CRC · California Residential Code

What clearances to combustibles apply to masonry heaters and what exceptions exist?

Masonry heaters normally require a 36‑inch ventilated clearance to combustibles (CRC § R1002.5). That clearance can be reduced to 4 inches only when the heater’s masonry and internal channel walls meet specified thicknesses, or when the heater is listed and installed per UL 1482 / EN 15250. Seismic anchorage rules are in § R1002.4 and do not change the clearance requirements.

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Last reviewed: July 6, 2026

What the code requires — 2–4 sentences

The California Residential Code requires that combustible materials not be placed within 36 inches (914 mm) of the outside surface of a masonry heater — and that the space around the heater be fully vented to permit free air flow. This is the basic clearance rule in § R1002.5.

Combustibles must be kept well away from masonry heaters — normally 36 inches (914 mm), unless a listed exception or minimum masonry thickness reductions apply (see § R1002.5).

Requirements in detail

Scope and controlling paragraph

The clearance rule for masonry heaters is established in § R1002.5. It references NFPA 211 (clearances for solid-fuel-burning appliances) and requires the required space to be fully vented so air can flow around all heater surfaces.

What “within” means and what must be vented

The measurement is from the outside surface of the masonry heater to combustible materials. The required space around the heater surfaces must be ventilated to permit free air flow — this is not a solid enclosure or filled airspace. Fully vented is part of the code requirement in § R1002.5.

Where the code allows smaller clearances (exceptions)

Two specific exceptions appear in § R1002.5:
1. If the heater has walls at least 8 inches (203 mm) thick of solid masonry and the heat-exchange-channel walls are at least 5 inches (127 mm) thick of solid masonry, combustible materials may be placed no closer than 4 inches (102 mm) to the outside surface of the heater. A clearance of not less than 8 inches (203 mm) is required between the gas‑tight capping slab of the heater and a combustible ceiling.
2. Masonry heaters listed and labeled in accordance with UL 1482 or CEN 15250 (EN 15250) may be installed per their listing and the manufacturer’s written instructions (i.e., the listed clearances/manufacturer instructions can govern).

Table — decision‑relevant clearances and where they come from

Situation / dimension	Required clearance (in / mm)	Notes / where measured	Code Reference
Default clearance from heater outside surface to combustibles	36 in (914 mm)	Space must be fully vented around heater surfaces	§ R1002.5
Exception — heavy solid-masonry heater walls	4 in (102 mm)	Heater wall ≥ 8 in (203 mm) solid masonry; heat‑exchange channel walls ≥ 5 in (127 mm)	§ R1002.5
Clearance between gas‑tight capping slab and combustible ceiling	8 in (203 mm)	Measured from gas‑tight capping slab to combustible ceiling	§ R1002.5
Listed or labeled masonry heater (UL 1482 / EN 15250)	Per listing/manufacturer	Manufacturer’s instructions/listing clearances apply	§ R1002.5

What R1002.4 says (and why it matters)

§ R1002.4 addresses seismic reinforcing and anchorage for masonry heaters (when and how they must be anchored), not clearance distances. It does not change the clearance requirements in § R1002.5, but is often relevant to the overall installation and structural detailing of the heater.

Exceptions & special cases

Reduced clearance to 4 inches (102 mm) is allowed only when the heater body and internal channel walls meet the specified minimum solid-masonry thicknesses (8 in / 5 in respectively). This is a construction‑based exception — you must be able to demonstrate the masonry thicknesses called out in § R1002.5.
If the unit is listed and labeled to UL 1482 or EN 15250, install per the listing/manufacturer instructions; those listed clearances can supersede the prescriptive 36‑inch rule if the listing permits less. § R1002.5 allows this.
The 8‑inch clearance from a gas‑tight capping slab to a combustible ceiling is a separate minimum even when the side clearance is reduced per the masonry thickness exception. § R1002.5 requires that specific ceiling clearance.
Note: any clearance reduction method that relies on different standards (e.g., NFPA or listing methods) must be used in the manner the code references; § R1002.5 refers to NFPA 211 for the basic approach.

Common mistakes

Assuming the 36‑inch rule can be filled or enclosed — the code requires the space to be fully vented so air can circulate; filling the airspace is not allowed. § R1002.5.
Using the 4‑inch reduction without verifying both masonry‑wall and channel‑wall thicknesses (8 in / 5 in). The reduction applies only when those thicknesses are met. § R1002.5.
Ignoring listed‑product instructions — if the heater is listed to UL 1482 or EN 15250, the manufacturer’s instructions and listing may permit different clearances; those must be followed per § R1002.5.
Confusing chimney clearance rules with masonry‑heater body clearances — masonry chimneys have their own clearance rules (see related provisions below); do not substitute chimney clearance numbers for heater clearances.

