Title 24 · California Energy Code

How does the Mechanical Code (CMC) prescribe duct leakage tests and allowable leakage?

The Mechanical Code requires certified technicians to pressure‑test representative duct sections and compare measured leakage to a code formula (Lmax = CL × P). Sampling starts at 10% (CMC) or 20% (Appendix E where used) and expands to 40% or 100% if failures occur; tests must be run at the duct design pressure class and documented per the referenced sections.

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Last reviewed: July 6, 2026

What the code requires — plain English (controlling §)

The Mechanical Code requires that duct leakage be measured with a certified technician and compared to a maximum permitted leakage calculated by the CMC formula in § 603.9.2. Representative sections of the duct system must be pressure‑tested (sampling steps: initial sample, expand to 40% if initial fails, then 100% if needed) and tests must be performed at the duct design pressure class. See § 603.9.2 for the formula and sampling rules and Appendix E (functional-testing procedures) for the field test and acceptance steps referenced by E805.4.

The single most important rule: allowed leakage is computed from the CMC formula Lmax = CL × P (with the code’s conversion factor) and tests must be run at the duct’s design pressure class; the tested sections must meet the sampling and pass/fail progression spelled out in § 603.9.2 and Appendix E.

Requirements in detail

Scope and who may test

Tests must be performed by a technician certified by AABC, NEBB, TABB, or an equivalent approved agency, per § 603.9.2.
Where Title 24 Part 6 (the California Energy Code) sets targets for a system, Part 6 procedures/targets apply; systems not covered by those Part 6 targets are to follow § 603.9.2.

Sampling / selection of sections to test

Under § 603.9.2 the initial representative sample must be not less than 10% of total installed duct area; if that sample fails, expand to 40%; if that fails, test 100%. Sections are selected by the building owner (or owner’s rep).
Appendix E (where functional testing for certain systems is specified) uses a different baseline sampling in some places: not less than 20% as the initial sample, then 40% then 100% per the Appendix E procedure (Equation and sampling text in E 503.4.7.2.1). Use the Appendix E sampling where E805.4 references it for functional testing.

Test pressure and permitted leakage (formula)

The maximum permitted leakage is given by the CMC formula in § 603.9.2:
- Lmax = CL × P [0.65] (Equation in § 603.9.2). Units are (ft3/min)/100 ft2 duct surface area. CL is the duct leakage class (the CMC sets CL = 6 at 1 in. w.c.); P is the test pressure, which must equal the design duct pressure class rating (in inches water column).
Appendix E repeats the same form for the permitted leakage (Equation E 503.4.7.2.1).

Field procedure references and acceptance (Appendix E / E805.4)

The field functional test procedure and acceptance criteria for air distribution systems are in Appendix E and specifically incorporated into § E805.4 (E805.4.1 test procedure; E805.4.2 acceptance criteria). E805.4.1.2 directs testers to perform the duct leakage test in accordance with E 503.4.7.2.1. Acceptance checks also include visual verification items (flex ducts not crushed, joints and seams sealed, insulation R-values where required).

Table — decision‑relevant values and where to find them

Decision / value	What to use / value	Why it matters	Code reference
Maximum permitted leakage formula	Lmax = CL × P [0.65]	Calculate allowable leakage per 100 ft² of duct surface	§ 603.9.2
Duct leakage class (CL)	6 (at 1 in. w.c.)	Base leakage class used in formula	§ 603.9.2
Test pressure (P)	Equal to design duct pressure class rating (in. w.c.)	Tests must be performed at design pressure	§ 603.9.2
Initial sampling (CMC)	10% of total installed duct area	Minimum sections to test under § 603.9.2	§ 603.9.2
Initial sampling (Appendix E)	20% of total installed duct area	Minimum sections to test under Appendix E functional test	E 503.4.7.2.1
Sampling escalation	If initial sample fails → test 40%; if 40% fails → test 100%	Ensures failures trigger broader testing	§ 603.9.2 and E 503.4.7.2.1
Who may select sections to test	Building owner or their designated representative	Owner-appointed selection governs which sections are sampled	§ 603.9.2
Technician qualification	AABC, NEBB, TABB or equivalent	Ensures test quality and traceability	§ 603.9.2
Appendix E acceptance checks	Flexible ducts not compressed; joints sealed; insulation R-values verified; sticker on air handler access	Additional acceptance items beyond leakage numbers	E 805.4.1.1 / E 805.4.2

Exceptions & special cases

Transfer air ducts operating at less than 1 in. w.c. are excepted from the § 603.9.2 requirement (see exception text in § 603.9.2).
If a system is covered by Title 24 Part 6 leakage targets (for example the 6% or 15% acceptance paths found in the Energy Code for certain systems), those Part 6 procedures and thresholds apply; otherwise CMC § 603.9.2 is the path. The Energy Code explicitly directs systems not subject to Part 6 testing to CMC § 603.9.2.
Appendix E functional testing rules apply where E805.4 is invoked (new systems and certain alterations): Appendix E uses the E 503.4.7.2.1 sampling and acceptance process rather than CMC’s 10% starting point in some contexts—read the applicable appendix section for the project type.

