You should check either with your local gas company or with your Town to find out about building codes, and to make sure you are not in violation. You probably need a new water heater. The useful life of these things is 20 years the most. They usually quit very suddenly and leave a big flood. You should consider yourself lucky that yours is "warning" you that it's time to replace it. The new gas heaters and water heaters are much more efficient than the old ones. Your gas bills will be lower. And, because of their higher efficiency, they emit less smoke. Therefore, even if your chimney is too small for your currect system, it may be fine for a new one. But you do need to get impartial assessment from either the gas company and/or your local municipality. Your system may have been approved according to codes that were used when the chimney was build years ago. But it may now be a violation because codes may have been updated. In that case, your system may be okay if there is a "grandfather" provision that avoids forcing all homeowners to update their systems.

I suppose this "vent" is for the flue gases? How do you know that the existing vent is ABS? Often installation manuals/instructions are online at the manufacturers website. In the title, you post PVC. In the message, you post CPVC. They are not interchangeable for physical size nor for most uses. How do you know which AO Smith specifies?

If you replaced your current atmospherically vented gas water heater (which I'm assuming is in your basement) with a direct-vent model or a power-vented model, you could eliminate the vent pipe snaking up through your walls altogether. The vent pipe on a power-vented water heater would simply go out horizontally through the top of the basement wall. A power-vented water heater doesn't even have to be near an outside wall. You'd just run the vent pipe from the water heater to the outside wall. Click on the link below then see page 6 in the pdf file that comes up. It will explain the different types of vent systems for gas water heaters. With a power-vent water heater, you can simply remove all of the existing vent pipe that's in your way. Problem solved! Here is a link that might be useful: Water Heater Information

Both the 22v50 and the direct vent 22dv50 have the same energy factor of .58 with a estimated av energy cost of $235. The 22v50 is your basic 50 gal gas water heater. Unless you don't have combustion air available, there is no reason to go with the direct vent. The direct vent would also require redoing all of your venting since they get their combustion air through a coaxial vent. Their largest tankless unit with 35 degree inlet water would put out just about 4.7 gal a minute at 105 degrees. That is about 2 showers at the same time. There are other units out there that can heat more water You are right about the gas line-- it needs to be 3/4". More importantly from a reasonable installation cost standpoint is that you need to keep the flue short since it is a special stainless steel vent. If you can relocate the heater to an outside wall and then vent through the wall it works best. You cannot reuse your existing vent if you were to go with a tankless. Your plumber might have the best idea, just keep it simple and cheap. Insulate the tank with a $15 tank insulation kit and insulate the pipes then call it good. Put your money towards saving energy somewhere else.