Boiler Leaks Only After No Heat

Hi. I hope someone can give me a clue what might be going on.
I have a new Burnham ES25 gas hot water boiler with indirect hot water. I have 3 zones for heat plus a 4th zone for indirect hot water. It's a 16500 0 BTU boiler. It was installed last November.
Here's my problem. It seems that after not providing heat for a while, if I ask it for heat, it leaks from somewhere under the boiler. It happened t wice. Once after I had a problem with the boiler and had no heat for a day (the connector for the flu damper was loose so the boiler wouldn't fire up ). And once in the Spring when I thought heating season was over and we ha d a cold night, so I turned the heat zones back on. Both times, after I tu rned the heat back on, I noticed a small puddle on the floor near the left front corner of the unit. There was also water underneath, it seems on the left side. But since the right side is near the wall, I couldn't really s ee if there was water under that side also.
If it was a leak, I would have leaking all the time. The boiler runs 24/7, even during the summer for hot water, and I never see water at all. When it was running constantly all winter, I never saw any water. Just those two times.
What could cause this?
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On Thu, 3 Oct 2013 19:36:16 -0700 (PDT), FTR

New enough to be under warranty?

Crack some place that leaks when cold and is closed when heat makes the metal expand? Just a guess. I have never had a boiler.
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On Friday, October 4, 2013 7:09:45 AM UTC-4, micky wrote:

Or a connection, seal, etc. When it's cold it contracts enough so that when re-started it leaks. Warmed up, it expands and the leak diminishes.
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Yes, it's under warranty.
The plumber who installed it is having a chimney guy come to look. He thinks the flu may be partially blocked and that would condensation.
But my question is: if there is a (partial) blockage, wouldn't the boiler not fire up at all? Aren't there safety controls that prevent that, because of the danger of CO build-up?
On Friday, October 4, 2013 7:09:45 AM UTC-4, micky wrote:

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FTR;3130080 Wrote:

There should be two safety controls on your boiler. One will be mounted on the flue hood above the boiler to detect flue gas spilling out the bottom of that hood. The other will be mounted near the bottom of the boiler to detect unburned gas spilling out the bottom of the boiler. If either of those safety devices detects a problem, it will not allow power to pass through it, and that prevents power from getting to the gas valve on your boiler so that the boiler shuts down.
I expect that if you have a partial blockage of your chimney, then you would get flue gas spillage out the bottom of the flue hood over the boiler. But, it's possible that even with a partially blocked chimney, your chimney could still draft sufficiently well to draw all the flue gas up and out the chimney.
Typically, the flue vent from your boiler won't connect to the bottom of the chimney. Instead, it will connect to the chimney a few feet above the bottom of the chimney. That's done so that leaves and brick mortar and dirt and stuff that falls into the chimney DOESN'T fall into your boiler and cause a potential blockage. All of that stuff will fall past the point at which the flue vent connects to the chimney and accumulate at the bottom of the chimney. If you look under the point on the wall where the flue vent from your boiler connects to the chimney, you should see a clean out plate. Removal of that clean out plate allows you access to the very bottom of the chimney so that you can clean out anything that has accumulated there.
The way inspectors check the condition of brick chimneys is by the amount of sand that's accumulated at the bottom of the chimney. Since flue gas condensate is corrosive, it tends to dissolve the lime and cement in the brick mortar, leaving only the sand behind. That sand collects at the bottom of the chimney, so the more sand you see at the bottom, the more deteriorated the mortar joints in the chimney are. (note that many brick chimneys use rectangular clay tiles inside them so there won't be any mortar deterioration and no sand at the bottom of the chimney)
The way inspectors check for a partially blocked chimney is by sticking a hand held mirror into the clean out at the bottom of the chimney. They hold the mirror at a 45 degree angle so they can look up the chimney. If they see "a light at the end of the tunnel", that's the daylight they're seeing, and if the light is square or round or rectangular like the chimney is, then they know the chimney is clear and unobstructed.
--
nestork

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