An improved wall thimble is useful as part of a venting apparatus for a direct vent heating appliance. Dual functionality is provided in a single wall thimble device by having a vent path for exhausting combustion by-products, and a separate air inlet path to draw in outside combustion air for use by the heating appliance. The wall thimble is formed as a two-part housing such that a first cylindrical portion can be inserted into a second cylindrical portion. When a smaller diameter exhaust pipe is routed through the cylindrical portions, an annular region is formed between the exhaust pipe and the cylindrical portions. A first air vent is formed in the first cylindrical portion to draw combustion air into the annular region from outside. A second air vent is formed in the second cylindrical portion to route the combustion air from the annular region to an opening in the housing where a combustion air inlet pipe is connected.