Worked example

Scenario: A homeowner is installing a custom-built masonry heater in a living room with a combustible wood-paneled wall 18 inches away, a combustible bookshelf to the side at 30 inches, and a combustible ceiling at 7 inches above the heater’s gas‑tight capping slab.

Default rule: 36 in (914 mm) clearance required to combustibles — the bookshelf at 30 in and wood‑paneled wall at 18 in do not meet the default clearance. § R1002.5 applies.
Check exception: Does the heater have ≥ 8 in (203 mm) solid‑masonry wall thickness and ≥ 5 in (127 mm) heat‑exchange channel walls? If yes, side clearances may be reduced to 4 in (102 mm). If the heater meets those thicknesses, the bookshelf at 30 in and wall at 18 in would be acceptable (both > 4 in). § R1002.5.
Ceiling clearance: even if the side clearance is reduced, the gas‑tight capping slab must be at least 8 in (203 mm) below any combustible ceiling. In this scenario the ceiling is only 7 in above the capping slab, so it fails the ceiling clearance requirement unless the ceiling is made noncombustible or the heater/manufacturer listing permits otherwise. § R1002.5.
Alternate route: If the manufacturer lists the heater to UL 1482 and their instructions allow the existing clearances, the homeowner may follow the listing/manufacturer instructions instead of the prescriptive numbers in § R1002.5.

Related provisions

§ R1002.4 — Seismic reinforcing and anchorage requirements for masonry heaters (structural, not clearance).
§ R1003.18 — Masonry chimney clearances to combustibles (separate rules for chimneys located inside vs. outside the building). Do not conflate chimney clearances with masonry heater clearances.
NFPA 211 (referenced by § R1002.5) — referenced for clearance methods for solid‑fuel appliances (see § R1002.5).

Code references

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

CRC § 2112.1 High relevance — show source text
SECTION 2112—MASONRY HEATERS

2112.1 Definition. A masonry heater is a heating appliance constructed of concrete or solid masonry, hereinafter referred to as “masonry,” which is designed to absorb and store heat from a solid fuel fire built in the firebox by routing the exhaust gases through internal heat exchange channels in which the flow path downstream of the firebox includes flow in either a horizontal or downward direction before entering the chimney and which delivers heat by radiation from the masonry surface of the heater.

2112.2 Installation. Masonry heaters shall be installed in accordance with this section and comply with one of the following:
1. Masonry heaters shall comply with the requirements of ASTM E1602.
2. Masonry heaters shall be listed and labeled in accordance with UL 1482 or EN 15250 and installed in accordance with the manufacturer’s instructions.
2112.3 Footings and foundation. The firebox floor of a masonry heater shall be a minimum thickness of 4 inches (102 mm) of noncombustible material and be supported on a noncombustible footing and foundation in accordance with Section 2113.2.

2112.4 Seismic reinforcing. In structures assigned to Seismic Design Category D, E or F, masonry heaters shall be anchored to the masonry foundation in accordance with Section 2113.3. Seismic reinforcing shall not be required within the body of a masonry heater with a height that is equal to or less than 3.5 times its body width and where the masonry chimney serving the heater is not

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supported by the body of the heater. Where the masonry chimney shares a common wall with the facing of the masonry heater, the chimney portion of the structure shall be reinforced in accordance with Section 2113.

2112.5 Masonry heater clearance. Combustible materials shall not be placed within 36 inches (914 mm) or the distance of the allowed reduction method from the outside surface of a masonry heater in accordance with NFPA 211, Section 12.6, and the required space between the heater and combustible material shall be fully vented to permit the free flow of air around all heater surfaces.

Exceptions:
1. Where the masonry heater wall thickness is not less than 8 inches (203 mm) of solid masonry and the wall thickness of the heat exchange channels is not less than 5 inches (127 mm) of solid masonry, combustible materials shall not be placed within 4 inches (102 mm) of the outside surface of a masonry heater. A clearance of not less than 8 inches (203 mm) shall be provided between the gas-tight capping slab of the heater and a combustible ceiling.
2. Masonry heaters listed and labeled in accordance with UL 1482 or EN 15250 and installed in accordance with the manufacturer’s instructions.
SECTION 2113—MASONRY CHIMNEYS

2113.1 General. The construction of masonry chimneys consisting of solid masonry units, hollow masonry units grouted solid, stone or concrete shall be in accordance with this section.
CRC § 16.2 High relevance — show source text
2113 A .16.2 Determination of minimum area. The minimum net cross-sectional area of the flue shall be determined in accordance with Figure 2113 A .16. A flue size providing not less than the equivalent net cross-sectional area shall be used. Crosssectional areas of clay flue linings are as provided in Tables 2113 A .16(1) and 2113 A .16(2) or as provided by the manufacturer or as measured in the field. The height of the chimney shall be measured from the firebox floor to the top of the chimney flue.

