Nat. Gas B-Vent sizing
If your water heater is 23yrs old, it wont be long before you have to replace it. Might want to do it now and not have to worry about it. 4in. is the most common for residential applications.
During cold weather the furnace will cycle on and off frquently during the day, and keep the vent or chimney warm and venting properly. Once that source of heat is removed, and vented elsewhere the water heater is the only source of heat and does not cycle as frequently, or not at all when water is not being drawn from it, and the vent or chimney cools off during the off cycle.
Once it does cycle on, warm/cold creates condensation. This condensation is acidic, and will attack the mortar of the chimney, or the metal in the vent. Typicaly a stainless steel liner is installed to prevent this. Maybe you should mention this to the furnace installer, and check with your local code enforcement to see if a liner is required. You may get another few years out of that old water heater. It will also come in handy when you do purchase a new heater, and give you another option to venting through the side wall.
Last edited by 8 Ball; 09-14-2008 at 07:22 AM.