Which type of chimney construction is described as factory-built?

The correct answer highlights that factory-built chimneys often consist of double or triple wall construction. This type of chimney system is designed to provide enhanced insulation and safety features compared to other chimney types. The double or triple wall construction typically incorporates an outer layer that provides a protective shell, while the inner layers contain insulating materials that help reduce the risk of heat transfer, making it safer for venting gas or wood-burning appliances.

Factory-built chimneys are manufactured to specific standards and are often prefabricated, which ensures consistent construction quality and compliance with safety regulations. Their design not only improves efficiency but also helps to prevent issues such as creosote buildup and potential fire hazards. This construction method is widely employed for modern heating appliances because of its ease of installation and superior performance in terms of maintaining the integrity of the chimney over time.

In this context, other chimney types such as single wall only or brick and mortar do not fit the description of factory-built chimneys as they either lack the enhanced insulation capabilities or are constructed on-site rather than manufactured in a factory setting. Uninsulated flues lack the necessary safety features for effective venting, making them less suitable for modern heating applications compared to double or triple wall constructions.