2113 A .17 Inlet. Inlets to masonry chimneys shall enter from the side. Inlets shall have a thimble of fireclay, rigid refractory material or metal that will prevent the connector from pulling out of the inlet or from extending beyond the wall of the liner.

2113 A .18 Masonry chimney cleanout openings. Cleanout openings shall be provided within 6 inches (152 mm) of the base of each flue within every masonry chimney. The upper edge of the cleanout shall be located not less than 6 inches (152 mm) below the lowest chimney inlet opening. The height of the opening shall be not less than 6 inches (152 mm). The cleanout shall be provided with a noncombustible cover.

Exception: Chimney flues serving masonry fireplaces, where cleaning is possible through the fireplace opening.

2113 A .19 Chimney clearances. Any portion of a masonry chimney located in the interior of the building or within the exterior wall of the building shall have a minimum airspace clearance to combustibles of 2 inches (51 mm). Chimneys located entirely outside the exterior walls of the building, including chimneys that pass through the soffit or cornice, shall have a minimum airspace clearance of 1 inch (25 mm). The airspace shall not be filled, except to provide fireblocking in accordance with Section 2113 A .20.

Exceptions:
1. Masonry chimneys equipped with a chimney lining system listed and labeled for use in chimneys in contact with combustibles in accordance with UL 1777, and installed in accordance with the manufacturer’s instructions, are permitted to have combustible material in contact with their exterior surfaces.
2. Where masonry chimneys are constructed as part of masonry or concrete walls, combustible materials shall not be in contact with the masonry or concrete wall less than 12 inches (305 mm) from the inside surface of the nearest flue lining.
3. Exposed combustible trim and the edges of sheathing materials, such as wood siding, are permitted to abut the masonry chimney sidewalls, in accordance with Figure 2113 A .19, provided that such combustible trim or sheathing is not less than 12 inches (305 mm) from the inside surface of the nearest flue lining. Combustible material and trim shall not overlap the corners of the chimney by more than 1 inch (25 mm).
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FIGURE 2113 A .19—ILLUSTRATION OF EXCEPTION THREE CHIMNEY CLEARANCE PROVISION

MASONRY ABUTTING

COMBUSTIBLE SHEATHING

12" FROM FLUE LINING
CRC § 8-7 High relevance — show source text
R1002.5 Masonry heater clearance. Combustible materials shall not be placed within 36 inches (914 mm) of the outside surface of a masonry heater in accordance with NFPA 211 Section 8-7 (clearances for solid-fuel-burning appliances), and the required space between the heater and combustible material shall be fully vented to permit the free flow of air around all heater surfaces.

Exceptions:
1. Where the masonry heater wall is not less than 8 inches (203 mm) thick of solid masonry and the wall of the heat exchange channels is not less than 5 inches (127 mm) thick of solid masonry, combustible materials shall not be placed within 4 inches (102 mm) of the outside surface of a masonry heater. A clearance of not less than 8 inches (203 mm) shall be provided between the gas-tight capping slab of the heater and a combustible ceiling.
2. Masonry heaters listed and labeled in accordance with UL 1482 or CEN 15250 shall be installed in accordance with the listing specifications and the manufacturer’s written instructions.
SECTION R1003—MASONRY CHIMNEYS

R1003.1 Definition. A masonry chimney is a chimney constructed of solid masonry units, hollow masonry units grouted solid, stone or concrete, hereinafter referred to as masonry. Masonry chimneys shall be constructed, anchored, supported and reinforced as required in this chapter.

R1003.2 Footings and foundations. Footings for masonry chimneys shall be constructed of concrete or solid masonry not less than 12 inches (305 mm) thick and shall extend not less than 6 inches (152 mm) beyond the face of the foundation or support wall on all sides. Footings shall be founded on natural undisturbed earth or engineered fill below frost depth. In areas not subjected to freezing, footings shall be not less than 12 inches (305 mm) below finished grade.

R1003.3 Seismic reinforcing. Masonry or concrete chimneys shall be constructed, anchored, supported and reinforced as required in this chapter. In all structures regulated by this code assigned to Seismic Design Category C, D 0, D 1 or D 2 masonry and concrete chimneys shall be reinforced and anchored as detailed in Sections R1003.3.1, R1003.3.2 and R1003.4. In Seismic Design Category A or B, reinforcement and seismic anchorage are not required.

R1003.3.1 Vertical reinforcing. For chimneys up to 40 inches (1016 mm) wide, four No. 4 continuous vertical bars, anchored in the foundation, shall be placed in the concrete, or between wythes of solid masonry, or within the cells of hollow unit masonry, and grouted in accordance with Section R608.1.1. Grout shall be prevented from bonding with the flue liner so that the flue liner is free to move with thermal expansion. For chimneys more than 40 inches (1016 mm) wide, two additional No. 4 vertical bars shall be installed for each additional 40 inches (1016 mm) in width or fraction thereof.
CRC § 10-6 High relevance — show source text
internal heat exchange channels in which the flow path downstream of the firebox includes flow in a horizontal or downward direction before entering the chimney and that delivers heat by radiation from the masonry surface of the heater.