Common mistakes

Testing at the wrong pressure: the test pressure must equal the design duct pressure class rating (P), not an arbitrary or arbitrary 25 Pa number. See § 603.9.2.
Confusing units: the CMC formula yields leakage expressed as (ft3/min) per 100 ft2 of duct surface area — the allowable leakage must be converted to the system’s total allowed cfm by scaling from the per‑100‑ft2 basis. See § 603.9.2.
Using the wrong sampling rule: Appendix E’s functional tests use a 20% initial sample in the places it applies; CMC § 603.9.2 uses 10%. Use the correct rule for the system and project—don’t mix them.
Not using a certified tester: the code explicitly requires certification (AABC, NEBB, TABB or equivalent) — an unqualified test report can be rejected. § 603.9.2.
Forgetting owner selection of sections: the building owner (or their rep) selects sample sections for testing; document this selection. § 603.9.2.

Worked example — concrete numbers

Scenario: A new HVAC system has a total duct surface area of 1,200 ft², design duct pressure class = 1 in. w.c., and CL = 6 (per the code).

Compute Lmax per 100 ft² using the CMC formula in § 603.9.2:
- Lmax = CL × P × 0.65 = 6 × 1 × 0.65 = 3.9 (ft³/min) per 100 ft².
Convert to total allowed leakage for the system:
- Number of 100 ft² units in 1,200 ft² = 1,200 / 100 = 12.
- Total allowed leakage = 3.9 × 12 = 46.8 cfm (total system leakage allowed).
Sampling and testing:
- Under § 603.9.2 test at least 10% of the system by duct surface area as representative sections (here 10% of 1,200 ft² = 120 ft² of duct to be tested first). If that sample passes (measured leakage ≤ permitted for the tested sections), the system passes. If the 10% sample fails, test 40% (480 ft²); if that fails, test 100%.
Field procedure:
- Have a certified AABC/NEBB/TABB technician perform the pressurization test at 1 in. w.c. and report measured cfm per 100 ft² for each tested section, compare to 3.9 cfm/100 ft² allowable, and scale to the system total as needed.

(If the project is under the Energy Code acceptance paths that specify 6% or 15% leak‑of‑fan‑flow limits, those Part 6 numbers and procedures replace or supplement the CMC approach for the systems they cover — see Title 24 guidance.)

Related provisions (quick list)

§ 603.9.2 — Duct leakage tests, formula, sampling and technician qualifications (CMC).
E 503.4.7.2.1 — Appendix E duct leakage test procedure and permitted leakage equation (Appendix E).
E 805.4 / E 805.4.1 / E 805.4.2 — Field test procedures and acceptance criteria for air distribution systems (functional testing) that reference E 503.4.7.2.1.
Title 24 / Part 6 references — where the Energy Code defines alternate acceptance paths (6% / 15% rules) and directs systems not covered there to § 603.9.2.
§ 603.9.1 — Joints, seams and closure systems requirements that should be satisfied prior to testing (sealing materials and markings).

Code references

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

§ 0.65 High relevance — show source text
Duct leakage tests shall be performed by a technician certified by the Associated Air Balance Council (AABC), the National Environmental Balancing Bureau (NEBB), the Testing, Adjusting and Balancing Bureau (TABB), or other equivalent approved agencies. Representative sections totaling not less than 10 percent of the total installed duct area shall be tested. Where the tested 10 percent fail to comply with the requirements of this section, then 40 percent of the total installed duct area shall be tested. Where the tested 40 percent fail to comply with the requirements of this section, then 100 percent of the total installed duct area shall be tested. Sections shall be selected by the building owner or designated representative of the building owner. Positive pressure leakage testing shall be permitted for negative pressure ductwork. The permitted duct leakage shall be not more than the following:

Lmax = CLP [0.65 ] (Equation 603.9.2)

Where:

Lmax = maximum permitted leakage, (ft [3] /min)/100 square feet [0.0001 (m [3] /s)/m [2] ] duct surface

area.

CL = six, duct leakage class, (ft [3] /min)/100 square feet [0.0001 (m [3] /s)/m [2] ] duct surface area at 1 inch water column (0.2 kPa).

P = test pressure, which shall be equal to the design duct pressure class rating, inch water column (kPa).

