I have a 19 year old atmospheric tank. I'm thinking of proactively replacing it. What sort of efficiency improvement should I expect in going from atmospheric to direct vent? Does it easily pay for the extra cost/complexity? Are the direct vent going to be less reliable or are they pretty good? I like the simplicity/reliability of my old tank, but I feel like I should proactively replace it before I have a problem. My basement is finished, so I don't want to take too many chances.