R1002.2 Installation. Masonry heaters shall be installed in accordance with this section and comply with one of the following:
1. Masonry heaters shall comply with the requirements of ASTM E1602.
2. Masonry heaters shall be listed and labeled in accordance with UL 1482 or CEN 15250 and installed in accordance with the manufacturer’s instructions.
R1002.3 Footings and foundation. The firebox floor of a masonry heater shall be a minimum thickness of 4 inches (102 mm) of noncombustible material and be supported on a noncombustible footing and foundation in accordance with Section R1003.2.

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R1002.4 Seismic reinforcing. In Seismic Design Categories D 0, D 1 and D 2, masonry heaters shall be anchored to the masonry foundation in accordance with Section R1003.3. Seismic reinforcing shall not be required within the body of a masonry heater whose height is equal to or less than 3.5 times its body width and where the masonry chimney serving the heater is not supported by the body of the heater. Where the masonry chimney shares a common wall with the facing of the masonry heater, the chimney portion of the structure shall be reinforced in accordance with Section R1003.

R1002.5 Masonry heater clearance. Combustible materials shall not be placed within 36 inches (914 mm) of the outside surface of a masonry heater in accordance with NFPA 211 Section 8-7 (clearances for solid-fuel-burning appliances), and the required space between the heater and combustible material shall be fully vented to permit the free flow of air around all heater surfaces.

Exceptions:
1. Where the masonry heater wall is not less than 8 inches (203 mm) thick of solid masonry and the wall of the heat exchange channels is not less than 5 inches (127 mm) thick of solid masonry, combustible materials shall not be placed within 4 inches (102 mm) of the outside surface of a masonry heater. A clearance of not less than 8 inches (203 mm) shall be provided between the gas-tight capping slab of the heater and a combustible ceiling.
2. Masonry heaters listed and labeled in accordance with UL 1482 or CEN 15250 shall be installed in accordance with the listing specifications and the manufacturer’s written instructions.
SECTION R1003—MASONRY CHIMNEYS

R1003.1 Definition. A masonry chimney is a chimney constructed of solid masonry units, hollow masonry units grouted solid, stone or concrete, hereinafter referred to as masonry. Masonry chimneys shall be constructed, anchored, supported and reinforced as required in this chapter.
CRC § 304.8 High relevance — show source text
For SI: 1 foot = 304.8 mm, 1 square inch = 645.16 mm [2] .

R1003.16 Inlet. Inlets to masonry chimneys shall enter from the side. Inlets shall have a thimble of fireclay, rigid refractory material or metal that will prevent the connector from pulling out of the inlet or from extending beyond the wall of the liner.

R1003.17 Masonry chimney cleanout openings. Cleanout openings shall be provided within 6 inches (152 mm) of the base of each flue within every masonry chimney. The upper edge of the cleanout shall be located not less than 6 inches (152 mm) below the lowest chimney inlet opening. The height of the opening shall be not less than 6 inches (152 mm). The cleanout shall be provided with a noncombustible cover.

Exception: Chimney flues serving masonry fireplaces where cleaning is possible through the fireplace opening.

R1003.18 Chimney clearances. Any portion of a masonry chimney located in the interior of the building or within the exterior wall of the building shall have a minimum airspace clearance to combustibles of 2 inches (51 mm). Chimneys located entirely outside the exterior walls of the building, including chimneys that pass through the soffit or cornice, shall have a minimum airspace clearance of 1 inch (25 mm). The airspace shall not be filled, except to provide fire blocking in accordance with Section R1003.19.

Exceptions:
1. Masonry chimneys equipped with a chimney lining system listed and labeled for use in chimneys in contact with combustibles in accordance with UL 1777 and installed in accordance with the manufacturer’s instructions are permitted to have combustible material in contact with their exterior surfaces.
2. Where masonry chimneys are constructed as part of masonry or concrete walls, combustible materials shall not be in contact with the masonry or concrete wall less than 8 inches (203 mm) from the inside surface of the nearest flue lining.
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1. Combustible materials shall be permitted to abut the masonry chimney side walls, in accordance with Figure R1003.18, provided such combustible material is not less than 8 inches (203 mm) from the inside surface of the nearest flue lining.
FIGURE R1003.18—CLEARANCE FROM COMBUSTIBLES

COMBUSTIBLE SHEATHING 8 IN. FROM FLUE LINING

1 IN. CLEARANCE (AIRSPACE) TO COMBUSTIBLE SHEATHING

For SI: 1 inch = 25.4 mm.

