I hope I am describing this well enough for people to understand.
I live in a house built between 1895 and 1905.
The house has two rooms with fireplaces that share common chimney.
In the ceiling about 2 feet in front of each of the fireplaces there is a circular hole that is covered with a round metal cover.
Directly above where these circular holes is the second floor and rooms.
I thought that at one time that they may have been a stove and the vent from the stove went thru the holes in the ceiling extended thru the second floor all the way to the roof.
The other day I started ripping out the carpeting on the second floor and to my surprise under it I found a vent register. I pulled up the register and saw what looked like to top of the round cover that is in the ceiling. This cover is held in place with a couple if springs. Once I popped the springs the cover fell down to the first floor and I could see all the way down.
Now my guess is that the register would be in the closed position in the summer and in the winter it would be open and the heat from the fireplace would go right upstairs thru this register.
Has anyone else come across something similar in their old home? Am I totally wrong in my thinking on its use?