Final answer:
B-Vent for gas or propane appliances requires a 2 inches clearance to combustibles as a safety measure to prevent fire hazards. This is a commonly applied standard for venting systems.
Step-by-step explanation:
Double wall B-vent for gas or propane appliances is a part of the heating system standards. As for the clearance to combustibles, which refers to the distance required to prevent nearby materials from igniting due to the heat, the answer would be B) 2 inches. It's essential that this standard is adhered to, because it's a major safety measure to prevent fire hazards. This standard is commonly applied for venting systems of gas or propane appliances, such as water heaters, furnaces, and boilers.
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