Water heater spilling water

My water heater will over-pressurize and open the blow-off valve and begin spilling water to the drain in the utility closet. I've tried stepping down the temp. incrementally to find a position that it likes, but nothing acceptable for showers will be acceptable for the heater. We've since been turning off the water supply valve to keep it from overflowing when we're not in need of hot water.
I called a plumber and the guy mentioned that they keep cranking up water pressure through our area to supply the folks farther out with decent pressure. He was wondering if we had a pressure reducing valve in the unit and I was going to check on that before I had a plumber physically come out. I believe it is more a problem with excessive water pressure coming to the tank because if we shut off the cold water supply to the unit it will not spew water even with the temperature elevated.
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Duke of Hazard wrote:

If city pressure *is* high (over 80 psi, for example), you will likely need a pressure reducing valve AND a thermal expansion tank. http://www.amtrol.com/thermxtrol.htm
Jim
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On Tue, 27 May 2008 10:43:24 -0700 (PDT), Duke of Hazard

Per home improvement show, you most likely have a one way valve in your supply line that is the culprit.
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Yes, the fix is simply to install an expansion tank in the cold water line, after the back flow valve.
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Duke of Hazard wrote:

Replace the T/P valve under the theory that the most likely culprit is the one that shows the symptom.
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I agree but my post was intended to cover all possible bases :)
I forgot a reminder - exercise those T/P valves annually, at the same time that you do a sediment flush and anode inspection (you do do all that stuff, right? RIGHT?)
nate
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around here exercising the valve means replace it
sediment flush gains nothing
anode inspection a great way to generate a leak.
water very hard, causes corrosion.. draing tank waste of time,.
how old is the OPs tank? if its gas and near end of life, say 10 years just replace it. new tank will be more efficent, and messing with old ones will likely cause leaks..........
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