Hot water heat acting weird

We have a hot water baseboard heater with 5 zones from 1994ish that recently started acting weird.
About 3 months ago, I noticed that the expansion tank was leaking at the seam...so I replaced it with new. For about 3 weeks, the system was very loud, even after purging as much air as I possibly could out of it. Eventually, all the air purged through the auto fill valve and the noise is gone.
Recently, the master bedroom zone temperature wasn't varying at all as it should with our programable thermostats. At night, the temperature is about 70 at 10pm, and rises to between 72 and 77 by 6am...then drops back to 70 during the day. We have it set to 58 during the day, 68 between 6 and 10, and 65 overnight. So I turned the thermostat off, but the same behavior exists. The baseboard is always warm to the touch.
The other zones were fine, until a few weeks ago. The dining room zone will not move off of 70, even though we have a similar heating patern programmed into the thermostat. I turned off the thermostat and the temperature dropped like you'd expect it to. Turn it back on and we're right back to 70 again.
Anyone have any ideas as to what might be happening? Our heating bills are nearly $400 a month now and I assume that the heating issue has something to do with it.
Thanks.
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Max wrote:

Put a good pressure gauge on the boiler. Did the existing gauge drop to zero when you removed the exp tank? It should have. Does it now register the correct elevation/psi for your system's height?
What I'm wondering is whether system pressure is waaaay above normal.
Is the water temp excesssively high? 140F should be enough. Very high temps might make the system circulate by gravity.
Just stabbing in the dark...
Jim
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Sounds like you may have a bad flow control valve.....

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Do you have five circulators and flow controls, or a combination of zone valves and circulators? and how many of each?

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Thanks for the quick replys. Here's some more info for you.
It's a Burnham furnace, pressure dropped to 0 when we changed the expansion tank. Current pressure is 20PSI, current temperature is a hair under 170.
We have 5 zones with Taco Cartridge Circs Model 007-F4
The controls are Argo ARM842 (3 zone) with 2 expansion zone controls, also Argo of course.
Sounds like the temp might be too high then? What would cause that and is there a way for me to adjust it without calling in a pro?
Thanks.
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Max wrote:

170F may be normal; all depends on the heat load the boiler must supply. You'd have to look if there is an adjustable temp sensor in the boiler (water). Leave the temp alone; that's probably a red herring.
Each circulator should prevent self-circulation in its own loop when not running. That may be a clue.
There is a helpful forum here you might post to: http://forum.doityourself.com/forumdisplay.php?f 8 Whip up a few photos of the zone pump layout.
Jim
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You can see a picture of the system here....I'll post to the other group when I get some free time.
http://www.advantechcny.com/zones.jpg
Thanks
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Max wrote:

Close the ball valve above the circulator for the zone which is overheating even though the circ isn't running. If that stops the zone heating, then you know hot water is bypassing (running thru) the circ.
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I'm not seeing flow checks in the system and the circulators he noted, don't have integral flow checks
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RBM wrote:

Apparently the system *did* work at one time though. Hmmmmm
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There are flow check valves installed...just a little further down the line. I had some trouble loosening all but one screw as they wouldn't budge. Not sure if they should be left open or screwed tight? One is open, the others are closed or mostly closed.
The system did work just fine.
I'm not sure if it's an issue, but the circ pumps pull, not push.
I have created a line diagram of the system you can see here: http://www.advantechcny.com/diag.pdf
Thanks for your help so far. I'll shut off the zone above the pump tonight and see what happens overnight.
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Max wrote:

The flow controls are set to balance the zones. Probably not critical on your system with individual zone pumps.
But one of them may have failed (corroded/debris) in such a way to allow gravity flow in that zone.
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Curious if the one that was open is on the affected zone. These valves prevent backflow and if left open, allow the zone to work by gravity. Try closing them all

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