What type of exhaust is approved for connection to a 6" flue stack venting a gas-fired furnace?

The correct answer is based on the compatibility and requirements for venting a gas-fired furnace. A water heater exhaust is approved for connection to a 6" flue stack because both gas-fired furnaces and water heaters generally utilize similar venting principles. They need to effectively vent combustion gases to the outside while ensuring safety and efficiency in operation.

This choice is appropriate because water heater exhaust systems are designed to handle the flue gases produced by gas combustion, ensuring proper clearance and draft to prevent backdraft and hazardous situations.

In terms of compatibility, standard furnace exhausts, boiler exhausts, and air conditioning exhausts do not have the same specifications or requirements for venting as those of water heaters. Therefore, they would not adhere to the necessary codes and would not ensure efficient or safe venting when connected to a 6" flue stack designed specifically for certain types of combustion appliances.