FFLLUUEE
LLIINNIINNGG
G Col2 Col3 Col4 Col5 Col6 Col7 Col8 Col9 Col10 Col11 Col12 Col13 Col14 Col15
CRC § 2113.17 High relevance — show source text
2113.17 Inlet. Inlets to masonry chimneys shall enter from the side. Inlets shall have a thimble of fireclay, rigid refractory material or metal that will prevent the connector from pulling out of the inlet or from extending beyond the wall of the liner.

2113.18 Masonry chimney cleanout openings. Cleanout openings shall be provided within 6 inches (152 mm) of the base of each flue within every masonry chimney. The upper edge of the cleanout shall be located not less than 6 inches (152 mm) below the lowest chimney inlet opening. The height of the opening shall be not less than 6 inches (152 mm). The cleanout shall be provided with a noncombustible cover.

Exception: Chimney flues serving masonry fireplaces, where cleaning is possible through the fireplace opening.

2113.19 Chimney clearances. Any portion of a masonry chimney located in the interior of the building or within the exterior wall of the building shall have a minimum airspace clearance to combustibles of 2 inches (51 mm). Chimneys located entirely outside the exterior walls of the building, including chimneys that pass through the soffit or cornice, shall have a minimum airspace clearance of 1 inch (25 mm). The airspace shall not be filled, except to provide fireblocking in accordance with Section 2113.20.

Exceptions:
1. Masonry chimneys equipped with a chimney lining system listed and labeled for use in chimneys in contact with combustibles in accordance with UL 1777, and installed in accordance with the manufacturer’s instructions, are permitted to have combustible material in contact with their exterior surfaces.
2. Where masonry chimneys are constructed as part of masonry or concrete walls, combustible materials shall not be in contact with the masonry or concrete wall less than 12 inches (305 mm) from the inside surface of the nearest flue lining.
3. Exposed combustible trim and the edges of sheathing materials, such as wood siding, are permitted to abut the masonry chimney sidewalls, in accordance with Figure 2113.19, provided that such combustible trim or sheathing is not less than 12 inches (305 mm) from the inside surface of the nearest flue lining. Combustible material and trim shall not overlap the corners of the chimney by more than 1 inch (25 mm).
FIGURE 2113.19—ILLUSTRATION OF EXCEPTION THREE CHIMNEY CLEARANCE PROVISION

MASONRY ABUTTING

COMBUSTIBLE SHEATHING

12" FROM FLUE LINING

Col1 Col2 Col3 Col4 Col5 Col6 Col7 Col8 Col9 M
C
12

1" CLEARANCE (AIRSPACE) TO COMBUSTIBLE SHEATHING

2113.20 Chimney fireblocking. All spaces between chimneys and floors and ceilings through which chimneys pass shall be fireblocked with noncombustible material securely fastened in place. The fireblocking of spaces between wood joists, beams or headers shall be self-supporting or be placed on strips of metal or metal lath laid across the spaces between combustible material and the chimney.

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CRC § 304.8 High relevance — show source text
)
140
110
76
70
53
32|269
214
168
124
82
58
37
91|||||||||||||| |224
187
MIN. CROSS-SECTIONAL AREA (SQ. IN.)
140
110
76
70
53
32|269
214
168
124
82
58
37
91|||||||||||||| |224
187
MIN. CROSS-SECTIONAL AREA (SQ. IN.)
140
110
76
70
53
32|269
214
168
124
82
58
37
91|||||||||||||| |224
187
MIN. CROSS-SECTIONAL AREA (SQ. IN.)
140
110
76
70
53
32|269
214
168
124
82
58
37
91|||||||||||||| |224
187
MIN. CROSS-SECTIONAL AREA (SQ. IN.)
140
110
76
70
53
32|269
214
168
124
82
58
37
91||||||||||||||

13 14 15 16 17 18 19 20 21 22 23 24 25

HEIGHT, MEASURED FROM FLOOR OF COMBUSTION CHAMBER TO TOP OF FLUE (FT)

For SI: 1 foot = 304.8 mm, 1 square inch = 645.16 mm [2] .

R1003.16 Inlet. Inlets to masonry chimneys shall enter from the side. Inlets shall have a thimble of fireclay, rigid refractory material or metal that will prevent the connector from pulling out of the inlet or from extending beyond the wall of the liner.

R1003.17 Masonry chimney cleanout openings. Cleanout openings shall be provided within 6 inches (152 mm) of the base of each flue within every masonry chimney. The upper edge of the cleanout shall be located not less than 6 inches (152 mm) below the lowest chimney inlet opening. The height of the opening shall be not less than 6 inches (152 mm). The cleanout shall be provided with a noncombustible cover.

Exception: Chimney flues serving masonry fireplaces where cleaning is possible through the fireplace opening.