Exception: Transfer air duct operating at less than 1 inch of water column (0.25 kPa). 603.9.2.1 Duct Leakage Tests for Buildings that Meet Air Distribution System Duct Leak- age Sealing Criteria in Title 24, Part 6. For duct leakage testing, see California Energy Code Sec- tions 150.0(m)(11) for single family buildings, Sec- tion 160.3(b) for multifamily buildings, and Section 120.4(g) for nonresidential and Hotel Motel build- ings. 603.10 Cross Contamination. Exhaust ducts that convey Class 4 air shall be negatively pressurized relative to ducts, plenums, or occupiable spaces through which the ducts pass. Exhaust ducts under positive pressure that convey Class 2 or Class 3 air shall not extend into or pass through ducts, plenums, or occupiable spaces other than the space from which the exhaust air is drawn.

603.11 Underground Installation. Ducts installed underground shall be approved for the installation and shall have a slope of not less than [1] ⁄ 8 inch per foot (10.4 mm/m) back to the main riser. Ducts, plenums, and fittings shall be permitted to be constructed of concrete, clay, or ceramics where installed in the ground or in a concrete slab, provided the joints are sealed and duct is secured in accordance with SMACNA

HVAC Duct Construction Standards – Metal and Flexible.

Metal ducts where installed in or under a concrete slab shall be encased in not less than 2 inches (51 mm) of concrete, secured in accordance with SMACNA HVAC Duct Con struction Standards – Metal and Flexible.

§ 603.9.1 High relevance — show source text

DUCT SYSTEMS

603.9.1 Closure Systems. Joints and seams and reinforcements for air ducts and plenums listed and labeled to UL 181, shall be in accordance with the manufacturer’s installation instructions. Closure systems for sealing air ducts and plenums shall be listed and labeled in accordance with UL 181A or UL 181B, and marked in accordance with Table 603.9.1.

TABLE 603.9.1

CLOSURE MARKINGS

TYPE OF DUCTWORK	STANDARD	TYPE OF CLO- SURE SYSTEM	MARKING
Rigid Metallic or Rigid Fiberglass	UL 181A	Pressure Sensitive Tape	181A-P
Rigid Metallic or Rigid Fiberglass	UL 181A	Mastic Tape	181A-M
Rigid Metallic or Rigid Fiberglass	UL 181A	Heat Sensitive Tape	181A–H
Flexible Air Ducts	UL 181B	Pressure Sensitive Tape*	181B-FX*
Flexible Air Ducts	UL 181B	Mastic*	181B-M*

Mechanical fasteners shall be used in conjunction with a listed pressure sensitive tape or mastic in accordance with UL 181. Nonmetallic mechanical fasteners shall be listed and labeled in accordance with UL 181B and

labeled “181B-C.”

603.9.2 Duct Leakage Tests. [Not adopted by HCD] Ductwork shall be leak-tested in accordance with the SMACNA HVAC Air Duct Leakage Test Manual. Duct leakage tests shall be performed by a technician certified by the Associated Air Balance Council (AABC), the National Environmental Balancing Bureau (NEBB), the Testing, Adjusting and Balancing Bureau (TABB), or other equivalent approved agencies. Representative sections totaling not less than 10 percent of the total installed duct area shall be tested. Where the tested 10 percent fail to comply with the requirements of this section, then 40 percent of the total installed duct area shall be tested. Where the tested 40 percent fail to comply with the requirements of this section, then 100 percent of the total installed duct area shall be tested. Sections shall be selected by the building owner or designated representative of the building owner. Positive pressure leakage testing shall be permitted for negative pressure ductwork. The permitted duct leakage shall be not more than the following:

Lmax = CLP [0.65 ] (Equation 603.9.2)

Where:

Lmax = maximum permitted leakage, (ft [3] /min)/100 square feet [0.0001 (m [3] /s)/m [2] ] duct surface

area.

CL = six, duct leakage class, (ft [3] /min)/100 square feet [0.0001 (m [3] /s)/m [2] ] duct surface area at 1 inch water column (0.2 kPa).

P = test pressure, which shall be equal to the design duct pressure class rating, inch water column (kPa).

Exception: Transfer air duct operating at less than 1 inch of water column (0.25 kPa).

§ 805.4 High relevance — show source text

New single zone supply ductwork must be less than 6% leakage rate per Section E 805.4 through Section E 805.4.2, existing single zone ductwork must be less than 15% leakage or other compliance path per Section E 805.4 through Section E 805.4.2.

Construction Inspection

Scope of test – New Buildings – this test required on New Buildings only if all check boxes 1(a) through 1(c) are checked.

Existing Buildings – this test required if 1(a) through 1(d) are checked. Ductwork conforms to the following (note if any of these are not checked, then this test is not required):  1(a) Connected to a constant volume, single zone air conditioners, heat pumps, or furnaces.

 1(b) Serves less than 5000 square feet of floor area.

