Hot Water Heater - Leaking from pressure valve

We discovered that we were getting water leaking out of the pressure relief valve/pipe... about a cup per day.
I called in a plumber and they suggested that our pressure in the tank was really high. They changed a main feed from outside and also changed the pressure valve. This was fine for about 2 weeks. We are now seeing water coming out of the valve again.
What else should be checked or what can be suspect? The water heater is 9 years old... is it that I need a whole new one?
Any advice would be great! Scott
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Did the plumber actually measure the pressure with a gauge?
Are you on city water or a well?
Is there a Pressure Reducing Valve on the supply from the city water main? Do your neighbors have PRV's?
Do you have a thermal expansion tank?( separate small tank connected to the cold water pipe near the water heater)
Explain what "main feed" they changed.
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He measured it and said it was above 100 ???? I am on city water. I live in an association and we all have similar feeds into our houses I am not sure if there is a pressure reducing valve from the main... I know he also replaced something in our closet that supposedly came in from the main feed. I do not think we have a thermal expansion tank....
Your thoughts and thanks for your time!
Scott

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On 12/11/03 "Whitespc" snipped-for-privacy@comcast.net writes in part:

Read this to help you understand the finer points of thermal expansion.
http://www.reevesjournal.com/rj/cda/articleinformation/features/bnp__featu res__item/0,,68876,00+en-uss_01dbc.html
It may work as a link, if not, copy and past it into your browser.
The fact that you've never had the problem before means absolutley squat!! The only thing that matters is what are the current conditions.
Good luck!!
Doug
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I have received great input... thank you.
The job is done. An expansion tank was now installed and I believe this is what was needed from the start.
So now, a new pressure regulator, a pressure relief, and an expansion tank were installed..... .not many other parts left. I am now over $400 in the red to fix a small leak. I am not sure this went exactly as it should have.

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