensor; or C. A 4-hour timer that can be manually operated.
CRC § 90.1 High relevance — show source text
Manufactured for nonstandard-size applications only: Not to be installed in new construction projects. [ASHRAE 90.1:6.4.1.6.2]
E 503.4.5 Load Calculations. Heating and cooling system design loads for the purpose of sizing systems and equipment shall be determined in accordance with ASHRAE/ACCA 183. [ASHRAE 90.1:6.4.2.1]
E 503.4.5.1 Pump Head. Pump differential pressure (head) for the purpose of sizing pumps shall be determined in accordance with generally accepted engineering standards and handbooks acceptable to the Authority Having Jurisdiction. The pressure drop through each device and pipe segment in the critical circuit at design conditions shall be calculated.
[ASHRAE 90.1:6.4.2.2]
E 503.4.6 Zone Thermostatic Controls. The supply of heating and cooling energy to each zone shall be individually controlled by thermostatic controls responding to temperature within the zone. For the purposes of Section E 503.4.6, a dwelling unit shall be permitted to be considered a single zone.
Exceptions: Independent perimeter systems that are designed to offset only building envelope loads shall be permitted to serve one or more zones also served by an interior system, provided that:
(1) the perimeter system includes not less than one thermostatic control zone for each building exposure having walls facing only one orientation for 50 contiguous feet (15 240 mm) or more and
(2) the perimeter system heating and cooling supply is controlled by thermostatic controls located within the zones served by the system.
Exterior walls and semiexterior walls are considered to have different orientations where the exposures they face differ by more than 45 degrees (0.79 rad).
[ASHRAE 90.1:6.4.3.1.1]
E 503.4.6.1 Dead Band. Where used to control both heating and cooling, zone thermostatic controls shall be capable of and configured to provide a temperature range or dead band of not less than 5°F (3°C) within which the supply of heating and cooling energy to the zone is shut off or reduced to a minimum.
Exceptions:
(1) Thermostats that require manual changeover between heating and cooling modes.
(2) Special occupancy or special applications where wide temperature ranges are not acceptable (such as retirement homes, process applications, museums, some areas of hospitals) and are approved by the Authority Having Jurisdiction. [ASHRAE 90.1:6.4.3.1.2]
E 503.4.6.2 Setpoint Overlap Restriction. Where heating and cooling to a zone are controlled by separate zone thermostatic controls located within
422 2025 CALIFORNIA MECHANICAL CODE
), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.
APPENDIX E
CRC § 160.3 High relevance — show source text
MULTIFAMILY BUILDINGS—MANDATORY REQUIREMENTS
Exception to Section 160.3(a)2Diib: Thermostat setup controls are not required in multifamily buildings in areas where the summer design dry-bulb 0.5-percent temperature determined in accordance with Section 170.2(c)1C is less than 100°F.
iii. Occupant sensing zone controls . Where the system providing space conditioning also provides the ventilation required by Section 160.2(c)3 and includes occupant sensor ventilation control as specified in Section 160.2(c)5E, the occupant sensing zone controls shall additionally comply with the following: a. Occupant sensing zone controls shall comply with the occupant sensor ventilation control device requirements of Section 160.3(c)5E and allow preoccupancy ventilation requirements of Section 160.3(c)5B; and b. In 5 minutes or less after entering occupied-standby mode as described in Section 160.2(c)5: I. Automatically set up the operating cooling temperature setpoint by 2°F or more and set back the operating heating temperature setpoint by 2°F or more; or II. For multiple zone systems with direct digital controls (DDC) to the zone level, set up the operating cooling temperature setpoint by 0.5°F or more and set back the operating heating temperature setpoint by 0.5°F or more. c. In 5 minutes or less after entering occupied-standby mode, mechanical ventilation to the zone shall remain off whenever the space temperature is between the active heating and cooling setpoints. Exception to Section 160.3(a)2Diii : Zones that are only ventilated by a natural ventilation system in accordance with Section 120.1(c)2. Exception 1 to Sections 160.3(a)2Di, ii and iii: Where it can be demonstrated to the satisfaction of the enforcing agency that the system serves an area that must operate continuously. Exception 2 to Sections 160.3(a)2Di, ii and iii: Systems with full load demands of 2 kW or less, if they have a readily accessible manual shut-off switch. E. Dampers for air supply and exhaust equipment. Outdoor air supply and exhaust equipment shall be installed with dampers that automatically close upon fan shutdown. Exception 1 to Section 160.3(a)2E: Equipment that serves an area that must operate continuously. Exception 2 to Section 160.3(a)2E: Gravity and other nonelectrical equipment that has readily accessible manual damper controls. Exception 3 to Section 160.3(a)2E: At combustion air intakes and shaft vents. Exception 4 to Section 160.3(a)2E: Where prohibited by other provisions of law. F. Isolation area devices . Each space-conditioning system serving multiple zones with a combined conditioned floor area of more than 25,000 square feet shall be designed, installed and controlled to serve isolation areas. i. Each zone, or any combination of zones not exceeding 25,000 square feet, shall be a separate isolation area. ii. Each isolation area shall be provided with isolation devices, such as valves or dampers that allow the supply of heating or cooling to be reduced or shut off independently of other isolation areas. iii. Each isolation area shall be controlled by a device meeting the requirements of Section 160.3(a)2Di. Exception to Section 160.3(a)2F: Zones designed to be conditioned continuously. G. **Automatic demand shed controls.
