Which of the following materials is not approved as a flue lining for low heat appliances?

Masonry brick is not approved as a flue lining for low heat appliances due to its porosity and inability to withstand the corrosive byproducts of combustion over time. In HVAC applications, especially those involving venting systems for low heat appliances, the flue lining must be made of materials that can endure high temperatures, resist corrosion, and maintain structural integrity.

In contrast, materials such as stainless steel and clay are suitable for flue linings. Stainless steel has a high resistance to corrosion and is durable enough for high-efficiency appliances, while clay has been traditionally used in chimney and flue applications because of its ability to handle heat and provide a robust lining. Fiberglass, although it may have applications in other areas, is also not a permissible material for flue linings in this context due to its thermal limitations and potential safety hazards when exposed to high heat.

Thus, masonry brick’s limitations in durability, resistance to corrosive elements, and heat tolerance are the primary reasons it does not qualify for use as a flue lining in low heat appliance applications.