Well, I was a bit early with my celebration of success. When I went down to the basement the next day to do some work, it smelled in the basement. I could also detect the smell in the house just a bit...
So, back to the hardware store to get some more parts and change the layout of the venting to better the flow path of the flue gases.
Below are the before and after layouts of the vent piping. Before, there was a "TEE" where the water heater tapped into the flue from the furnace. When the water heater was on, flue gases would get to the "tee" and go both directions.
Now, I've re-routed both to a "TEE" up higher. This prevents backdraft of flue gases.
So far, so good