CMC · California Mechanical Code

What are the return‑air limitations and prohibited sources for HVAC systems?

In short: the CMC requires HVAC systems to get return or outside air from safe, adequately sized locations — not from hazardous areas, kitchens, bathrooms, closets, garages, unconditioned attics, or rooms that are too small — and it expressly forbids returning air from one dwelling unit into another; see **§ 311.1**, **§ 311.3**, and **§ 311.4** for the full rules.

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Last reviewed: July 6, 2026

What the code requires — 2–4 sentences

A heating or cooling air system must be provided with return air, outside air, or both, and any return/outside‑air intake must avoid certain prohibited locations and meet volume/separation limits. Key prohibitions and limitations are stated in § 311.1, § 311.3, and § 311.4 of the California Mechanical Code. The code also protects against cross‑contamination between dwelling units by forbidding return air from one dwelling unit to discharge into another.

The single most important plain‑English rule: do not take return or outside air from hazardous, unsanitary, very small, or certain occupied rooms — and never return air from one dwelling into another. § 311.3 and § 311.4 state this directly.

Requirements in detail

Basic source requirement

A heating or cooling air system must have return air, outside air, or both; systems intended to replace required ventilation must deliver at least the outside air specified in Chapter 4. See § 311.1.

Prohibited sources — where you may NOT take outside or return air

The code lists specific locations that are not permitted as the source of outside or return air for HVAC systems. The table below summarizes the decision‑relevant source types and the controlling code citation.

Prohibited source (plain English)	Why it matters (decision dimension)	Code Reference
Less than required separation distances to vents	Intake too close to vents can draw contaminants; comply with vent separation rules	§ 311.3 (1)
Less than separation distances required by § 402.4	Other code separation requirements apply to intakes	§ 311.3 (2)
Hazardous or insanitary locations; refrigeration machinery room	Contains contaminants or refrigerant hazards	§ 311.3 (3)
An area whose volume is < 25% of the total served (unless opening to equal 25%)	Too small a source can concentrate contaminants or starve the system	§ 311.3 (4)
Closets, bathrooms, toilet rooms, kitchens, garages, boiler/furnace rooms, unconditioned attics	These spaces contain odors, combustion products or contaminants	§ 311.3 (5)
Rooms/spaces containing fuel‑burning appliances (when used as return source)	Risk of backdrafting or carbon monoxide entrainment	§ 311.3 (6)
Return air from one dwelling unit into another	Cross‑contamination between separate dwelling units is prohibited	§ 311.4

Numerical and placement limitations (decision dimensions and thresholds)

Dimension / threshold	Value / rule	Code Reference
Minimum source volume as % of served volume	25% (source area must be ≥ 25% of entire volume served by the system unless permanent opening to area equal to 25%)	§ 311.3 (4)
Dwelling‑unit exception for openings	Opening may be reduced to 50% of required area when remaining return is from a room/hall with ≥ 3 doors to other rooms served	§ 311.3 (4) Exception
Volume required when return is from a room with fuel‑burning appliances	1 cu ft per 10 Btu/h of appliance fuel input rating (i.e., room volume must exceed that)	§ 311.3 (6) Exception (3)(a)
Supply air that must be discharged back into same room for blower‑type systems	≥ 75% of supply air must be discharged back into same room	§ 311.3 (6) Exception (3)(b)
Minimum distance from appliance firebox/draft diverter for return inlets	10 feet (3048 mm)	§ 311.3 (6) Exception (3)(c)

Fuel‑burning appliance rooms — permitted exceptions

Section § 311.3 prohibits taking return air from rooms containing fuel‑burning appliances, but provides exceptions. The notable exceptions are:

Fireplaces, residential cooking appliances, direct‑vent appliances, enclosed furnaces, and domestic‑type clothes dryers are exempt from the prohibition. § 311.3 exceptions apply.
Gravity‑type or listed vented wall heating/cooling systems are exempt. § 311.3 exceptions apply.
A blower‑type heating/cooling system may take return air from such a room if it meets all three sub‑requirements: the room volume meets the 1 cu ft per 10 Btu/h rule, at least 75% of supply air returns to the same room, and return inlets are not within 10 ft of the appliance firebox/draft diverter. § 311.3 (6) Exceptions (3)(a)–(c)

Dwelling‑unit separation

§ 311.4 is unambiguous: return air from one dwelling unit shall not discharge into another dwelling unit through the heating or cooling air system. This is a strict prohibition to prevent air transfer between dwelling units.

Exceptions & special cases

The 25% volume restriction has a dwelling‑unit exception that allows the required opening to be reduced to 50% of the required area when the remainder of return air is taken from a room or hall with at least three doors to other rooms served by the furnace (useful in some residential layouts). § 311.3 (4) Exception.
The fuel‑burning appliance prohibition does not apply to several appliance types (fireplaces, direct‑vent, enclosed furnaces, domestic clothes dryers, residential cooking appliances). § 311.3 exceptions.
Gravity or listed vented wall systems are exempt from the fuel‑burning‑room prohibition. § 311.3 exceptions.
Blower‑type systems serving rooms with fuel‑burning appliances are allowed only when the three specific subcriteria listed above are met. § 311.3 (6) Exceptions (3).

