If you find that your new gas water heater has a smaller vent than your existing gas vent, don’t assume that you have to completely replace the venting system. If you can’t find a unit that has the same size vent opening as your current water heater (or you just want a smaller unit), buy one with a vent opening that’s just 1 in. smaller, then use a “reducer” to connect to the vent as shown. Use clamps or sheet metal screws and aluminum tape to connect the reducer. Install it on the horizontal part of the vent, at least 12 in. away from the elbow. Reducers are available at home centers for about $5.
Be sure to check building codes (this project requires a permit) and manufacturer’s specifications for any additional venting requirements.