Is it the Expansion Tank?

I have a closed hydronic loop with no obvious leaks in a boiler heating system. The expansion tank is quite old. Lately I noticed the pressure dropping even when the circulator is not circulating. What starts out at 15 psi will be down to 7 by the end of the day. There are no known leaks anywhere in the piping. While the expansion tank seems to be the culprit by a process of elimination, some signs point otherwise. For example, when I let water into the system the gauge rises slowly as if the bladder is expanding. Also the top of the tank is warm (when the therm calls for heat) and the bottom seems cold which suggests the bladder isn't leaking.. Before I replace the tank, could someone give me their thoughts on this? I sincerely appreciate your help. Thanks. Frank
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Would you expect an increase of 8 psig due to expansion? I haven't observed this for awhile, so you could be right. I'll try draining the tank anyway. Thank you.
On Tue, 25 Oct 2011 19:45:37 -0400, "Stormin Mormon"

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We searced for such a leak for 2-3 weeks. Turned out to be inside the unit, leaking into the condensate dump pipe.
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You guys will have a laugh over my stupidity. One of the baseboard bleed valves was leaking. It was running down the pipe into the dirt cellar, but I didn't notice because somehow it wasn't draining onto the floor. Some good ideas from all of you. I'll make a list of them for next time. Thanks again. Frank
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frank1492 posted for all of us...

You are NOT stupid! Extra points for posting what the problem was!
Thank you.
--
Tekkie

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