Furnace Flue Device - What Is It?

On my natural gas furnace there is some kind of box on the furnace exhaust pipe wired to the main unit. What is this and what is it supposed to do? Does it measure heat going up the chimney or gasses? How does the furnace respond based on the reading? What are the implications for maintenance.
I realize this is probably more complex than it seems, but I'm trying to get a handle on how the furnace operates. It's a Peerless boiler furnace, about 7 years old.
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Hi, probably a damper? It is closed when furnace is off to keep heat from escaping. When furnace comes on, it opens. Usually little motor moves it on/off. Tony
John Smith wrote:

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That makes sense! I just patched a 1 foot diameter hole in the chimney left by the prior owner so I could keep exhaust gasses and cold out of the basement. I guess it just didn't occur to me such an intelligent thing might be on the furnace under such circumstances :)

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It's hard to tell without looking at it, but it's probably a blocked flue/spillage switch. If the flue ever gets blocked ( by squirrels building a nest or the chimney collapsing) the switch will open up and keep the unit from working.
If the unit has stopped working because of the switch, call a pro to look at it. Make sure s/he checks the chimney. Don't jump it out or you and/or your family make wake up dead....

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Probably says Honeywell on it too..
Might have 2, or 4 wires leaving it, and its got a cover that pops off and has a board behind it right?
Thats a flue dampner. Required in some areas for code, and it should open fully when the thermostat calls for heat, and not allow the furnace to fire until it is open fully.

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