 1(c) Has more than 25% duct surface area located in one or more of the following spaces.

– Outdoors. – A space directly under a roof where the U-factor of the roof is greater than U-factor of the ceiling. – A space directly under a roof with fixed vents or openings to the outside or unconditioned spaces. – An unconditioned crawl space. – Other unconditioned spaces.

 1(d) A duct is extended or any of the following replaced: air handler, outdoor condensing unit of a split system, cooling or heating coil, or the furnace heat exchanger. 2. Instrumentation to perform test includes: a. Duct Pressure Test.

Material and Installation. Complying new duct systems shall have a checked box for all of the following categories (a) through (g):

a. Choice of drawbands. (check one of the following)

Col1		Stainless steel worm-drive hose clamps.
		UV-resistant nylon duct ties.
	b. Flexible ducts are not constricted in any way.	b. Flexible ducts are not constricted in any way.
	c. Duct leakage tests performed before access to ductwork and connections are blocked.	c. Duct leakage tests performed before access to ductwork and connections are blocked.
	d. Joints and seams are not sealed with cloth back rubber adhesive tape unless used in combination with mastic and drawbands.	d. Joints and seams are not sealed with cloth back rubber adhesive tape unless used in combination with mastic and drawbands.
	e. Duct R-values are verified R-8 per Section E 805.4 through Section E 805.4.2.	e. Duct R-values are verified R-8 per Section E 805.4 through Section E 805.4.2.
	f. Ductwork located outdoors has insulation that is protected from damage and suitable for outdoor service.	f. Ductwork located outdoors has insulation that is protected from damage and suitable for outdoor service.
	g. A sticker has been affixed to the exterior surface of the air handler access door per Section E 805.4 through Section E 805.4.2.	g. A sticker has been affixed to the exterior surface of the air handler access door per Section E 805.4 through Section E 805.4.2.

For SI units: 1 square foot = 0.0929 m [2]

§ 21.7 High relevance — show source text

kBtu/h [ (Line # 1b) x 21.7 ft3/min/kBtu/h].||| |3.|Total calculated supply fan flow 2(a) or 2(b) ft3/min.|||

NEW CONSTRUCTION OR ENTIRE NEW DUCT SYSTEM ALTERATION:	Col2	Col3	Col4
	Duct pressurization test results (ft3/min @ 25 Pa).	Duct pressurization test results (ft3/min @ 25 Pa).	Duct pressurization test results (ft3/min @ 25 Pa).
4.	Enter tested leakage flow in ft3/min:		P P
5.	Pass if leakage percentage≤6%: [( Line #4)/(Line #3 )] x 100	%	 Pass Fail
ALTERATIONS: Pre-existing duct system with duct alteration and/or HVAC equipment change-out.	ALTERATIONS: Pre-existing duct system with duct alteration and/or HVAC equipment change-out.	ALTERATIONS: Pre-existing duct system with duct alteration and/or HVAC equipment change-out.	ALTERATIONS: Pre-existing duct system with duct alteration and/or HVAC equipment change-out.
6.	Enter tested leakage flow (cubic feet per minute): Pre-test of existing duct system prior to duct system alteration, equipment change-out, or both.
7.	Enter tested leakage flow (cubic feet per minute): Final test of new duct system or altered duct system for duct system alteration, equipment change-out, or both.
TEST OR VERIFICATION STANDARDS: For altered duct system and/or HVAC equipment change-out use one of the following three tests or verification standards for compliance:	TEST OR VERIFICATION STANDARDS: For altered duct system and/or HVAC equipment change-out use one of the following three tests or verification standards for compliance:	TEST OR VERIFICATION STANDARDS: For altered duct system and/or HVAC equipment change-out use one of the following three tests or verification standards for compliance:	TEST OR VERIFICATION STANDARDS: For altered duct system and/or HVAC equipment change-out use one of the following three tests or verification standards for compliance:
8.	Pass if leakage percentage <15% [ (Line # 7) / (Line # 3 )] x 100	%	 Pass Fail
9.	Pass if leakage reduction percentage >60% Leakage reduction = [1 - [ (Line#7) / (Line#6 )] } x 100	%	 Pass Fail
10.	Pass if all accessible leaks are sealed as confirmed by visual inspection and verifica- tion by HERS rater (sampling rate 100%).	%	 Pass Fail
	Pass if One of Lines #8 through #10 pass		 Pass Fail

For SI units: 1000 British thermal units per hour = 0.293 kW, 1 cubic foot per minute = 0.00047 m [3] /s, 1 metric ton = 1000 kg

