fireplace question

We bought this ~55 y.o. house a couple of years ago. It came with brick fireplace.
Last night I peered up flue to open it to start a fire. I noticed what appeared to be a thermocouple going to the right side of the chimney.
On the left and right side of the fireplace there is a vent near the floor and one a couple of feet higher. I removed the covers to see if there was any motor in there - which there wasn't. Firebox is metal and there is a gap of at least an inch between firebox and more bricks.
Anyone have any ideas what that thermocouple could control? Or what it might have gone too? I see no power going into the fireplace from the basement. Previous owner blew out a town to unknown place. So I cannot ask him. And the owners before them are deceased.
TIA.
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Sounds like at one time the fireplace may have had a ventilator. The left-right vents blew air out, and the middle vent sucked air in. I have one of these myself. It could be that the unit was simply removed at some point in time, and that is why you don't see a fan blower motor.
Of course, it could be something else too, but that is what it sounds like to me.
--James--
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snipped-for-privacy@comcast.net wrote:

The left and right vents sound like a passive heating system, where room air goes in the bottom vent, is heated by the exterior of the firebox (i.e. goes up between the metal firebox and the surrounding brick) and exits the top vent.
As for the thermocouple, it may serve (or have once served) a powered vent system such as a power flue vent, or a powered heat-recovery system. I suppose possibly it could serve to shut something else (like the furnace) OFF while the fireplace is being used, although I have never seen that. -- H
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