What would you call this?

[Ross Neag]

Looking for best description, 1965 Rheem boiler vent run to chimney with a little pit stop. :o

[David Bell]

Is the bottom of the box open?

[wayne soper]

W.T.F.
is that Asbestos?

[John Goad]

My guess would be some type of heat recovery unit.

[john_opwin]

Very difficult to make out from this ........

[Bob Elliott]

Ross was the box open on the bottom.?

I asked because it is most likely a Draft Hood.
Here is a picture from one I had last year in a north side home.

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[Ross Neag]

It was a draft hood, the terminology was escaping me. Thanks...