If a situation is not clearly covered in § 311.1, § 311.3, or § 311.4, the CMC text in the provided files should be consulted for related chapters (e.g., Chapter 4 ventilation requirements referenced in § 311.1) or the Authority Having Jurisdiction for interpretation. The material cited here is taken from the CMC excerpts available in the project files.

Common mistakes

Assuming any interior room is an acceptable source of return air — kitchens, bathrooms, garages, attics and closets are specifically prohibited. § 311.3 (5).
Failing to account for the 25% minimum volume rule (or the 50% dwelling exception) when designing returns for smaller suites or zones — leads to undersized return source and noncompliance. § 311.3 (4).
Taking return air from a room with a fuel‑burning appliance without verifying the 1 cu ft per 10 Btu/h volume rule, the 75% supply recirculation, and the 10 ft clearance. § 311.3 (6) Exceptions (3)(a)–(c).
Allowing return air to transfer from one apartment to another in multifamily buildings — prohibited by § 311.4.

Worked example — concrete scenario with numbers

Scenario: A single‑family furnace (blower type) is in an enclosed utility room. The furnace and a water heater are in that room. The furnace burner input rating is 40,000 Btu/h. The builder wants to take return air from that room.

Apply the code tests from § 311.3:

Room volume required: 1 cu ft per 10 Btu/h → 40,000 Btu/h ÷ 10 = 4,000 cu ft.
- If the utility room is 20 ft × 20 ft × 10 ft = 4,000 cu ft, the room meets the volume test (Exception (3)(a)). § 311.3 (6) Exception (3)(a).
Percent of supply discharged back to same room: the HVAC design must ensure ≥ 75% of supply air is discharged back into this same room (Exception (3)(b)). § 311.3 (6) Exception (3)(b).
Return inlet location: return inlets must be ≥ 10 ft from the appliance firebox or draft diverter (Exception (3)(c)). § 311.3 (6) Exception (3)(c).

If all three conditions are satisfied, the blower‑type system may take return air from that room despite it containing fuel‑burning appliances. If any test fails, the room cannot be used as the return source; an alternate return location or dedicated outside air intake is required. § 311.3 (6) Exceptions.

Related provisions

§ 311.1 — Source requirement that systems be provided with return or outside air and discharge the required outside air for ventilation.
§ 311.2 / § 311.2.1 — Air filter requirements and MERV 13 minimum for mechanically ventilated buildings (filters on outside and return air). While not the primary topic, filtration interacts with return/outside air decisions.
§ 402.4 — (Referenced by § 311.3) Separation distances that affect intake location. See § 311.3 (2) for cross‑reference.
Chapter 4 (Ventilation requirements) — required outside air quantities that § 311.1 refers to when the system replaces ventilation.

Code references

Grounded in the retrieved California Mechanical Code — click a citation to read the verbatim passage:

CMC § 311.2.1 High relevance — show source text
(2) Air filters used in listed appliances and in accordance with the manufacturer’s instructions.

311.2.1 Minimum Filtration. In mechanically ventilated buildings, occupied areas of the building shall be provided with air filtration media for outside and return air that provides not less than a Minimum Efficiency Reporting Value (MERV) of 13 or as required by the Authority Having Jurisdiction. Installed filters shall be clearly labeled by the manufacturer indicating the MERV rating.

311.3 Prohibited Sources. Outside or return air for a heating or cooling air system shall not be taken from the following locations:

(1) Less than the separation distances to vents required by Section 802.6.1 and Section 802.8.

(2) Less than the separation distances required by Section 402.4.

(3) A hazardous or insanitary location, or a refrigeration machinery room as defined in this code.

(4) An area, the volume of which is less than 25 percent of the entire volume served by such system, unless there is a permanent opening to an area the volume of which is equal to 25 percent of the entire volume served.

Exception: Such openings where used for a heating or cooling air system in a dwelling unit shall be permitted to be reduced to not less than 50 percent of the required area, provided the balance of the required return air is taken from a room or hall having not less than three doors leading to other rooms served by the furnace.

(5) A closet, bathroom, toilet room, kitchen, garage, boiler room, furnace room, or unconditioned attic.

(6) Rooms or spaces containing a fuel-burning appliance therein. Where such room or space serves as source of return-air.

Exceptions:

(1) This shall not apply to fireplaces, fireplace appliances, residential cooking appliances, direct vent appliances, enclosed furnaces, and domestic-type clothes dryers installed within the room or space.

(2) This shall not apply to a gravity-type or listed vented wall heating or cooling air system.

