Boiler TP valve problems?

This is a wierd one. I have a residential oil-fired boiler, with a backflow preventer and PRV set at 12PSI, expansion tank charged to 12 psi, city water at around 80 psi, and typical temp-pressure relief valve that relieves at 30 psi. Normally, everything works fine.
When I (the owner) leave home for more than a couple of days, I like to turn off the main city water shutoff valve to the house to lessen the risk of having a flooded basement. Last winter when I did this, I returned home to find the boiler TP relieving water all over the floor, and boiler pressure gauge reading 40 psi. I checked the expansion tank, found it had been charged to too high a pressure, and reduced it to 12 psig. The expansion tank was checked out later by a local boiler serviceman, and found to be OK (half full of water).
I suspected a faulty PRV valve, and closed the makeup water valve to the boiler, and kept an eye on the gauge for a few days. The boiler pressure krept up and the boiler TP valve started to drip.
I suspected that I had a small leak in the system (I've found one that I haven't fixed yet), and that when I close off the source of makeup water, eventually I loose pressure and get air into the system, which expands the next time the boiler fires and overpressurized things.
To test this, last weekend, I closed the city water main valve, and turned off the boiler burner circuit when I left. I returned home to find the boiler cold (as it should be), but the boiler TP was once again flooding water all over the basement, and the boiler pressure gauge read 40 psig.
I should add that I have no problem for months at a time with the boiler TP valve as long as I leave the city water main valve and the boiler makeup water valves open. Once one of them is shut, whether the boiler is on or not, the pressure in the boiler system starts to climb.
I'm confused!! I'm a heating systems design engineer, but that ain't helping me figure this out. Any ideas?
Thank you very much!
Linc Vannah Stratton, Maine
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does your pressure tank have a bladder, that could be the problem
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snipped-for-privacy@webtv.net (m Ransley) wrote in message

Yes, the pressure tank has a bladder. I don't get any water out of the shraeder valve at the bottom of the tank though, so I believe that the bladder is OK.
Linc
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Linc Vannah wrote:

(The boiler safety relief should be/is pressure only, not T&P.)
Do you have a separate storage water heater? Was it firing during this last test while you were away? If so, is it fitted with an expansion tank? Is the tank plumbed in such a way that overpressure (due to expansion) can be telegraphed to the boiler feed circuit?
Jim
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No, my domestic hot water is heated by a "sidearm heater" (heat exchanger that is installed in the boiler).
The boiler system (and as a result, the domestic hot water heat exchanger) was off while I was away.
The only expansion tank is the one on the boiler. That tank is charged to 12 psig. I set the pressure of that tank (through the shraeder valve on the bottom) while the boiler was pressurized. Should I have set it when the boiler is depressurized (by closing the make up water valve and draining some heating water from the boiler?)
Thanks,
Linc

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How do you heat your domestic hot water? Is it a coil inside the boiler?

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Yes, the domestic hot water is heated by a coil inside the boiler.
Linc
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boiler?
That's most likely your problem. You have a pinhole leak in the coil and it's causing the pressure to increase inside the boiler.
You possibly can have the indirect coil replaced, but I'd recommend either an indirect tank or a separate water heater.
For someone that really knows wet heat, check out this link. http://www.heatinghelp.com/getlisted.cfm
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Thanks for the response, HeatMan. If I had a pinhole leak in the heat exchanger, why would I only see a rise in boiler system pressure when I turn off the city water supply valve?
Thank you,
Linc

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For starters, I can only guess since I can't see it from here.
The 'TP' valve is a safety. That's it. If it blows off, you have a problem. It could possibly be trouble with the expansion tank.

and
either
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