Water Pressure Low - But Builds Up

All,
I own a condo that was built in the mid to late 70's. Recently, we had individual water meters installed at each unit, and a plumbing company installed PRV's at each unit as well. Started noticing some strange things after this happened. First - the water pressure was cut in half...which is not all that bad, being that our pressure was around 105 pounds prior. Over the past few weeks, I have noticed that our pressure seems to build. For example, after some time not using the water, I can flip on the kitchen sink and the pressure is extremely high - then levels out.
The plumber came out again and replaced the PRV - still notice the problem!
Anyone have any suggestions as to what might be the problem here? I would also mention that our hot water heater is quite old (not sure how old...but during the inspection we were told that it probably had a couple of years left). Could this be a potential culprit?
Thanks
Chris
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Chris wrote:

Anytime a PRV is installed, a thermal expansion tank is *required* on the water heater. The PRV prevents the high pressure created by expansion from returning to the street main. More: http://www.stateind.com/expansion/expansion.htm
If you get flack on this, call the city inspector to get their official position.
Jim
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Interesting...
I have an electric water heater...does the expansion tank still apply?
Chris
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You are still heating water, water expands when heated. Pressure is a factor of volume in a given area. Since with all the water taps off, and the PRV in the line, you have a 'closed' area. Increasing the temperature of the water increases it's volume, the area is fixed, so the pressure goes up.
--
Anthony

You can't 'idiot proof' anything....every time you try, they just make
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