(3) This shall not apply to a blower-type heating or cooling air system installed in accordance with the following requirements:

(a) Where the return air is taken from a room or space having a volume exceeding 1 cubic foot (0.03 m [3] ) for each 10 Btu/h (0.003 kW) fuel input rating of fuel-burning appliances therein.

(b) Not less than 75 percent of the supply air is discharged back into the same room or space.

(c) Return-air inlets shall not be located within 10 feet (3048 mm) from an appliance firebox or draft diverter in the same enclosed room or confined space.

311.4 Return-Air Limitations. Return air from one dwelling unit shall not discharge into another dwelling unit through the heating or cooling air system.

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GENERAL REGULATIONS
CMC § 310.5 High relevance — show source text
310.5 Point of Discharge. Air-conditioning condensate waste pipes shall connect indirectly to the drainage system through an air gap or air break to trapped and vented receptors, dry wells, mop sinks, leach pits, or the tailpiece of plumbing fixtures. An individual condensate drain shall be trapped in accordance with the appliance manufacturer’s instructions or in accordance with Section 310.4.

310.6 Condensate Waste from Air-Conditioning Coils. Where the condensate waste from air-conditioning coils discharges by direct connection to a lavatory tailpiece or to an approved accessible inlet on a bathtub overflow, the connection shall be located in the area controlled by the same person controlling the air-conditioned space. 310.7 Female Plastic Connections. Female plastic screwed fittings shall be used with plastic male fittings and plastic male threads. Female plastic threaded connections shall not be allowed to be used when threaded onto a male

metallic connection.

311.0 Heating or Cooling Air System. 311.1 Source. A heating or cooling air system shall be provided with return air, outside air, or both. A heating or cooling air system regulated by this code and designed to replace required ventilation shall be arranged to discharge into a conditioned space not less than the amount of outside air specified in Chapter 4.

311.2 Air Filters. Air filters shall be installed in a heating, cooling, or makeup air system. Media-type air filters shall comply with UL 900. Electrostatic and high efficiency particulate filters shall comply with Section 935.0.

Exceptions:

(1) Air filters used in systems serving single guest rooms or dwelling units shall not be required to be listed.

(2) Air filters used in listed appliances and in accordance with the manufacturer’s instructions.

311.2.1 Minimum Filtration. In mechanically ventilated buildings, occupied areas of the building shall be provided with air filtration media for outside and return air that provides not less than a Minimum Efficiency Reporting Value (MERV) of 13 or as required by the Authority Having Jurisdiction. Installed filters shall be clearly labeled by the manufacturer indicating the MERV rating.

311.3 Prohibited Sources. Outside or return air for a heating or cooling air system shall not be taken from the following locations:

(1) Less than the separation distances to vents required by Section 802.6.1 and Section 802.8.

(2) Less than the separation distances required by Section 402.4.

(3) A hazardous or insanitary location, or a refrigeration machinery room as defined in this code.

(4) An area, the volume of which is less than 25 percent of the entire volume served by such system, unless there is a permanent opening to an area the volume of which is equal to 25 percent of the entire volume served.

Exception: Such openings where used for a heating or cooling air system in a dwelling unit shall be permitted to be reduced to not less than 50 percent of the required area, provided the balance of the required return air is taken from a room or hall having not less than three doors leading to other rooms served by the furnace.

(5) A closet, bathroom, toilet room, kitchen, garage, boiler room, furnace room, or unconditioned attic.

(6) Rooms or spaces containing a fuel-burning appliance therein. Where such room or space serves as source of return-air.

Exceptions:
CMC § 310.1 High relevance — show source text
310.1 Condensate Disposal . . . . . . . . . . . 57

