Which of the following is acceptable when a fireplace insert is installed?

The use of a metal chimney liner when installing a fireplace insert is considered acceptable because it provides a safe and efficient means of venting the exhaust gases produced by combustion. Metal chimney liners, typically made of stainless steel or aluminum, are designed to withstand high temperatures and corrosive elements, helping to prevent chimney fires and ensure proper draft. They are also adaptable to different flue sizes and can improve the performance of the fireplace insert by enhancing airflow and reducing the chances of blockages.

In contrast, utilizing an existing fireplace flue might not always meet modern safety standards, as it may be compromised or not adequately sized for the new insert. Using a BW vent could also be inappropriate depending on the specific requirements and safety codes applicable to the installation. A tile-lined fireplace flue, while potentially suitable in some instances, may not provide the same level of protection against heat and corrosion that a metal liner offers, making it a less preferred option for new installations.