A vent connector shall NOT be connected to a chimney flue serving a fireplace unless the fireplace flue opening is _____________.

The correct answer is that the fireplace flue opening must be permanently sealed when a vent connector is connected to a chimney flue serving a fireplace. This is critical for safety and efficiency. If the fireplace flue is not sealed, there is a risk that combustion products from the appliance could infiltrate the living space, potentially causing hazardous situations such as carbon monoxide buildup.

Permanently sealing the fireplace flue prevents the possibility of back drafting, where air and gases could flow the wrong way, undermining the venting system. This not only protects the occupants by reducing the risk of harmful gases entering the home but also ensures that the exhaust from the connected appliance is directed safely outside.

Options related to maintenance accessibility, size comparisons between flue openings, and dual usage create scenarios that do not address the fundamental safety requirement of sealing off the fireplace flue in this context. Sealing is a specific safety measure vital for the proper functioning of the combined venting system.