310.2 Condensate Control . . . . . . . . . . . . 57

310.3 Condensate Waste Pipe Material and Sizing. . . . . . . . . . . . . . . . . . . . 57

Table 310.3 Minimum Condensate Pipe Size. . . 57

310.4 Appliance Condensate Drains . . . . 58

310.5 Point of Discharge . . . . . . . . . . . . . 58

310.6 Condensate Waste from

Air-Conditioning Coils . . . . . . . . . . . 58

310.7 Female Plastic Connections . . . . . . 58

311.0 Heating or Cooling Air System . . . . 58

311.1 Source . . . . . . . . . . . . . . . . . . . . . . 58

311.2 Air Filters . . . . . . . . . . . . . . . . . . . . 58

311.3 Prohibited Sources . . . . . . . . . . . . . 58

311.4 Return-Air Limitations . . . . . . . . . . . 58

311.5 California Energy Code Requirements for Residential

Air Filtration . . . . . . . . . . . . . . . . . . 59

312.0 Plumbing Connections . . . . . . . . . . 59

312.1 General. . . . . . . . . . . . . . . . . . . . . . 59

313.0 Hangers, Supports, and Anchors . . 59

313.1 General. . . . . . . . . . . . . . . . . . . . . . 59

313.2 Material. . . . . . . . . . . . . . . . . . . . . . 59

313.3 Suspended Piping . . . . . . . . . . . . . 59

313.4 Alignment . . . . . . . . . . . . . . . . . . . . 59

313.5 Underground Installation . . . . . . . . 59

313.6 Hanger Rod Sizes . . . . . . . . . . . . . 59

Table 313.6 Hanger Rod Sizes . . . . . . . . . . . . . 59

313.7 Gas Piping . . . . . . . . . . . . . . . . . . . 59

314.0 Balancing . . . . . . . . . . . . . . . . . . . . 59

314.1 General. . . . . . . . . . . . . . . . . . . . . . 59

315.0 Louvers in Hurricane Prone

Regions . . . . . . . . . . . . . . . . . . . . . 59

315.1 General. . . . . . . . . . . . . . . . . . . . . . 59

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316.0 Protection of Piping, Tubing, Materials, and Structures . . . . . . . . 59

316.1 General. . . . . . . . . . . . . . . . . . . . . . 59
CMC § 305.6 High relevance — show source text
305.6 Outdoor Locations . . . . . . . . . . . . . 55

306.0 Automatic Control Devices . . . . . . . 55

306.1 General. . . . . . . . . . . . . . . . . . . . . . 55

306.2 Building Automation Systems . . . . 56

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TABLE OF CONTENTS

307.0 Labeling . . . . . . . . . . . . . . . . . . . . . 56

307.1 Fuel-Burning Appliances . . . . . . . . 56

307.2 Electric Heating Appliances . . . . . . 56

307.3 Heat Pump and Electric Cooling Appliances . . . . . . . . . . . . . . . . . . . 56

307.4 Absorption Units . . . . . . . . . . . . . . . 56

308.0 Improper Location. . . . . . . . . . . . . . 57

308.1 General. . . . . . . . . . . . . . . . . . . . . . 57

309.0 Workmanship . . . . . . . . . . . . . . . . . 57

309.1 Engineering Practices. . . . . . . . . . . 57

309.2 Concealing Imperfections . . . . . . . . 57

309.3 Installation Practices. . . . . . . . . . . . 57

310.0 Condensate Wastes and Control . . 57

310.1 Condensate Disposal . . . . . . . . . . . 57

310.2 Condensate Control . . . . . . . . . . . . 57

310.3 Condensate Waste Pipe Material and Sizing. . . . . . . . . . . . . . . . . . . . 57

Table 310.3 Minimum Condensate Pipe Size. . . 57

310.4 Appliance Condensate Drains . . . . 58

310.5 Point of Discharge . . . . . . . . . . . . . 58

310.6 Condensate Waste from

Air-Conditioning Coils . . . . . . . . . . . 58

310.7 Female Plastic Connections . . . . . . 58

311.0 Heating or Cooling Air System . . . . 58

311.1 Source . . . . . . . . . . . . . . . . . . . . . . 58

311.2 Air Filters . . . . . . . . . . . . . . . . . . . . 58

311.3 Prohibited Sources . . . . . . . . . . . . . 58

311.4 Return-Air Limitations . . . . . . . . . . . 58

311.5 California Energy Code Requirements for Residential

Air Filtration . . . . . . . . . . . . . . . . . . 59

312.0 Plumbing Connections . . . . . . . . . . 59

312.1 General. . . . . . . . . . . . . . . . . . . . . . 59
CMC § 52.2 Medium relevance — show source text
ing within the required temperature range for material and equipment installation. Where required, a supplemental HVAC system shall be used during construction, return air shall be equipped with filters with a minimum efficiency reporting value (MERV) of 8, in accordance with ASHRAE 52.2, or an average efficiency of 30 percent in accordance with ASHRAE 52.2. Before occupancy, filters shall be replaced with filters having a MERV 13 rating in accordance with Section E 603.3.

Exception: Embedded hydronics system shall be permitted to be used to condition the building during construction.

E 603.1.2 Indoor Air Quality After Construction. After construction ends and interior finishes are installed, flush-out the building to reduce contaminant concentrations by supplying a total outdoor air volume of 14 000 cubic feet per square foot (ft [3] /ft [2] ) (4267.2 m [3] /m [2] ) of occupiable building area. An internal temperature of not less than 60°F (16°C) and relative humidity not higher than 60 percent shall be maintained during the flush-out process. Occupancy shall begin on condition of 3500 ft [3] /ft [2] (1066.8 m [3] /m [2] ) of building area, with the remaining 10 500 ft [3] /ft [2] (3200.4 m [3] /m [2] ) being accomplished as soon as possible.

Exception: Other means of reducing the contaminant concentration levels shall be permitted where approved by the Authority Having Jurisdiction. E 603.1.3 Covering of Duct Openings and Pro- tection of Mechanical Equipment During Con- struction. At the time of rough installation, or during storage on the construction site and until final startup of the heating and cooling equipment, duct and other related air distribution component openings shall be covered

with tape, plastic, sheet metal, or other methods acceptable to the Authority Having Jurisdiction to reduce the amount of dust or debris that collects in the system.

