Burnham boiler using water like crazy

I inherited a hydronic heating system (Burnham Series 2, installed circa 1996) in the home we purchased in February. We have had problems with it ever since. We have been called by the gas company asking to do a gas leak check because we used so much more natural gas than last year. Additionally, the water company actually took time and highlighted our water bill last month, because the usage was so out of control.
We have been renovating the house, a 4-level split in Iowa, so it is vacant, and have had all four zones set at 55F, after replacing all of the old mercury-based thermostats with new electronic thermostats. I called a local plumber listed on the Burnham Web site, but he knew little about the system, and recommended replacing the thermostats. In the upstairs zone, that made a difference, because requests for more heat actually fill the radiators now, previously that did not happen.
Our water usage is still out of control, though. It is warm enough to turn the boiler off, which we have done, and have also shut off the water supply valve to the boiler. Prior to that, when it was turned on, it sounded like the water was constantly running, like when somebody has a faucet turned on. There was water in the closet near the boiler, so maybe there is a leak in one of the pipes, but I didn't see any visible drips or wet copper pipes. ANother plumber suggested that maybe a copper pipe has burst under the basement floor, and it is constantly requesting water, but I don't show any evidence of that scale of a leak.
I am working with Burnham to locate who installed the unit, so I can contact them to investigate the problem. What types of things are likely candidates for all that water usage, though? Could it be a valve problem or something like that? Also, what annual maintenance would a boiler pro recommend performing annually/end of season?
Thanks for any help!
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Austin Lorenzen wrote:

<SNIP>
If you are using water, there is a leak. (Sounds like a massive leak.) The leak is discharging *heated* water, which explains the high gas useage also.
Is this an old house? Slab on grade? If old, it's possible that the original black iron pipe buried in-ground is still in use as part of the loop, even though exposed portions were re-done with copper. Shouldn't be a big mystery for a local guy familiar with Hydronic.
Jim
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To add to what Jim said, the person that put the boiler in may have not know a lot about wet heat.....
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This house is a 4-level split, built in 1967. I don't think that any black iron pipe was buried in ground. Unfortunately for our area, which is somewhat rural, there are not a lot of qualified experts on hydronic heating. I think that we'll enjoy it, though, once we get everything fixed. Thanks for the advice, that's what I thought it may be.
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Find someone here. http://www.heatinghelp.com/getListed.cfm

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If it's a Burnam V7 they had a lot casting troubles resulting in leaks on that model -- have your heating contractor contact them. Post at: The Wall on www.heatinghelp.com too Burnham hangs out there among some of the best in the hydronics steam and radient business.

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They said it was a Series 2......

gas
leak.)
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