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The requirement for unlisted unit heaters to maintain clearances to combustible materials is essential for ensuring safety and preventing fire hazards. A clearance of 18 inches is specified to allow adequate space for heat dissipation and to minimize the risk of igniting any nearby combustible objects. This distance helps in maintaining an effective thermal barrier between the heater and any materials that can catch fire, ensuring that the heat generated does not reach a level that could cause ignition.

Clearance requirements are designed based on industry standards that consider the potential temperature rise in surrounding areas due to the heater's operation. Keeping an 18-inch distance reduces the chances of heat buildup in enclosed areas where combustible materials might be present. It is crucial for both residential and commercial installations to adhere to these safety standards to protect occupants and property from fire risks.