E 603.2 Isolation of Pollutant Sources. Rooms where activities produce hazardous fumes or chemicals, including commercial kitchens, garages, janitorial or laundry rooms, and copy or printing rooms, shall be exhausted and isolated from adjacent spaces in accordance with this code.

E 603.3 Filters. In mechanically ventilated buildings, particle filters, or air-cleaning devices shall be provided to clean outdoor and return air prior to its delivery to occupied spaces. The particle or air cleaner shall have a MERV of 13.

Exception: A filter or air cleaning device with a lower MERV value shall be permitted provided it is the highest value commercially available for the specific equipment that is installed.

E 603.4 Ozone Depletion and Global Warming Reductions. Installations of HVAC and refrigeration shall not contain CFCs and shall be in accordance with this code.

E 604.0 Indoor Moisture Control.

E 604.1 Rainwater Control. Roof drainage systems shall discharge to a place of disposal in accordance with the plumbing code. Storm water shall be directed away from the building.
CMC § 90.1 Medium relevance — show source text
(1) Monitor zone damper positions or other indicator of need for static pressure.

(2) Automatically detect those zones that are capable of excessively driving the reset logic and generate an alarm to the system operator.

(3) Readily allow operator removal of zones from the reset algorithm. [ASHRAE 90.1:6.5.3.2.3]

E 503.5.6.2.3 Return and Relief Fan Con-

trol. Return and relief fans used to meet Section E 503.5.1.4 shall comply with all of the following:

(1) Relief air rate shall be controlled to maintain building pressure either directly, or indirectly through differential supplyreturn 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.

(2) Fans shall have variable-speed control or other devices that will result in total return/relief fan system demand of no more than 30 percent of total design power at 50 percent of total design fan flow.

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APPENDIX E

Exceptions:

(1) Return or relief fans with total motor size less than or equal to 0.5 hp (0.37 kW).

(2) Staged relief fans with a minimum of four stages. [ASHRAE 90.1:6.5.3.2.4]

E 503.5.6.3 Multiple-Zone VAV System Venti- lation 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 in accordance with Section 404.0 or ASHRAE 62.1.

Exceptions:

(1) 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 fanpowered terminal units.

(2) Systems where total design exhaust airflow is more than 70 percent of total design outdoor air intake flow requirements. [ASHRAE 90.1:6.5.3.3]

E 503.5.6.4 Supply Air Temperature Reset Controls. Multiple zone HVAC systems shall include controls that are capable of and configured to automatically reset the supply air temperature in response to representative building loads, or to outdoor air temperature. The controls shall reset the supply air temperature to at least 25 percent of the difference between the design supply air temperature and the design room air temperature. Controls that adjust the reset based on zone humidity shall be permitted in Climate Zones 0B, 1B, 2B, 3B, 3C, and 4 through 8. HVAC zone that are expected to experience relatively constant loads shall have maximum airflow designed to accommodate the fully reset supply air temperature.

HVAC zones that are expected to experience relatively constant loads typically include electronic equipment rooms and interior zones.

Exceptions:

(1) Systems in Climate Zones 0A, 1A, and 3A with less than 3000 cubic feet per minute (1.4 m [3] /s) of design outdoor air.

CMC § 602.1.2 Medium relevance — show source text

E 602.1.2 Prohibited. Continuous burning pilot lights and the use of indoor air for cooling a firebox jacket, where the indoor air is vented to the outside of the building, are prohibited.

E 603.0 Pollutant Control.

E 603.1 Indoor Air Quality During Construction. Indoor air quality of a building shall be maintained in accordance with Section E 603.1.1 through Section E 603.1.3. E 603.1.1 Temporary Ventilation During Con- struction. Temporary ventilation during construction shall be provided in accordance with the following: (1) Ventilation during construction shall be achieved through openings in the building shell using fans to produce not less than three air changes per hour. (2) During dust-producing operations, the supply and return HVAC system openings shall be protected from dust in accordance with Section E 603.1.3.

(3) Where the building is occupied during demolition or construction, ventilation shall be provided in accordance with the Control Measures of the SMACNA IAQ Guidelines for Occupied Buildings Under Construction.

