As I said, I'm not a plumber, but it is my understanding that sediment can accumulate in the bottom of the tank and that this can decrease the efficiency of the unit by taking heat away from the water, or some-such mechanism. If you look at your tank, you should have what looks like a regular exterior hose faucet (or sim.) down near the bottom. You need to drain some water from that at least once per year. An article at http://www.popularmechanics.com/popmech/homei/9410HIHOAD.html makes mention of this and says just a few quarts. I probably go a little overboard and do it twice a year and take out several gallons by hooking up a garden hose to it. Since the end goes up hill and out my cellar stairs, I first screw an inline gate valve on the end of it that goes toward the heater, and close it before removing the hose so that the water that is in the hose doesn't drain out in my basement. I do more than a few quarts b/c there is iron in my water and I fear it collecting in there. Maybe someone else will give us their thoughts. The only possible downside to doing this and/or testing your overpressure valve is the possiblility that since they haven't been used often that they might possibly not close entirely b/c of the crud that is in there. I think the odds of this happening are small and the gains are definate.