What combination of material does not reduce the clearance of an unlisted residential fired furnace from 18" to 6"?

The correct answer is identified as the combination of materials that does not reduce the required clearance of an unlisted residential fired furnace from 18 inches to 6 inches. The specification of maintaining proper clearance from combustibles is crucial for safety in HVAC installations.

In this case, the combination of 1/2 inch insulating board over 1 inch of fiberglass is recognized as an appropriate insulating layer that does not sufficiently reduce the clearance requirement. Insulating materials are generally assessed for their ability to withstand heat and provide thermal barriers, but in this particular combination, they do not contribute to reducing the clearance requirement as effectively as needed.

Other combinations, such as 1 inch drywall over 1/2 inch plywood, 0.5 inch metal sheeting over 1 inch insulation, and 1 inch particle board, are often capable of providing effective fire resistance or insulating properties that allow some reduction in clearance. These materials, when applied together, may have thermal properties or fire ratings that facilitate a lower minimum clearance from the furnace.

Therefore, the combination of insulating board and fiberglass is the right choice because it does not achieve the necessary thermal protection or clearance reduction required for safety with an unlisted furnace. Understanding the properties of these materials and their fire-resistance