R1003.18 Chimney clearances. Any portion of a masonry chimney located in the interior of the building or within the exterior wall of the building shall have a minimum airspace clearance to combustibles of 2 inches (51 mm). Chimneys located entirely outside the exterior walls of the building, including chimneys that pass through the soffit or cornice, shall have a minimum airspace clearance of 1 inch (25 mm). The airspace shall not be filled, except to provide fire blocking in accordance with Section R1003.19.

Exceptions:
1. Masonry chimneys equipped with a chimney lining system listed and labeled for use in chimneys in contact with combustibles in accordance with UL 1777 and installed in accordance with the manufacturer’s instructions are permitted to have combustible material in contact with their exterior surfaces.
CRC § 21-13 High relevance — show source text
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supported by the body of the heater. Where the masonry chimney shares a common wall with the facing of the masonry heater, the chimney portion of the structure shall be reinforced in accordance with Section 2113.

2112.5 Masonry heater clearance. Combustible materials shall not be placed within 36 inches (914 mm) or the distance of the allowed reduction method from the outside surface of a masonry heater in accordance with NFPA 211, Section 12.6, and the required space between the heater and combustible material shall be fully vented to permit the free flow of air around all heater surfaces.

Exceptions:
1. Where the masonry heater wall thickness is not less than 8 inches (203 mm) of solid masonry and the wall thickness of the heat exchange channels is not less than 5 inches (127 mm) of solid masonry, combustible materials shall not be placed within 4 inches (102 mm) of the outside surface of a masonry heater. A clearance of not less than 8 inches (203 mm) shall be provided between the gas-tight capping slab of the heater and a combustible ceiling.
2. Masonry heaters listed and labeled in accordance with UL 1482 or EN 15250 and installed in accordance with the manufacturer’s instructions.
SECTION 2113—MASONRY CHIMNEYS

2113.1 General. The construction of masonry chimneys consisting of solid masonry units, hollow masonry units grouted solid, stone or concrete shall be in accordance with this section.

2113.2 Footings and foundations. Footings for masonry chimneys shall be constructed of concrete or solid masonry not less than 12 inches (305 mm) thick and shall extend not less than 6 inches (152 mm) beyond the face of the foundation or support wall on all sides. Footings shall be founded on natural undisturbed earth or engineered fill below frost depth. In areas not subjected to freezing, footings shall be not less than 12 inches (305 mm) below finished grade.

2113.3 Seismic reinforcement. In structures assigned to Seismic Design Category A or B, seismic reinforcement is not required. In structures assigned to Seismic Design Category C or D, masonry chimneys shall be reinforced and anchored in accordance with Sections 2113.3.1, 2113.3.2 and 2113.4. In structures assigned to Seismic Design Category E or F, masonry chimneys shall be reinforced in accordance with the requirements of Sections 2101 through 2108 and anchored in accordance with Section 2113.4.

2113.3.1 Vertical reinforcement. For chimneys up to 40 inches (1016 mm) wide, four No. 4 continuous vertical bars anchored in the foundation shall be placed in the concrete between wythes of solid masonry or within the cells of hollow unit masonry and grouted in accordance with Section 2103.3. Grout shall be prevented from bonding with the flue liner so that the flue liner is free to move with thermal expansion. For chimneys greater than 40 inches (1016 mm) wide, two additional No. 4 vertical bars shall be provided for each additional 40 inches (1016 mm) in width or fraction thereof. **2113.3.2 Horizontal reinforcement.
CRC § 3.5 High relevance — show source text
2112 A .2 Installation. Masonry heaters shall be installed in accordance with this section and comply with one of the following:
1. Masonry heaters shall comply with the requirements of ASTM E1602.
2. Masonry heaters shall be listed and labeled in accordance with UL 1482 or EN 15250 and installed in accordance with the manufacturer’s instructions.
2112 A .3 Footings and foundation. The firebox floor of a masonry heater shall be a minimum thickness of 4 inches (102 mm) of noncombustible material and be supported on a noncombustible footing and foundation in accordance with Section 2113 A .2.

2112 A .4 Seismic reinforcing. In structures assigned to Seismic Design Category D, E or F, masonry heaters shall be anchored to the masonry foundation in accordance with Section 2113 A .3. Seismic reinforcing shall not be required within the body of a masonry heater with a height that is equal to or less than 3.5 times its body width and where the masonry chimney serving the heater is not supported by the body of the heater. Where the masonry chimney shares a common wall with the facing of the masonry heater, the chimney portion of the structure shall be reinforced in accordance with Section 2113 A .

2112 A .5 Masonry heater clearance. Combustible materials shall not be placed within 36 inches (914 mm) or the distance of the allowed reduction method from the outside surface of a masonry heater in accordance with NFPA 211, Section 12.6, and the required space between the heater and combustible material shall be fully vented to permit the free flow of air around all heater surfaces.

