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904.3 Assembly and Installation
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A central-heating boiler or furnace shall be installed in accordance with the manufacturer's instructions in one of the following manners:
- On a floor of noncombustible construction with noncombustible flooring, and surface finish and with no combustible material against the underside thereof.
- On fire-resistive slabs or arches having no combustible material against the underside thereof.
- Appliances listed for installation on a combustible floor.
- Installation on a floor protected in an approved manner. [NFPA 54:10.3.3]
Where a furnace is supported by the ground, it shall be installed on a concrete slab not less than 3 inches (76 mm) above the adjoining ground level.
Where a furnace is supported from above, a clearance of not less than 6 inches (152 mm) shall be provided from finished grade.