What must a masonry chimney have if it is to be used for a gas appliance?

A masonry chimney intended for use with a gas appliance must have a clay tile liner. This requirement is crucial because the clay lining helps to protect the masonry from the corrosive byproducts of gas combustion, which can lead to deterioration over time. The liner also ensures that the flue is properly sized and shaped to facilitate efficient venting of flue gases, improving the overall safety and performance of the gas appliance.

In addition, a clay tile liner provides thermal resistance, which is important because gas appliances can produce high-temperature exhaust gases. The liner helps to maintain the necessary temperature within the flue to prevent condensation and potential damage to the chimney system. Furthermore, the smooth surface of a clay liner promotes better draft performance, ensuring that combustion gases are vented effectively.

In contrast, the other options do not meet the specific requirements for safely venting gas appliances through a masonry chimney. For example, simply having "nothing special" would not provide adequate protection or performance. Steel casing or insulated pipe may be relevant in different venting scenarios, but they do not address the specific needs for a masonry chimney meant for gas appliances. Thus, the clay tile liner is the correct solution for ensuring safety and efficiency in this context.