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APPENDIX E

§ 120.4 High relevance — show source text
The system is not a single package air-cooled commercial unitary air conditioner or heat pump; and 2. The cooling capacity of the system is less than 54,000 Btu/h. D. Altered duct systems. New or replacement space-conditioning system ducts installed to serve an existing building shall meet the requirements of Section 120.4 (a) through (f) and meet i, ii or iii below: i. Entirely new or complete replacement duct systems installed as part of an alteration shall be leakage tested in accordance with Section 120.4(g). This applies to replacement duct systems installed as part of an alteration that are constructed of at least 75 percent new duct material. Up to 25 percent of that alteration may consist of reused parts from the building’s existing duct system (including registers, grilles, boots, air handlers, coils, plenums and ducts) if the reused parts are accessible and can be sealed to prevent leakage. ii. If the new ducts are an extension of an existing duct system and the combined new and existing duct system meets the criteria in Subsections 1, 2, 3 and 4 below, the duct system shall be sealed to a leakage rate not to exceed 15 percent of the nominal air handler airflow rate as confirmed through acceptance testing, in accordance with the applicable procedures in Reference Nonresidential Appendix NA7.5.3:
1. The duct system does not serve a healthcare facility;
2. The duct system provides conditioned air to an occupiable space for a constant volume, single zone, space-conditioning system;
3. The space-conditioning system serves less than 5,000 square feet of conditioned floor area; and
4. The combined surface area of the ducts located outdoors or in unconditioned space is more than 25 percent of the total surface area of the entire duct system. Exception 1 to Section 141.0(b)2Dii: When it is not possible to achieve the duct leakage criteria in Section 141.0(b)2Dii, then all accessible leaks shall be sealed and verified through a visual inspection and a smoke test performed by a certified mechanical acceptance test technician utilizing the methods specified in Reference Nonresidential Appendix NA7.5.3. Exception 2 to Section 141.0(b)2Dii: Duct sealing. Existing duct systems that are extended, which are constructed insulated or sealed with asbestos are not required to comply with Subsection 141.0(b)2Dii. iii. If new ducts installed as part of an alteration are not required to comply with leakage testing specified by Section 141.0(b)2Di or 141.0(b)2Dii, then the new ducts shall meet the duct leakage testing requirements of CMC Section 603.9.2.
E. Altered space-conditioning systems. When a space-conditioning system is altered by the installation or replacement of space-conditioning system equipment (including replacement of the air handler, outdoor condensing unit of a split system air conditioner or heat pump, or cooling or heating coil: i. For all altered units where the existing thermostat does not comply with the requirements for demand responsive controls specified in Section 110.12, the existing thermostat shall be replaced with a demand responsive thermostat that complies with Section 110.12. All newly installed space-conditioning systems requiring a thermostat shall be equipped with a demand responsive thermostat that complies with Section 110.12; and ii. The duct system that is connected to the new or replaced space-conditioning system equipment shall be sealed in accordance with Section 141.0(b)2Dii.
§ 503.4.7.1.4 High relevance — show source text
E 503.4.7.1.4 Radiant Floor Heating. The bottom surfaces of floor structures incorporating radiant heating shall be insulated with a minimum of R-3.5. Adjacent building envelope insulation counts toward this requirement.

Exception: Heated slab-on-grade floors incorporating radiant heating shall be in accordance with ASHRAE 90.1. [ASHRAE 90.1:6.4.4.1.5]

E 503.4.7.2 Ductwork and Plenum Leakage. Transverse joints, longitudinal seams, and duct wall penetrations shall be sealed. Pressure-sensitive tape shall not be used as the primary sealant, unless it has been certified to comply with UL 181A or UL 181B by an independent testing laboratory and the tape is used in accordance with that certification. All other connections shall be considered transverse joints, including but not limited to spin-ins, taps, other branch connections, access door frames and jambs, and duct connections to equipment.

Exceptions:

(1) Rods that penetrate the duct wall that shall be permitted to move in order to function properly (control rod for volume damper) shall not be sealed in a fashion that prevents them from working properly.

(2) Spiral lock seams in a round or flat oval duct.

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APPENDIX E

E 503.4.7.2.1 Duct Leakage Tests. Ductwork shall be leak-tested in accordance with the SMACNA HVAC Air Duct Leakage Test Manual. Representative sections totaling not less than 20 percent of the total installed duct area shall be tested. Where the tested 20 percent fail to comply with the requirements of this section, then 40 percent of the total installed duct area shall be tested. Where the tested 40 percent fail to comply with the requirements of this section, then 100 percent of the total installed duct area shall be tested. Sections shall be selected by the building owner or designated representative of the building owner. Positive pressure leakage testing shall be permitted for negative pressure ductwork. The permitted duct leakage shall be not more than the following: Lmax = CLP [0.65] (Equation E 503.4.7.2.1)

Where:

Lmax = maximum permitted leakage, (ft [3] /min)/100 square feet [0.0001 (m [3] /s)/m [2] ] duct surface area.