(4) The permanent HVAC system shall not be used during construction to condition and ventilate the build

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APPENDIX E

TABLE E 503.7.1(1) ELECTRICALLY OPERATED UNITARY AIR CONDITIONERS AND CONDENSING UNITS—MINIMUM EFFICIENCY REQUIREMENTS

[ASHRAE 90.1: TABLE 6.8.1-1]

EQUIPMENT TYPE	SIZE CATEGORY	HEATING SECTION TYPE	SUBCATEGORY OR RATING CONDITION	MINIMUM EFFICIENCY	TEST PROCEDURE1
Air conditioners, air cooled	<65 000 Btu/h2	All	Split system, three phase and applications outside U.S. single phase2	13.0_SEER_ before 1/1/2023 13.4_SEER2_ after 1/1/2023	AHRI 210/240- 2017 before 1/1/2023 AHRI 210/240- 2023 after 1/1/2023
Air conditioners, air cooled	<65 000 Btu/h2	All	Single package, three phase and applications outside U.S. single phase2	14.0_SEER_ before 1/1/2023 13.4_SEER2_ after 1/1/2023	14.0_SEER_ before 1/1/2023 13.4_SEER2_ after 1/1/2023
Space constrained, air cooled	≤30 000 Btu/h2	All	Split system, three phase and applications outside U.S.

CMC § 90.1 Medium relevance — show source text

Exception: Units that heat the airstream using only series energy recovery when representative building loads or outdoor air temperature indicate that the majority of zones require cooling in Climate Zones 0A, 1A, 2A, 3A, and 4A. [ASHRAE 90.1:6.5.2.6]

E 503.5.6 Air System Design and Control. HVAC air system design and control shall be in accordance with the provisions of Section E 503.5.6.1 through Section E 503.5.6.8.

E 503.5.6.1 Fan System Power and Efficiency. Each HVAC system having a total fan system motor nameplate horsepower (kW) exceeding 5 hp (3.7 kW) at fan system design conditions shall not exceed the allowable fan system motor nameplate horsepower (kW) (Option 1) or fan system brake horsepower (kW) (Option 2) as shown in Table E 503.5.6.1(1). This shall include supply fans, return or relief fans, exhaust fans, and fan-powered terminal units associated with systems providing heating or cooling capability that operate at fan system design conditions. Single-zone VAV systems shall comply with the constant-volume fan power limitation.

Exceptions:

(1) Hospital, vivarium, and laboratory systems that utilize flow control devices on exhaust, return, or both to maintain space pressure relationships

TABLE E 503.5.6.1(1) FAN POWER LIMITATION*

[ASHRAE 90.1: TABLE 6.5.3.1-1]

Col1	LIMIT	CONSTANT VOLUME	VARIABLE VOLUME
Option 1: Fan system motor nameplate (hp)	Allowable motor nameplate (hp)	hp≤CFMS• 0.0011	hp≤_CFMS _• 0.0015
Option 2: Fan system (bhp)	Allowable fan system (bhp)	bhp≤CFMS• 0.00094 +A	_ bhp_≤CFMS• 0.0013 +A

For SI units: 1 horsepower = 0.746 kW, 1 cubic foot per minute = 0.00047 m [3] /s, 1 cubic foot per minute = 0.4719 L/s

Where:

A = sum of ( PD x CFMD /4131) bhp = the maximum combined fan brake horsepower (kW) CFMD = the design airflow through each applicable device from Table E 503.5.6.1(2) in CFM (m [3] /s) CFMS = the maximum design supply airflow rate to conditioned spaces served by the system in CFM (m [3] /s) hp = the maximum combined motor nameplate horsepower (kW) PD = each applicable pressure drop adjustment from Table E 503.5.6.1(2) in inch water column (kPa)

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APPENDIX E

TABLE E 503.5.6.1(2) FAN POWER LIMITATION PRESSURE DROP ADJUSTMENT

[ASHRAE 90.1: TABLE 6.5.3.1-2]

CMC § 0.03 Medium relevance — show source text
(5) A closet, bathroom, toilet room, kitchen, garage, boiler room, furnace room, or unconditioned attic.

(6) Rooms or spaces containing a fuel-burning appliance therein. Where such room or space serves as source of return-air.

Exceptions:

(1) This shall not apply to fireplaces, fireplace appliances, residential cooking appliances, direct vent appliances, enclosed furnaces, and domestic-type clothes dryers installed within the room or space.

(2) This shall not apply to a gravity-type or listed vented wall heating or cooling air system.

(3) This shall not apply to a blower-type heating or cooling air system installed in accordance with the following requirements:

(a) Where the return air is taken from a room or space having a volume exceeding 1 cubic foot (0.03 m [3] ) for each 10 Btu/h (0.003 kW) fuel input rating of fuel-burning appliances therein.

(b) Not less than 75 percent of the supply air is discharged back into the same room or space.

(c) Return-air inlets shall not be located within 10 feet (3048 mm) from an appliance firebox or draft diverter in the same enclosed room or confined space.

311.4 Return-Air Limitations. Return air from one dwelling unit shall not discharge into another dwelling unit through the heating or cooling air system.

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GENERAL REGULATIONS

311.5 California Energy Code Requirements for Res- idential Air Filtration. In mechanically ventilated residen- tial buildings, air filters shall be provided to clean outdoor air and return air prior to its delivery to occupied spaces where specified in California Energy Code Sections 150.0(m)12 and 160.2(b). The air filters shall comply with California Energy Code requirements for labeling, ventilation system design and installation, air filter efficacy, and air filter pressure drop.

