Final answer:
The maximum reduction of a vent from a 12" flue collar on a gas-fired appliance is subject to building codes and manufacturer's specifications, which ensure proper venting and safety. The exact reduction limit is generally provided by relevant codes or the manufacturer's instructions.
Step-by-step explanation:
The reduction of a vent from a 12" flue collar on a gas-fired appliance is typically subject to building codes and manufacturer's specifications, which are designed to ensure proper venting and prevent the backflow of exhaust gases into the living space. While the information provided does not expressly state the maximum allowable reduction, as a general rule, any reduction should comply with the National Fuel Gas Code or local building codes, which often limit the reduction in flue size to ensure adequate venting. It's important always to consult the appliance manufacturer's instructions and local codes for specific requirements.