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

A metal chimney liner is considered an acceptable and safe option when installing a fireplace insert because it provides an effective way to ensure proper venting of combustion gases. Metal liners are designed to withstand high temperatures and are resistant to corrosion, making them a suitable choice for improving the efficiency of the venting system. They also create a smooth surface for the exhaust gases to travel through, minimizing the risk of creosote buildup, which can lead to chimney fires.

In contrast, using existing fireplace flues or tile-lined flues may not guarantee the same level of safety or efficiency. Existing flues might not be adequately sized or conditioned for the new insert, and tile-lined flues can have less versatility in handling the heat and gases produced by modern fireplace inserts. BW vents, while sometimes used in other applications, would not meet the specific needs of a fireplace insert installation.