G2409.2 (308.2) Reduction Table
The allowable clearance reduction shall be based on one of the methods specified in Table G2409.2 or shall utilize a reduced clearance protective assembly listed and labeled in accordance with UL 1618. Where required clearances are not listed in Table G2409.2, the reduced clearances shall be determined by linear interpolation between the distances listed in the table. Reduced clearances shall not be derived by extrapolation below the range of the table. The reduction of the required clearances to combustibles for listed and labeled appliances and equipment shall be in accordance with the requirements of this section, except that such clearances shall not be reduced where reduction is specifically prohibited by the terms of the appliance or equipment listing [see Figures G2409.2(1) through 2409.2(3)].
NOTES:
A = the clearance without protection.
B = the reduced clearance permitted in accordance with Table G2409.2. The protection applied to the construction using combustible material shall extend far enough in each direction to make "C" equal to "A."
FIGURE G2409.2(1) [308.2(1)]
EXTENT OF PROTECTION NECESSARY TO REDUCE CLEARANCES FROM GAS EQUIPMENT OR VENT CONNECTORS
For SI: 1 inch = 25.4 mm.
FIGURE G2409.2(2) [308.2(2)]
WALL PROTECTOR CLEARANCE REDUCTION SYSTEM
For SI: 1 inch = 25.4 mm.
FIGURE G2409.2(3) [308.2(3)]
MASONRY CLEARANCE REDUCTION SYSTEM
TABLE G2409.2 (308.2)a through k
REDUCTION OF CLEARANCES WITH SPECIFIED FORMS OF PROTECTION
| TYPE OF PROTECTION APPLIED TO AND COVERING ALL SURFACES OF COMBUSTIBLE MATERIAL WITHIN THE DISTANCE SPECIFIED AS THE REQUIRED CLEARANCE WITH NO PROTECTION [see Figures G2409.2(1), G2409.2(2), and G2409.2(3)] | ||||||||||
| WHERE THE REQUIRED CLEARANCE WITH NO PROTECTION FROM APPLIANCE, VENT CONNECTOR, OR SINGLE-WALL METAL PIPE IS: (inches) | ||||||||||
| 36 | 18 | 12 | 9 | 6 | ||||||
| Allowable clearances with specified protection (inches) | ||||||||||
| Use Column 1 for clearances above appliance or horizontal connector. Use Column 2 for clearances from appliance, vertical connector and single-wall metal pipe. | ||||||||||
| Above Col. 1 | Sides and rear Col. 2 | Above Col. 1 | Sides and rear Col. 2 | Above Col. 1 | Sides and rear Col. 2 | Above Col. 1 | Sides and rear Col. 2 | Above Col. 1 | Sides and rear Col. 2 | |
| 1. 31/2-inch-thick masonry wall without ventilated airspace | — | 24 | — | 12 | — | 9 | — | 6 | — | 5 |
| 2. 1/2-inch insulation board over 1-inch glass fiber or mineral wool batts | 24 | 18 | 12 | 9 | 9 | 6 | 6 | 5 | 4 | 3 |
| 3. 0.024-inch (nominal 24 gage) sheet metal over 1-inch glass fiber or mineral wool batts reinforced with wire on rear face with ventilated airspace | 18 | 12 | 9 | 6 | 6 | 4 | 5 | 3 | 3 | 3 |
| 4. 31/2-inch-thick masonry wall with ventilated airspace | — | 12 | — | 6 | — | 6 | — | 6 | — | 6 |
| 5. 0.024-inch (nominal 24 gage) sheet metal with ventilated airspace | 18 | 12 | 9 | 6 | 6 | 4 | 5 | 3 | 3 | 2 |
| 6. 1/2-inch-thick insulation board with ventilated airspace | 18 | 12 | 9 | 6 | 6 | 4 | 5 | 3 | 3 | 3 |
| 7. 0.024-inch (nominal 24 gage) sheet metal with ventilated airspace over 0.024-inch (nominal 24 gage) sheet metal with ventilated airspace | 18 | 12 | 9 | 6 | 6 | 4 | 5 | 3 | 3 | 3 |
| 8. 1-inch glass fiber or mineral wool batts sandwiched between two sheets 0.024-inch (nominal 24 gage) sheet metal with ventilated airspace | 18 | 12 | 9 | 6 | 6 | 4 | 5 | 3 | 3 | 3 |
For SI: 1 inch = 25.4 mm, °C = [(°F - 32)/1.8], 1 pound per cubic foot = 16.02 kg/m3, 1 Btu per inch per square foot per hour per °F = 0.144 W/m2 × K.
- Reduction of clearances from combustible materials shall not interfere with combustion air, draft hood clearance and relief, and accessibility of servicing.
- Clearances shall be measured from the outer surface of the combustible material to the nearest point on the surface of the appliance, disregarding any intervening protection applied to the combustible material.
- Spacers and ties shall be of noncombustible material. A spacer or tie shall not be used directly opposite an appliance or connector.
- For all clearance reduction systems using a ventilated airspace, adequate provision for air circulation shall be provided as described [see Figures G2409.2(2) and G2409.2(3)].
- There shall be not less than 1 inch between clearance reduction systems and combustible walls and ceilings for reduction systems using ventilated airspace.
- Where a wall protector is mounted on a single flat wall away from corners, it shall have a minimum 1-inch air gap. To provide air circulation, the bottom and top edges, or only the side and top edges, or all edges shall be left open.
- Mineral wool batts (blanket or board) shall have a density of 8 pounds per cubic foot and a minimum melting point of 1500°F.
- Insulation material used as part of a clearance reduction system shall have a thermal conductivity of 1.0 Btu per inch per square foot per hour per °F or less.
- There shall be not less than 1 inch between the appliance and the protector. In no case shall the clearance between the appliance and the combustible surface be reduced below that allowed in this table.
- Clearances and thicknesses are minimum; larger clearances and thicknesses are acceptable.
- Listed single-wall connectors shall be installed in accordance with the manufacturer's instructions.