312.0 Plumbing Connections. 312.1 General. Water supply, sanitary drainage, and backflow protection shall be in accordance with the California Plumbing Code .

313.0 Hangers, Supports, and Anchors. 313.1 General. Piping, tubing, appliances, and appurtenances shall be supported in accordance with this code, the manufacturer’s installation instructions, and in accordance with the Authority Having Jurisdiction. Seismic restraints shall be in accordance with the building code. 313.2 Material. Hangers, supports, and anchors shall be of sufficient strength to support the weight of the pipe or tubing and its contents. Piping or tubing shall be isolated from incompatible materials.

313.3 Suspended Piping. Suspended piping or tubing shall be supported at intervals not to exceed those shown in Table 313.3.

313.4 Alignment. Piping or tubing shall be supported in such a manner as to maintain its alignment and prevent sagging.

313.5 Underground Installation. Piping or tubing in the ground shall be laid on a firm bed for its entire length; where other support is otherwise provided, it shall be approved in accordance with Section 302.0.

313.6 Hanger Rod Sizes. Hanger rod sizes shall be not smaller than those shown in Table 313.6.

CMC § 90.1 Medium relevance — show source text

[ASHRAE 90.1: TABLE 6.4.3.4.3]

Col1	OUTDOOR AIR INTAKE (CFM/ft2)	Col3	EXHAUST/RELIEF (CFM/ft2)	Col5
CLIMATE ZONE	NONMOTORIZED1	MOTORIZED	NONMOTORIZED3	MOTORIZED
0, 1, 2	0, 1, 2	0, 1, 2	0, 1, 2	0, 1, 2
Any height	20	4	20	4
3	3	3	3	3
Any height	20	10	20	10
4, 5B, 5C	4, 5B, 5C	4, 5B, 5C	4, 5B, 5C	4, 5B, 5C
Fewer than three stories	204	10	20	10
Three or more stories	204	10	204	10
5A, 6, 7, 8	5A, 6, 7, 8	5A, 6, 7, 8	5A, 6, 7, 8	5A, 6, 7, 8
Fewer than three stories	204	4	20	4
Three or more stories	204	4	204	4

For SI units: 1 square foot = 0.0929 m [2], 1 cubic foot per minute = 0.00047 m [3] /s, 1 cubic foot per minute = 0.4719 L/s, 1 cubic foot per minute per square foot = 5.08 [(L/s)/m [2] ], 1 inch water gauge = 0.249 kPa

Notes:

1 When tested in accordance with AMCA 500D.

2 Dampers smaller than 12 inches (305 mm) in height, width, or diameter need not be tested but shall be of the same design and construction as the smallest tested damper meeting the listed leakage rate requirement.

3 Nonmotorized dampers smaller than 24 inches (610 mm) in height, width, or diameter shall be permitted to have a leakage rate of 40 CFM/ft 2 [0.203 (m 3 /s)/m 2 ].

4 Where permitted by Section E 503.4.6.4.1, exception 2.

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APPENDIX E

(2) Systems serving zones where humidity levels are required to be maintained with precision of not more than ±5 percent relative humidity to comply with applicable codes or accreditation standards or as approved by the Authority Having Jurisdiction.

[ASHRAE 90.1:6.4.3.6.2]

CMC § 503.8 Medium relevance — show source text
E 503.8 Alternative Compliance Path, Computer Room Systems. HVAC systems only serving the heating, cooling, or ventilating needs of a computer room with IT equipment load greater than 13.4 hp (10 kW) shall be in accordance with ASHRAE 90.4. [ASHRAE 90.1:6.6.1]

E 504.0 Solar Energy Systems.

E 504.1 General. Solar energy systems shall be installed in accordance with the Uniform Solar, Hydronics and Geothermal Code (USHGC).

E 601.0 Indoor Environment.

E 601.1 Scope. The provisions of this section shall establish the means of reducing the quantity of air contaminants that are odorous, irritating, or harmful to the comfort and wellbeing of a building’s installers, occupants, and neighbors.

E 602.0 Fireplaces.

E 602.1 Requirements. A direct vent sealed-combustion gas or sealed wood-burning fireplace, or a sealed wood stove shall be installed. The fireplace shall comply with Section E 602.1.1 and Section E 602.1.2.

E 602.1.1 Masonry or Factory-Built Fireplace. Masonry and factory-built fireplaces located in conditioned spaces shall be in accordance with Section E 602.1.1.1 through Section E 602.1.1.3. E 602.1.1.1 Opening Cover. Closeable metal or glass doors covering the entire opening of the firebox shall be installed.