CRC § 0.293 High relevance — show source text
For SI units: 1000 British thermal units = 0.293 kW
- For mechanical cooling stage control that does not use variable compressor displacement the percent displacement shall be equivalent to the mechanical cooling capacity reduction evaluated at the full load rating conditions for the compressor.
E 503.5.4 Economizer Heating System Impact. HVAC system design and economizer controls shall be such that economizer operation does not increase the building heating energy use during normal operation.
Exception: Economizers on variable air valve (VAV) systems that cause zone level heating to increase due to a reduction in supply air temperature. [ASHRAE 90.1:6.5.1.4] E 503.5.4.1 Economizer Humidification Sys- tem Impact. Systems with hydronic cooling and humidification systems designed to maintain inside humidity at a dew-point temperature more than 35°F (2°C) shall use a fluid economizer where an economizer is required in accordance with Section E 503.5 through Section E 503.5.4.1. [ASHRAE 90.1:6.5.1.5] E 503.5.5 Simultaneous Heating and Cooling Limitation, Zone Controls. Zone thermostatic controls shall prevent the following:
(1) Reheating.
(2) Recooling.
(3) Mixing or simultaneously supplying air that has been previously mechanically heated and air that has
2025 CALIFORNIA MECHANICAL CODE 433
), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.
APPENDIX E
point up to a maximum setpoint 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 E 503.5.11.3.
(4) Zones where at least 75 percent 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. [ASHRAE 90.1:6.5.2.1]
E 503.5.5.1 Supply Air Temperature Reheat Limit. Where reheating is permitted in accordance with this appendix, zones that have both supply and return or exhaust air openings more than 6 feet (1829 mm) above the floor shall not supply heating air more than 20°F (11°C) above the space temperature setpoint.
Exceptions:
(1) Laboratory exhaust systems in accordance with Section E 503.5.11.3.
(2) During preoccupancy building warm-up and setback. [ASHRAE 90.1:6.5.2.1.1]
E 503.5.5.2 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 Section
E 503.5.5.2.1 through Section E 503.5.5.2.3.
[ASHRAE 90.1:6.5.2.2]
CRC § 503.4.6.11 High relevance — show source text
E 503.4.6.11, E 503.5.11.1,
E 503.11.2.2, I 801.1.4
Variable Air Volume . . . . . . . . . . . . . . . . . . . . . . . . 407.5
Ventilation . . . . . . . . . . . . . . . . . . . . Chapter 4, 701.9.3,
902.2, 914.5.2, 915.3.1.1,
925.4, Appendix E
AIRBORNE INFECTION ISOLATION ROOMS . . . 414.0, 417.0
Alarms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321.7, 416.0
AIR CIRCULATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407.4
AIR CONDITIONED
Definition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203.0
Zone isolation . . . . . . . . . . . . . . . . . . . . . . E 503.4.6.3.4
AIR CONDITIONING
Air Conditioning and Heating Systems [OSHPD] . . 320.0
Appliances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 903.0
Coils. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 903.2.6
Gas fired type . . . . . . . . . . . . . . . . . . . . . . . . . . . . 903.2
General requirements . . . . . . . . . . . . . . . . . . Chapter 3
Refrigerant port protection . . . . . . . . . . . . . . . . . 1105.11
System test . . . . . . . . . . . . . . . . . . . . . . . . . . . . E 805.0
System water use . . . . . . . . . . . . . . . . . . . . . . . E 403.0
AIR-HANDLING UNIT
Definition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203.0
Ducts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 6
Exposed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 904.9.1
Permit Fees . . . . . . . . . . . . . . . . . . . . . . . . . Table 104.5
System test . . . . . . . . . . . . . . . . . . . . . . . . . . . . E 805.0
AIR, EXHAUST
Frequently asked questions
Do thermostats have to be programmable to meet off‑hour controls?
Not necessarily — you must implement one of the methods in § E 503.4.6.3.1 (7‑day programmable schedule with the listed backup/override features, an occupancy sensor that can shut off after up to 30 minutes, a manual timer adjustable up to 2 hours, or security interlock). The schedule option has additional retention/override requirements.
What if heating and cooling are provided by separate systems in the same zone?
You must prevent setpoint overlap by mechanical stops, limit switches, or DDC logic so the heating setpoint cannot exceed the cooling setpoint (see § E 503.4.6.2).
How large can an isolation area be?
An isolation area may be up to 25,000 ft² (2,322.6 m²) and must not include more than one floor. If zones are intended to operate nonsimultaneously, they must be broken into such isolation areas equipped with automatic isolation devices. § E 503.4.6.3.4.
Are there exceptions for small systems or continuously operating spaces?
Yes. Systems intended to operate continuously are exempt from off‑hour shutdown requirements. Small nonresidential systems (< 7,000 Btu/h) with a readily accessible manual ON/OFF control also have limited exemptions; see § E 503.4.6.3 exceptions.
If I have radiant floor heating, can I use a smaller setback?
Yes — the code permits radiant heating systems configured for setback to use a 4°F (2°C) heating setback instead of the standard 10°F. § E 503.4.6.3.2 (Exception).
More in California Residential Code
Ask about the CRC
Get cited, plain-English answers on the California Residential Code for your project — any code section, any scenario.
Start Free Trial