On Tue, 27 May 2008 10:43:24 -0700 (PDT), Duke of Hazard
wrote:

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.

Per home improvement show, you most likely have a one way valve in
your supply line that is the culprit.