Unexplained internal chinmey vents

In my 1955 house the fireplace in the lounge has been blocked up and a vent placed at ground level.
In the adjoining room there is a vent some 3' up the wall which I expected to be there to facilitate a circulation of air in the flue. However, when I take the grill off it reveals an open 3" asbestos pipe which appears to enter the flue and turn in a downwards direction.
There is a similar vent to the chimney at first floor level but I have not yet removed the grill to examine behind it. In both cases there is a narrow shelf above the grill.
The removed grill has an integral box section at the rear which has been cut into the bricks of the wall.
Any knowledgeable thought on the purpose of the vents would be appreciated.
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vent
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appreciated.
Warm air circulation between rooms from one centrally based heating appliance. Easy way to distribute heat around the rest of the house ?
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I did consider this as an option but personally would have been concerned with having what I presume would have been an open fire as the heat source.
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source.
And you're sure the two chimneys are connected by the asbestos pipe ?
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concerned
Only one chimney!

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?
Then this idea has probably been an after thought to spread the hot air around when another heating system was installed. Is the rest of the chimney sealed around the asbestos pipe, or has it been left open to draw to the pot on the roof ?
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As the original fireplace opening has been closed off I cannot determine how the other end of the pipe fits into the scheme of things but from your assumptions and those of Mich it appears the warm air circulation theory looks good.
My thanks to both of you for taking the time to respond.

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?
Its a type of warm air circulation system based on the idea that the heat from the central fire could be redirected to another room/area. It was an idea tried out in the 1950's. I was not particularly successful I dont think.
I had one in my old house which was built in 1955. ( ex council - my LA put them in all the post war build houses of the time) It didnt work but what the hell?
I closed the vent off in the 1980's.
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