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The normal requirement states
that a hood and associated ductwork must have an 18" clearance from combustible
materials. Since this is too great in most installations, BOCA, NFPA and the
Uniform Mechanical Code have produced approved methods of protecting exhaust
ducts from surrounding combustibles to reduce this clearance to 3". Please
observe the method approved by local authorities governing your area.
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BOCA
SECTION M-1101.0 - REDUCED CLEARANCES
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M-1101.1 Labeled reduction: The required clearance to
combustibles may be reduced by methods that have been tested and
bear the label of an approved agency.
M-1101.2 Reduction table: The required clearance to combustibles may be reduced when
protected in accordance with one of the methods specified in Table M-1101.2. The
reduced clearance distances in the table shall be measured from the device to
the face of the protection.
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Table M-1 101.2 - REDUCED CLEARANCE TO COMBUSTIBLES
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| Reduced clearance protection* |
Reduced clearance based on required
clearance to combustibles (inches)** |
| 36
| 18
| 9
| 6
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| 1/8-inch unpainted
aluminum plate spaced 1 inch off the wall |
3 |
3 |
3 |
3 |
| 1/8-inch painted aluminum
plate spaced 1 inch off the wall |
28 |
9 |
3 |
3 |
Unpainted galvanized sheet
metal Manufacturer's Standard
Gage No. 28 mounted on 1/2-inch inorganic insulating
board
attached directly to the wall |
3 |
3 |
3 |
3 |
Painted galvanized sheet
metal Manufacturer's Standard
Gage No.28 spaced 1 inch off the wall |
24 |
6 |
3 |
3 |
2 layers of painted
galvanized sheet metal Manufacturer's
Standard Gage No. 28 having a 1-inch air space between
layer,
spaced 1 inch off the wall |
3 |
3 |
3 |
3 |
1/4-inch inorganic
insulating board having painted galvanized
sheet metal Manufacturer's Standard Gage No. 28 attached
on
both sides spaced 1 inch off the wall |
3 |
3 |
3 |
3 |
| 1/4-inch inorganic
insulating board spaced 1 inch off the wall |
30 |
9 |
3 |
3 |
2 layers of painted
galvanized sheet metal Manufacturer's
Standard Gage No. 24 having 1 inch of fiberglass
insulation
between layers spaced 1 inch off the wall
3-1/2-inch thick wall space 1 inch off the wall |
3
18 |
3
5 |
3
3 |
3
0 |
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Note: * The spacers shall be noncombustible
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Note: ** 1 inch - 25.4mm
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Uniform Mechanical Code
TABLE NO. 5-B - CLEARANCES, INCHES, WITH SPECIFIED
FORMS OF PROTECTION*,**
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TYPE OF PROTECTION
Applied to the Combustible Material Unless
Otherwise Specified
and Covering All Surfaces Within the Distance Specified
as the Required Clearance With No Protection
(Thicknesses Are Minimum) |
WHERE THE
STANDARD CLEARANCE IN TABLE NO. 5-A WITH NO PROTECTION
IS: |
| 36 inches |
18 inches |
9 inches |
6 inches |
| Above |
Sides
& Rear |
Chimney
or Vent
Connector |
Above |
Sides
& Rear |
Chimney
or Vent
Connector |
Above |
Sides
& Rear |
Chimney
or Vent
Connector |
Above |
Sides
& Rear |
Chimney
or Vent
Connector |
a) 1/4" insulating
millboard spaced
out 1"*** |
30 |
18 |
30 |
15 |
9 |
12 |
9 |
6 |
6 |
3 |
2 |
3 |
b) 0.013" (No. 28 manufacturer's standard gage)
steel sheet on 1/4" insulating millboard |
24 |
18 |
24 |
23 |
9 |
12 |
9 |
6 |
4 |
3 |
2 |
2 |
c) 0.013" (No. 28
manufacturer's standard gage)
steel sheet spaced out 1"*** |
18 |
12 |
18 |
9 |
6 |
9 |
6 |
4 |
4 |
2 |
2 |
2 |
d) 0.013" (No. 28
manufacturer's standard gage)
steel sheet on 1/8" insulating millboard spaced out
1"*** |
18 |
12 |
18 |
9 |
6 |
9 |
6 |
4 |
4 |
2 |
2 |
2 |
| e) 1/2" insulating cement
covering on heating appliance |
18 |
12 |
36 |
9 |
6 |
18 |
6 |
4 |
9 |
2 |
1 |
6 |
f) 1/4" insulating millboard on
1" mineral fiber bats
reinforced with wire mesh or equivalent |
18 |
12 |
18 |
6 |
6 |
6 |
4 |
4 |
4 |
2 |
2 |
2 |
g) 0.027 (No. 22 manufacturer's
standard gage) steel sheet
on 1" mineral fiber bats reinforced with wire or
equivalent |
18 |
12 |
12 |
4 |
3 |
3 |
2 |
2 |
2 |
2 |
2 |
2 |
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*For appliances complying with Section 504(b) and (c)
**Except for the protection described in (3), all
clearances shall be measured from the outer surface of the
appliance to the combustible material, disregarding any
intervening protection applied to the combustible material
***Spaces shall be on noncombustible material
NOTE: Insulating millboard is a factory made product formed of
noncombustible materials, normally fibers, and having a thermal
conductivity of 1 Btu-inch per square foot per degree F, or less
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NFPA 96
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Appendix A: This Appendix is not a part of the
requirements of this NFPA document, but it is included for
information purposes only.
A-1 Where 18 in. (457.2 mm) clearance is required to
unprotected combustible material, the clearance may be reduced if
the combustible material is protected by an engineered
construction system acceptable to the authority having
jurisdiction, or by the use of materials or products listed for
protection purposes, or by the use of materials listed below:
Type of Protection
1. 0.013-in. (.33mm) (28 gage) sheet metal spaced out 1 in. (25.4
mm) on noncombustible spacers. Clearance: 9 inches (228.6
mm)
2. 0.027 (.69 mm) (22 gage) sheet metal on 1 in. (25.4 mm)
mineral wool bats reinforced with wire mesh or equivalent spaced
out 1 in. (25.4 mm) on noncombustible spacers Clearance: 3
inches (76.2 mm)
A-2 Materials and products
listed for the purpose of reducing clearance to combustibles shall be installed
in accordance with the condition of the listing and the manufacturer's
instruction.
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