CL = Six, duct leakage class, (ft [3] /min)/100 square feet [0.0001 (m [3] /s)/m [2] ] duct surface area at 1 inch water column (0.2 kPa).

P = test pressure, which shall be equal to the design duct pressure class rating, inch water column (kPa).

E 503.4.8 Walk-In Coolers and Walk-In Freezers.

Site-assembled or site-constructed walk-in coolers and walk-in freezers shall conform to the following requirements:

(1) Shall be equipped with automatic door closers that firmly close walk-in doors that have been closed to within 1 inch (25.4 mm) of full closure.

Exception: Doors wider than 45 inches (1143 mm) or taller than 7 feet (2134 mm).
§ 110.8. High relevance — show source text
(c) All duct insulation product R -values shall be based on insulation only (excluding air films, vapor retarders or other duct components) and tested C -values at 75°F mean temperature at the installed thickness, in accordance with ASTM C518 or ASTM C177, incorporated herein by reference, and certified pursuant to Section 110.8.

(d) The installed thickness of duct insulation used to determine its R -value shall be determined as follows:
1. For duct board, duct liner and factory-made rigid ducts not normally subjected to compression, the nominal insulation thickness shall be used.
2. For duct wrap, installed thickness shall be assumed to be 75 percent (25 percent compression) of nominal thickness.
3. For factory-made flexible air ducts, the installed thickness shall be determined by dividing the difference between the actual outside diameter and nominal inside diameter by two.
(e) Insulated flexible duct products installed to meet this requirement must include labels, in maximum intervals of 3 feet, showing the thermal performance R -value for the duct insulation itself (excluding air films, vapor retarder or other duct components), based on the tests in Section 120.4(c) and the installed thickness determined by Section 120.4(d)3.

(f) Protection of insulation. Insulation shall be protected from damage, including that due to sunlight, moisture, equipment maintenance and wind, but not limited to the following:

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NONRESIDENTIAL, HOTEL/MOTEL OCCUPANCIES, AND COVERED PROCESSES—MANDATORY REQUIREMENTS

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 that can cause degradation of the material.

(g) Duct sealing. Duct systems shall comply with Subsection 1 or 2 below:
1. New duct systems that meet the criteria in Subsections A, B, C and D below shall be sealed to a leakage rate not to exceed 6 percent of the nominal air handler airflow rate as confirmed through acceptance testing, in accordance with Reference Nonresidential Appendix NA7.5.3; A. The duct system does not serve a healthcare facility; and B. The duct system provides conditioned air to an occupiable space for a constant volume, single zone, space-conditioning system; and C. The space-conditioning system serves less than 5,000 square feet of conditioned floor area; and D. The combined surface area of the ducts located outdoors or in unconditioned space is more than 25 percent of the total surface area of the entire duct system.
2. New duct systems that are not subject to testing under Section 120.4(g)1 shall instead meet the duct leakage testing requirements of CMC Section 603.9.2.
Note: Authority: Sections 25213, 25218, 25218.5, 25402 and 25402.1, Public Resources Code. Reference: Sections 25007, 25008, 25218.5, 25310, 25402, 25402.1, 25402.4, 25402.8, and 25943, Public Resources Code.

SECTION 120.5—REQUIRED NONRESIDENTIAL MECHANICAL SYSTEM ACCEPTANCE

§ 605.1.3.1 High relevance — show source text

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APPENDIX E

Col1	TABLE E 605.1.3.1 VENTILATION AIR REQUIREMENTS, (cubic foot per minute) [ASHRAE 62.2: TABLE 4-1a]	Col3	Col4	Col5	Col6
FLOOR AREA (ft2)	BEDROOMS	BEDROOMS	BEDROOMS	BEDROOMS	BEDROOMS
FLOOR AREA (ft2)	1	2	3	4	5
<500	30	38	45	53	60
501-1000	45	53	60	68	75
1001-1500	60	68	75	83	90
1501-2000	75	83	90	98	105
2001-2500	90	98	105	113	120
2501-3000	105	113	120	128	135
3001-3500	120	128	135	143	150
3501-4000	135	143	150	158	165
4001-4500	150	158	165	173	180
4501-5000	165	173	180	188	195

For SI units: 1 square foot = 0.0929 m [2], 1 cubic foot per minute = 0.00047 m [3] /s

Exception: Where Qfan, calculated for unbalanced ventilation, is less than or equal to 15 CFM (7 L/s), a dwelling-unit ventilation system is not required.

