The home I inspected today use to have a wood burning stove. A class A vent is installed. A free standing gas fireplace has replaced the wood burning. The gas fireplace vents into the Class A vent. Is this ok. I understand Class A is generally used for high temperature fuels such as wood or oil. Since the class A is designed for high temperatures is there any reason why it could not be used for gas which is cooler. I feel as though it is ok but as you see in the left side of the photo, the neighbors had the same conversion and their installer ran a B vent through the Class A.