Exceptions:
1. Where the masonry heater wall thickness is not less than 8 inches (203 mm) of solid masonry and the wall thickness of the heat exchange channels is not less than 5 inches (127 mm) of solid masonry, combustible materials shall not be placed within 4 inches (102 mm) of the outside surface of a masonry heater. A clearance of not less than 8 inches (203 mm) shall be provided between the gas-tight capping slab of the heater and a combustible ceiling.
2. Masonry heaters listed and labeled in accordance with UL 1482 or EN 15250 and installed in accordance with the manufacturer’s instructions.
SECTION 2113 A —MASONRY CHIMNEYS

2113 A .1 General. The construction of masonry chimneys consisting of solid masonry units, hollow masonry units grouted solid, stone or concrete shall be in accordance with this section.

2113 A .2 Footings and foundations. Footings for masonry chimneys shall be constructed of concrete or solid masonry not less than 12 inches (305 mm) thick and shall extend not less than 6 inches (152 mm) beyond the face of the foundation or support wall on all sides. Footings shall be founded on natural undisturbed earth or engineered fill below frost depth. In areas not subjected to freezing, footings shall be not less than 12 inches (305 mm) below finished grade.
CRC § 9.2.2 High relevance — show source text
** Appliances and their vent connectors shall be installed with clearances from combustible material so their operation does not create a hazard to persons or property. Minimum clearances between combustible walls and the back and sides of various conventional types of appliances and their vent connectors are specified in Chapter 8 and Chapter 9, or NFPA 211.

[NFPA 54:9.2.2] Where not provided in this code, listed and unlisted equipment or appliances shall be installed to maintain the required clearances for servicing and to combustible construction in accordance with the listing and the manufacturer’s installation instructions.

303.10.1 Clearance Reduction. Reduced clearances to combustible construction for listed equipment and appliances shall comply with the listing and Table 303.10.1. Where permitted by the manufacturer, and not provided in this code, reduced clearances to combustible construction for unlisted equipment and appliances shall comply with Table 303.10.1. 303.10.1.1 Type I Hood Exhaust System. Reduced clearances for Type I exhaust systems shall be in accordance with Section 507.4.2 through Section 507.4.2.3. Clearances from the duct or the

exhaust fan to the interior surface of enclosures of

combustible construction shall be in accordance with

Section 510.7.3 and clearances shall not be reduced.

303.10.1.2 Product Conveying Ducts. Reduced clearances to combustibles construction for product conveying ducts shall be in accordance with Section 506.10.3 through Section 506.11.6.3.

2025 CALIFORNIA MECHANICAL CODE 53

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GENERAL REGULATIONS

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303.10.1.3 Solid-Fuel-Burning Appliances. For solid-fuel-burning appliances, the clearance shall not be less than 12 inches (305 mm) to combustible walls and not less than 18 inches (457 mm) to combustible ceilings. The clearance, after reduction, shall be permitted to be less than 12 inches (305 mm) to combustible walls and less than 18 inches (457 mm) to combustible ceilings. Solid-fuel-burning appliances listed for lesser clearances shall be permitted to be installed in accordance with the manufacturer’s instructions and their listing. 303.11 Installation in Commercial Garages. Appliances installed in commercial garages shall be in accordance with Section 303.11.1 through Section 303.11.2. 303.11.1 Parking Structures. Appliances installed in enclosed, basement, and underground parking structures shall be installed in accordance with NFPA 88A. [NFPA 54:9.1.11.1] 303.11.2 Repair Garages. Appliances installed in repair garages shall be installed in accordance with NFPA 30A. [NFPA 54:9.1.11.2]

303.12 Installation in Aircraft Hangars. Heaters in aircraft hangars shall be installed in accordance with NFPA 409.

[NFPA 54:9.1.12]

CRC § 25.4 High relevance — show source text

For SI: 1 inch = 25.4 mm.

masonry, that is designed to absorb and store heat from a solid-fuel fire built in the firebox by routing the exhaust gases through