E 602.1.1.2 Combustion Air Intake. A combus tion air intake to draw air from the outside of the building directly into the firebox, which is an area of not less than 6 square inches (0.004 m [2] ) and is equipped with a readily accessible, operable, and tight-fitting damper or combustion air control device. E 602.1.1.3 Accessible Damper Control. The flue damper shall have a readily accessible control. Exception: Where a gas log, log lighter, or decorative gas appliance is installed in a fireplace, the flue damper shall be blocked open where required by this code or the manufacturer’s installation instructions.

E 602.1.2 Prohibited. Continuous burning pilot lights and the use of indoor air for cooling a firebox jacket, where the indoor air is vented to the outside of the building, are prohibited.

E 603.0 Pollutant Control.

E 603.1 Indoor Air Quality During Construction. Indoor air quality of a building shall be maintained in accordance with Section E 603.1.1 through Section E 603.1.3. E 603.1.1 Temporary Ventilation During Con- struction. Temporary ventilation during construction shall be provided in accordance with the following: (1) Ventilation during construction shall be achieved through openings in the building shell using fans to produce not less than three air changes per hour. (2) During dust-producing operations, the supply and return HVAC system openings shall be protected from dust in accordance with Section E 603.1.3.

(3) Where the building is occupied during demolition or construction, ventilation shall be provided in accordance with the Control Measures of the SMACNA IAQ Guidelines for Occupied Buildings Under Construction.

(4) The permanent HVAC system shall not be used during construction to condition and ventilate the build
CMC § 1124.1 Medium relevance — show source text
1124.1 General. . . . . . . . . . . . . . . . . . . . . 247

1125.0 Refrigerants and Hazardous

Fluids . . . . . . . . . . . . . . . . . . . . . . 247

1125.1 General. . . . . . . . . . . . . . . . . . . . . 247

1126.0 Drift Eliminators . . . . . . . . . . . . . . 248

1126.1 General. . . . . . . . . . . . . . . . . . . . . 248

1127.0 Water Supply . . . . . . . . . . . . . . . . 248

1127.1 General . . . . . . . . . . . . . . . . . . . . 248

Table 1102.3 Refrigerant Groups, Properties, and Allowable Quantities . . . . . . . 249

CHAPTER 12 HYDRONICS . . . . . . . . . . . . . . . . 255

1201.0 General. . . . . . . . . . . . . . . . . . . . . 257

1201.1 Applicability. . . . . . . . . . . . . . . . . . 257

1201.2 Insulation . . . . . . . . . . . . . . . . . . . 257

1201.3 Water Hammer Protection . . . . . . 257

1201.4 Terminal Units . . . . . . . . . . . . . . . 257

1201.5 Return-Water Low-Temperature

Protection . . . . . . . . . . . . . . . . . . . 257

1201.6 Heat Transfer Fluid Quality. . . . . . 257

1201.7 Heat Emitters . . . . . . . . . . . . . . . . 257

1201.8 Mechanical Devices . . . . . . . . . . . 257

1201.9 Flexible Connectors . . . . . . . . . . . 257

1201.10 Freeze Protection . . . . . . . . . . . . . 257

Table 1201.10 Percent Glycol Mixtures . . . . . . . . 257

1202.0 Protection of Potable Water

Supply. . . . . . . . . . . . . . . . . . . . . . 257

1202.1 Prohibited Sources . . . . . . . . . . . . 257

1202.2 Chemical Injection . . . . . . . . . . . . 257

1202.3 Compatibility . . . . . . . . . . . . . . . . . 258

1203.0 Capacity of Heat Source. . . . . . . . 258

1203.1 Heat Source . . . . . . . . . . . . . . . . . 258

1203.2 Dual Purpose Water Heaters . . . . 258

Table 1203.2 Water Heaters . . . . . . . . . . . . . . . 258

1203.3 Tankless Water Heaters . . . . . . . 258

1204.0 Identification of Potable and

Nonpotable Water Systems . . . . . 258

Frequently asked questions

Can a return air grille be placed in a bathroom or kitchen?

No. Bathrooms and kitchens are listed as prohibited sources for outside or return air per § 311.3 (5). Use a compliant return location or dedicated outside air per the ventilation rules.

May I use attic air as return air?

No. An unconditioned attic is specifically listed as a prohibited source in § 311.3 (5). Do not use attic air as the return source.

When is it acceptable to take return air from a room with a furnace?

Only when the listed exceptions in § 311.3 (6) Exception (3) are met: the room volume meets 1 cu ft per 10 Btu/h, ≥ 75% of supply air returns to the same room, and the return inlet is ≥ 10 ft from the appliance firebox/draft diverter.

Can return air from one apartment flow into another via the HVAC system?

No. § 311.4 expressly prohibits return air from one dwelling unit discharging into another. Design separate return/air boundaries.

If a source area is less than 25% of the served volume, what can I do?

You must provide a permanent opening to an area whose volume equals 25% of the served volume, or else the intake from that small area is not allowed (see § 311.3 (4)). In dwellings, a reduced opening to 50% is permitted under the specific dwelling exception coupled with a room/hall having three doors to other rooms.

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