Which material is prohibited for single wall connectors according to NFPA 211?

Galvanized steel is prohibited for single wall connectors according to NFPA 211 due to concerns over its performance and the potential release of toxic fumes when exposed to high heat or combustion gases. Single wall connectors need to withstand elevated temperatures while also being safe for use in venting applications. Galvanized steel coatings can break down when exposed to the high temperatures associated with venting flue gas from heating appliances, leading to the failure of the connector and posing a safety risk.

The other materials mentioned—steel, copper, and aluminum—are permissible because they can handle the conditions required for single wall connectors without the risks associated with galvanized steel. Steel provides strength and durability, while copper and aluminum can offer suitable thermal properties when used appropriately in specific appliances.