e fireplace opening is 6 square feet (0.6 m2) or larger, such combustible tri masonry fireplace sidewalls and hearth extension provided that such co inches (305 mm) from the inside surface of the nearest firebox lining. ustible mantels or trim is permitted to be placed directly on the masonry f ng providing such combustible materials are not placed within 6 inches (1 aterial within 12 inches (306 mm) of the fireplace opening shall not projec mm) distance from such an opening. FIGURE R1001.11—CLEARANCE FROM COMBUSTIBLES MASONRY COMBUSTIBLE SHEATHING EDGE ABUTTING MASONRY 12 IN. FROM FIREBOX FRAME WALL 2 IN. CLEARANCE (AIRSPA TO COMBUSTIBLE FRAMIN WOOD MANTEL cking. Fireplace fireblocking shall comply with the provisions of Section R ories. Listed and labeled fireplace accessories shall be installed in accorda r’s instructions. Fireplace accessories shall comply with UL 907. SECTION R1002—MASONRY HEATERS onry heater is a heating appliance constructed of concrete or solid maso	Col2	Col3	Col4	Col5	Col6	Col7	Col8	Col9	Col10	Col11	Col12	Col13	Col14	Col15	Col16	Col17	Col18	Col19	Col20	Col21	Col22	Col23	Col24	Col25	Col26
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CRC § 902.2. High relevance — show source text
(2) Where approved, one listed wall-mounted unvented room heater equipped with an oxygen depletion safety shutoff system shall be permitted to be installed in a bedroom, provided that the input rating does not exceed 10 000 Btu/h (3 kW) and combustion and ventilation air is provided as specified in Section 902.2. [NFPA 54:10.21.2]

(3) Portable oil fired unvented heating appliances used as supplemental heating in storage occupancies, utility occupancies, and in accordance with the fire code.

[HCD 1 & HCD 2] Unvented fuel-burning room heaters shall not be installed, used, maintained, or permitted to exist in a Group R Occupancy.

915.3.2 Installations in Institutions. Room heaters shall not be installed in the following occupancies:

(1) Residential board and care

(2) Health care [NFPA 54:10.21.3] 915.3.3 Clearance. A room heater shall be placed so as not to cause a hazard to walls, floors, curtains, furniture, doors where open, and to the free movements of persons within the room. Heaters designed and marked, “For use in noncombustible fireplace only,” shall not be installed elsewhere. Listed room heaters shall be installed in accordance with their listings and the manufacturer’s installation instructions. In no case shall the clearances be

2025 CALIFORNIA MECHANICAL CODE 207

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INSTALLATION OF SPECIFIC APPLIANCES

(1) A listed unit heater shall be installed with clearances from combustible material at the back and one side of not less than 6 inches (152 mm). Where the flue gases are vented horizontally, the 6 inch (152 mm) clearance shall be measured from the draft hood or

vent instead of the rear wall of the unit heater. A unit

heater listed for reduced clearances shall be installed in accordance with its listing and the manufacturer’s installation instructions.

(2) Floor-mounted-type unit heaters installed on combustible floors shall be listed for such installation.

(3) Combustible floors under unlisted floor-mounted unit heaters shall be protected in an approved man ner.

(4) Clearances for servicing shall be in accordance with the manufacturer’s instructions.

916.4 Combustion and Circulating Air. Combustion and circulating air shall be provided in accordance with Section 701.0. [NFPA 54:10.24.4]

916.5 Ductwork. A unit heater shall not be attached to a warm air duct system unless listed and marked for such installation. [NFPA 54:10.24.5] 916.6 Installation in Commercial Garages and Air- craft Hangars. Unit heaters installed in garages for more than three motor vehicles or in aircraft hangars shall be installed in accordance with Section 303.11 and Section 303.12. [NFPA 54:10.24.6]

916.7 Oil-Fired Unit Heaters. Oil-fired unit heaters shall comply with UL 731 and installed in accordance with the manufacturer’s installation instructions.

917.0 Food Service Appliance, Floor-Mounted. 917.1 Application. Floor-mounted food service appliances shall be listed in accordance with ANSI Z83.11/CSA 1.8.

Frequently asked questions

Can I reduce the 36‑inch clearance by adding heat shielding?

Not in the text of § R1002.5 for masonry heaters — the code provides a masonry‑thickness exception (4 in) and an option to follow manufacturer/listing instructions. The section does not describe a generic wall‑shield reduction method; listed products or NFPA‑based methods referenced by the listing would be the path to approved reductions.

My masonry heater is listed to UL 1482 — do I still need 36 inches?

If the heater is listed and labeled to UL 1482 (or EN 15250), § R1002.5 lets you install it in accordance with the listing and the manufacturer’s instructions — follow those listed clearances.

What if the heater’s wall thickness is nonuniform?

The 4‑inch reduced clearance in § R1002.5 requires the heater wall to be not less than 8 inches (203 mm) thick and the heat‑exchange channel walls not less than 5 inches (127 mm) thick. If those minimums are not met everywhere required by the installation, you cannot apply the 4‑inch reduction.

Does § R1002.4 change clearances when seismic anchorage is required?

No. § R1002.4 covers seismic reinforcing/anchorage for masonry heaters (structural requirements). It does not modify the combustible‑clearance requirements in § R1002.5.

Is the airspace around the heater allowed to be filled with insulation or enclosure?

No — § R1002.5 requires the space between the heater and combustible material to be fully vented to permit free flow of air around heater surfaces. Filling the airspace is not compliant.

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