[Richard]

I think both ideas are strong contributors, in addition to that, let’s look at the air flow. We don’t normally think about air as having mass like water or any other fluid flowing but it does. So a fellow named Newton said that an object in motion wants to stay in motion in the same direction unless something changes that direction. Well, that’s close enough to what he said. The air from the generator is flowing merrily along when it encounters the Y junction of the two exhaust pipes. Because it is a Y junction the air flow would have to reverse direction to flow down the AH pipe. Wait, it gets better. Because the Y is actually a crude Venturi setup, the flow of the generator exhaust actually creates a small vacuum on the AH exhaust pipe.
Those factors along with the heat rising phenom mentioned above all work to help the flow upwards and out the the stack.

Now if the gen exhaust were teed into the AH exhaust, that would be a different situation, and you would get gen exhaust trying to go into the AH exhaust.

I don’t know when the design changed, but the 95 I owned had separate vertical pipes. The 99 has the Y combined into one vertical.