Water still running after shower turned off

I just had a new hot water heater installed a week ago. The plumber recommended an Expansion Tank, and he installed one of those as well.
Everything is fine. However, once my shower is turned off, or a toilet is flushed and fills back up, I still hear water flowing through my pipes through out the house for about 30 seconds or so. Nothing is leaking anywhere, and this behavior did not exist before the new hot water heater and new Expansion Tank were installed. My plumber is baffled. He came out and replaced the Expansion Tank (thinking that was the problem) but it hasn't helped.
Any idea why this is happening? Is it the new Expansion Tank and, if so, is this normal behavior?
Please feel free to post here, or e-mail me directly at snipped-for-privacy@comcast.net.
Thanks.
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Robert Spike wrote:

It could indeed be due to the exp tank, but it's not necessarily a problem.
If the system pressure drops considerably with useage (toilet refill, etc.), water will be forced *out* of the exp tank. When the toilet has filled, the pressure builds again and now the exp tank fills up.
You might experience this on a well pump system; less likely on city water unless there is a restriction which drops pressure markedly.
Jim
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I am baffled why the plumber doesn't know. Why did he recommend the expansion tank unless you had low water flow? So now that you have the expansion tank you should get more flow with the extra supplied from the tank. But after you turn off the valves it takes a while for it to fill again. If there is a way to turn off the line to the expansion tank I expect turning it off would eliminate the problem.
If the sound itself is exceptionally loud that is something else.
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So - - - to confirm what I think I understand here. The sound of the water rushing through the pipes for 20 seconds or so *after* the shower/sink/toilet shuts off is actually normal as the water is refilling itself back up into the Expansion Tank? I actually know absolutely nothing about plumbing, so all this information is very helpful to me.
I'm not sure why the plumber recommended this for me. It was pretty inexpensive, and I recall having one put on in my last house (in the same Township) after the water company installed a "back flow" device. But, this behavior didn't exist in my old house - - - Just my new one since 6 days ago when the new water heater and Expansion Tank was installed.
I'm just trying to reach a conclusive decision and feel comfortable that my house isn't going to flood from a pipe burst. :)
Thanks, again. I'll look forward to further responses.
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Robert Spike wrote:

Yes, it appears to be normal. As I said earlier though, there must be *some* kind of restriction causing the pressure to drop (sounds like you are again on town water supply). But if you're happy with the way the fixtures work, I wouldn't worry any more about the refill noise.
Jim
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Since posting my last message, I contacted the Water Company. My home currently DOES NOT have a Back-Flow device installed, but it DOES have a Pressure Reducing Valve. They suggested having the plumber inspect the Pressure Reducing Valve and adjust it if necessary. Is this consistent with the problem that I am describing? Do these things need adjusting?
In speaking with my plumber again, he wants to come out and remove the Expansion Tank so see if that helps the problem. If he does that, and the problem goes away, what then? How necessary is the Expansion Tank? The house is 16 years old and has never had an Expansion Tank before.
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The only time I have seen them used is if you have trouble with low water pressure or perhaps noise in the pipes.
But appears there may be many other reasons. A Google for expansion tanks resulted in thousands of hits:
http://www.stateind.com/expansion/expansion.htm
http://www.contractormag.com/articles/column.cfm?columnidu
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Robert Spike wrote:

A thermal expansion tank is *required* when there is a pressure reducing valve or backflow preventer installed. So, the plumber was acting on his best instincts.
One possibility is that the reducing valve is not closing tightly. This means that once the set pressure is reached, a small amount of water continues to flow past the valve seat and the pressure slowly climbs to max street pressure. Easily test this theory with a pressure gauge. It sure would explain the noise.
Jim
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The plumber returned to my home and corrected the problem. I thought I would post my findings here.
He took a pressure reading and it revealed a reading of 105 PSI. He tried to adjust the Pressure Reducing Valve but it wouldn't go below 100 PSI. He said this was way too high. He replaced the Pressure Reducing Valve, adjusted the pressure to 75 PSI and now all is fixed. Now when I flush a toilet, turn the shower off etc. the water stops immediately and I don't hear the noise anymore.
Thanks for all the information.
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