How many inches of clearance must unlisted suspended-type unit heaters have from combustible material?

Unlisted suspended-type unit heaters are required to have a minimum clearance from combustible materials to ensure safety and prevent fires. The accepted standard for unlisted suspended-type unit heaters is 18 inches of clearance from combustible materials. This requirement is in place to allow adequate space for heat dissipation and to reduce the risk of overheating materials that could ignite.

In comparison, the other options reflect either lesser or greater clearances that do not conform to the established safety guidelines for these heaters. Adhering to the 18 inches distance ensures maximum efficiency of the heater while prioritizing safety in its operation.