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506.11 Clearance Reduction Methods
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It shall be permitted to protect stored combustible material or combustible construction from ductwork in accordance with Table 506.11 and Section 506.11.1 through Section 506.11.6. In no case shall the clearance between the duct and the combustible surface be reduced below that allowed in Table 506.11. [NFPA 91:4.7.4, 4.7.4.1]
| FORM OF PROTECTION* | MAXIMUM ALLOWABLE REDUCTION IN CLEARANCE (percent) | |
|---|---|---|
| AS WALL PROTECTOR OR VERTICAL SURFACE | AS CEILING PROTECTOR OR HORIZONTAL SURFACE | |
| 31/2 inch thick masonry wall without ventilated air space | 33 | None |
| 1/2 inch thick noncombustible insulation board over 1 inch glass fiber or mineral wool batts without ventilated air space | 50 | 33 |
| 0.024 inch (24 gauge) sheet metal over 1 inch glass fiber or mineral wool batts reinforced with wire, or equivalent on rear face with at least a 1 inch air gap | 66 | 66 |
| 31/2 inch (90 mm) thick masonry wall with at least a 1 inch air gap | 66 | None |
| 0.024 inch (24 gauge) sheet metal with at least a 1 inch air gap | 66 | 50 |
| 1/2 inch thick noncombustible insulation board with at least a 1 inch air gap | 66 | 50 |
| 0.024 inch (24 gauge) sheet metal with ventilated air space over at least 0.024 inch (24 gauge) sheet metal with at least a 1 inch air gap | 66 | 50 |
| 1 inch glass fiber or mineral wool batts sandwiched between two sheets of 0.024 inch (24 gauge) sheet metal with at least a 1 inch air gap | 66 | 50 |
For SI units: 1 inch = 25.4 mm
* Clearance reduction applied to and covering all combustible surfaces within the distance specified as required clearance with no protection in Table 506.10.4.
Spacers and ties for protection materials shall be of noncombustible material and shall not be installed on the duct side of the protection system. [NFPA 91:4.7.4.2]
Mineral wool batts (blanket or board) shall have a density of not less than 8 pounds per cubic feet (lb/ft3) (128 kg/m3) and have a melting point of not less than 1500°F (816°C). [NFPA 91:4.7.4.3]
Insulation board used as a part of a clearance-reduction system shall meet the following criteria:
- Have a thermal conductivity of 1 British thermal unit inch per hour square foot degree Fahrenheit [Btu•in/(h•ft2•°F)] [0.14 W/(m•K)] or less.
- Be formed of noncombustible material. [NFPA 91:4.7.4.4]
With all clearance reduction systems, not less than 1 inch (25.4 mm) clear space shall be provided between the duct and the thermal shield. [NFPA 91:4.7.4.5]
When using clearance reduction systems that include an air gap, not less than 1 inch (25.4 mm) clear space shall be provided between the thermal shield and the combustible surface. [NFPA 91:4.7.4.6]
When using clearance reduction systems that include an air gap between the combustible surface and the selected means of protection, air circulation shall be provided by one of the methods in accordance with Section 506.11.6.1 through Section 506.11.6.2. [NFPA 91:4.7.4.7]
Air circulation shall be permitted to be provided by leaving all edges of the wall protecting system open with at least a 1 inch (25.4 mm) air gap. [NFPA 91:4.7.4.7.1]
If the means for protection is mounted on a single flat wall away from corners, air circulation shall be permitted to be provided by one of the following:
- Leaving only the top and bottom edges open to circulation by maintaining the 1 inch (25.4 mm) air gap.
- Leaving the top and both side edges open to circulation by maintaining the 1 inch (25.4 mm) air gap. [NFPA 91:4.7.4.7.2]
Thermal shielding that covers two walls in a corner shall be permitted to be open at the top and bottom edges with not less than 1 inch (25.4 mm) air gap. [NFPA 91:4.7.4.7.3]
Related Code Sections
506.11 Exhaust Systems, Clearance Reduction Methods
506.11. [NFPA 91:4.7.4, 4.7.4.1]
TABLE 506.11 REDUCTION OF DUCT CLEARANCE WITH SPECIFIED FORMS OF PROTECTION [NFPA 91:TABLE 4.7.4]
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2018 Uniform Mechanical Code of Nevada > 5 Exhaust Systems > 506.0 Product-Conveying Ducts > 506.11 Clearance Reduction Methods
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When using clearance reduction systems that include an air gap, not less than 1 inch (25.4 mm) clear space shall be provided between the thermal ...
2018 Uniform Mechanical Code of Nevada > 5 Exhaust Systems > 506.0 Product-Conveying Ducts > 506.11 Clearance Reduction Methods > 506.11.5 Thermal Shield and Combustible Surface
506.11.6.1 Exhaust Systems, Air Circulation
Air circulation shall be permitted to be provided by leaving all edges of the wall protecting system open with at least a 1 inch (25.4 mm) air gap ...
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506.11.6.2 Exhaust Systems, Single Flat Wall
If the means for protection is mounted on a single flat wall away from corners, air circulation shall be permitted to be provided by one ...
2018 Uniform Mechanical Code of Nevada > 5 Exhaust Systems > 506.0 Product-Conveying Ducts > 506.11 Clearance Reduction Methods > 506.11.6 Reduce Clearance With Air Gaps > 506.11.6.2 Single Flat Wall
506.11.6.3 Exhaust Systems, Thermal Shielding
Thermal shielding that covers two walls in a corner shall be permitted to be open at the top and bottom edges with not less than 1 inch (25.4 mm) air ...
2018 Uniform Mechanical Code of Nevada > 5 Exhaust Systems > 506.0 Product-Conveying Ducts > 506.11 Clearance Reduction Methods > 506.11.6 Reduce Clearance With Air Gaps > 506.11.6.3 Thermal Shielding