[ASHRAE 62.2:4.1.2] E 605.1.3.4 Effective Annual Average Infiltra- tion Rate ( Qinf ) Using a Multipoint Envelope Leakage Test. Effective Annual Average Infiltration Rate ( Qinf ) shall be calculated using the normalized leakage calculated from measurements of envelope leakage using a multipoint test from either ASTM E779 in accordance with Section E 605.1.3.4(A) or CGSB 149.10 in accordance with Section E 605.1.3.4(B). E 605.1.3.4(A) ASTM Procedure. To calculate the effective leak area ( ELA ) from ASTM E779, the leakage area for pressurization and depressurization (using a 0.0006 psi [4 Pa] reference pressure) shall be averaged using Equation E 605.1.3.4(A):

[Equation E 605.1.3.4(A)]

ELA = ( Lpress + Ldepress )/ 2

Where:

ELA = effective leakage area, ft [2] (m [2] ) Lpress = leakage area from pressurization, ft [2 ] (m [2] ) Ldepress = leakage area from depressurization, ft [2 ] (m [2] )

For SI units: 1 square foot = 0.0929 m [2]

§ 180.2 High relevance — show source text
G. Insulation shall be protected from damage, including that due to sunlight, moisture, equipment maintenance and wind but not limited to the following: 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 that can cause degradation of the material. H. Duct systems shall be tested in accordance with i or ii below: i. New duct systems that meet the criteria in Subsections a, b and c below or ductwork that is part of a system that meets the criteria of Section 180.2(b)2B shall be sealed to a leakage rate not to exceed 6 percent of the nominal air handler airflow rate as confirmed through acceptance testing, in accordance with Reference Nonresidential Appendix NA7.5.3. a. The duct system provides conditioned air to an occupiable space for a constant volume, single zone, space-conditioning system; and b. The space-conditioning system serves less than 5,000 square feet of conditioned floor area; and c. The combined surface area of the ducts located in the following spaces is more than 25 percent of the total surface area of the entire duct system:

I. Outdoors; or

II. In a space directly under a roof that has a U- factor greater than the U- factor of the ceiling, or if the roof does not meet the requirements of Section 170.2(a)1; or III. In a space directly under a roof that has fixed vents or openings to the outside or unconditioned

spaces; or IV. In an unconditioned crawl space; or V. In other unconditioned spaces. ii. All duct systems that do not meet the criteria in Section 160.3(c)2H shall meet the duct leakage testing requirements of CMC Section 603.9.2.

(d) Mechanical acceptance testing.
1. Common areas. Before an occupancy permit is granted, the following systems and equipment serving multifamily common areas shall be certified as meeting the Acceptance Requirements for Code Compliance, as specified by Reference Nonresidential Appendix NA7. These systems and equipment shall also comply with the applicable requirements of Section
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MULTIFAMILY BUILDINGS—MANDATORY REQUIREMENTS

160.3(d)3. A Certificate of Acceptance shall be submitted to the enforcement agency that certifies that the equipment and systems meet the acceptance requirements: A. Outdoor air ventilation systems shall be tested in accordance with NA7.5.1. B. Constant volume, single zone air conditioning and heat pump unit controls shall be tested in accordance with NA7.5.2. C. Duct systems shall be tested in accordance with NA7.5.3 where either: a. They are new duct systems; or b. They are part of an altered system. D. Air economizers, DOAS, HRV or ERV systems shall be tested in accordance with NA7.5.4. Exception to Section 160.3(d)1D: Air economizers installed by the HVAC system manufacturer and certified to the Commission as being factory calibrated and tested are not required to comply with the Functional Testing section of the Air Economizer Controls acceptance test as described in NA7.5.4.2. E. Demand control ventilation systems required by Section 160.2(c)3 shall be tested in accordance with NA7.5.5.

§ 805.4 High relevance — show source text

Col1		Stainless steel worm-drive hose clamps.
		UV-resistant nylon duct ties.
	b. Flexible ducts are not constricted in any way.	b. Flexible ducts are not constricted in any way.
	c. Duct leakage tests performed before access to ductwork and connections are blocked.	c. Duct leakage tests performed before access to ductwork and connections are blocked.
	d. Joints and seams are not sealed with cloth back rubber adhesive tape unless used in combination with mastic and drawbands.	d. Joints and seams are not sealed with cloth back rubber adhesive tape unless used in combination with mastic and drawbands.
	e. Duct R-values are verified R-8 per Section E 805.4 through Section E 805.4.2.	e. Duct R-values are verified R-8 per Section E 805.4 through Section E 805.4.2.
	f. Ductwork located outdoors has insulation that is protected from damage and suitable for outdoor service.	f. Ductwork located outdoors has insulation that is protected from damage and suitable for outdoor service.
	g. A sticker has been affixed to the exterior surface of the air handler access door per Section E 805.4 through Section E 805.4.2.	g. A sticker has been affixed to the exterior surface of the air handler access door per Section E 805.4 through Section E 805.4.2.

