A heater in series will give more water, but do you have a large enough gas line and flue to handle two heaters, especially since a larger one will also have a larger flue outlet.: We are always running out of hot water. I have a eight year old gas-fired hot water heater and am thinking of adding a second hot water heater. If my goal is to never run out of haot water should I just get the biggest hot water heater I can find and have it hooked it up in series with my exisiting unit?: Thanks