For SI units: 1 square foot = 0.0929 m [2]

500 2025 CALIFORNIA MECHANICAL CODE

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APPENDIX E

CERTIFICATE OF ACCEPTANCE MECH-4A	Col2
Air Distribution Systems Acceptance (Page 3 of 3)	Air Distribution Systems Acceptance (Page 3 of 3)
Project Name/Address:	Project Name/Address:
System Name or Identification/Tag:	System Location or Area Served:

Air Distribution System Leakage Diagnostic. The installing contractor must pressure test every new HVAC systems that meet the requirements of Section E 805.4 through Section E 805.4.2 and every retrofit to existing HVAC systems that meet the requirements of Section E 805.4 through Section E 805.4.2.	Col2	Col3	Col4
RATED FAN FLOW (applies to all systems)	RATED FAN FLOW (applies to all systems)	Measured Values	Measured Values
1.	Cooling capacity or for heating only units heating capacity.
	(a) Cooling capacity (for all units but heating only units) in tons.
	(b) Heating capacity (for heating only units) kBtu/h.
2.	Fan flow calculation
	(a) Cooling capacity in tons [ (Line # 1a) x 400 ft3/min/ton].
	(b) Heating only cap.

§ 90.1 High 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.

2025 CALIFORNIA MECHANICAL CODE 425

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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]

§ 510.5.3.2 High relevance — show source text
510.5.3.2 Welded Duct Connection. Acceptable duct-to-duct connection shall be as follows:

116 2025 CALIFORNIA MECHANICAL CODE

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EXHAUST SYSTEMS

(1) Telescoping joint, as shown in Figure 510.5.3.2(1).

(2) Bell-type joint, as shown in Figure 510.5.3.2(2).

(3) Flange with edge weld, as shown in Figure 510.5.3.2(3).

(4) Flange with lap joint weld, as shown in Figure 510.5.3.2(4). [NFPA 96:7.5.5.1]

FIGURE 510.5.3.2(3)

FLANGE WITH EDGE WELD DUCT CONNECTION

[NFPA 96: FIGURE 7.5.5.1(c)]

For SI units: 1 inch = 25.4 mm

Notes:

1 Duct size decreases (going upward) with each telescope.

2 Smaller (inside) duct section shall be above or uphill (on sloped duct), to be self draining into larger (outside) duct.

FIGURE 510.5.3.2(1)

TELESCOPING -TYPE DUCT CONNECTION

[NFPA 96: FIGURE 7.5.5.1(a)]

For SI units: 1 inch = 25.4 mm

Notes:

1 Duct size stays the same throughout the duct system.

2 Smaller (inside) male duct end is always above or uphill (on sloped duct), to be self draining into larger (outside) female duct end.

FIGURE 510.5.3.2(2)

BELL-TYPE DUCT CONNECTION

[NFPA 96: FIGURE 7.5.5.1(b)]

FIGURE 510.5.3.2(4)

FLANGE WITH LAP JOINT WELD DUCT CONNECTION

[NFPA 96: FIGURE 7.5.5.1(d)]

510.5.4 Butt Welded Connections. Butt welded connections shall not be permitted. [NFPA 96:7.5.5.2]

510.5.5 Telescoping and Bell-Type Connections. For telescoping and bell-type connections, the inside duct section shall always be uphill of the outside duct section.

[NFPA 96:7.5.5.3] The overlap shall not exceed 2 inches (51 mm) as shown in Figure 510.5.3.2(1).

510.5.6 Duct Leakage Test. Prior to the use of or concealment of a grease duct system, a leakage test shall be performed to determine that welded joints and seams are liquid tight. The leakage test shall consist of a light test, water pressure test, or an approved equivalent test. The permit holder shall be responsible for providing the necessary equipment and for performing the test. Such test shall be conducted in accordance with ASHRAE 154.

Frequently asked questions

Who must perform the duct leakage test?

A technician certified by AABC, NEBB, TABB, or an equivalent approved agency, as required in § 603.9.2.

At what pressure must the duct be tested?

At the design duct pressure class rating for the system (P in the formula), per § 603.9.2.

How do I know whether to use the CMC 10% sampling or Appendix E 20% sampling?

Use § 603.9.2 (10% initial sample) for systems covered by the CMC path. If functional testing under Appendix E / E805.4 applies to your project, follow Appendix E’s sampling (20% initial) and the Appendix E acceptance steps. Check the Energy Code and Appendix E invocation for your project type.

How is the allowed leakage number expressed?

The code expresses allowed leakage as (ft3/min) per 100 ft² duct surface area using the formula in § 603.9.2; convert that per‑100‑ft² number to a total cfm for the system by scaling to total duct surface area.

If the initial sample fails, what happens?

If the initial sample fails, test 40% of the duct area; if 40% fails, test 100%, per § 603.9.2 (and the same escalation appears in Appendix E where it applies).

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