Block off an internal chimney vent

[Lamentable]

Hi, in my 1930's house both upstairs bedrooms used to have fire places. In both of the rooms there are vents just above the skirting board covering the old fire places. The vents are white plastic and about 30 x 30cm and can be opened.

I am refurbishing the rooms and wondering if can block these vents?

To give more context: If you imagine a typical 1930's property - two downstairs reception rooms with fire places (now gas) and above those two bedrooms with fire places (were also converted to gas at some point). Externally there are two chimneys with external breasts, one for the front rooms and one for the back rooms, with two "pots" on the top of each one. I'm guessing 1 pot is for the upstairs room and 1 for the downstairs on each one.

If I can't completely block up the vents in the bedrooms could I fit a smaller airbrick or vent?

Thanks in advance.

Edit: Another idea - could I put an external air brick?

 

[denso13]

Another idea - could I put an external air brick?

Yes, as long as it is ventilated somewhere.

 

[Lamentable]

Is that something I could do? I would be worried if I remove a brick externally the whole thing would collapse!

 

[denso13]

I would be worried if I remove a brick externally the whole thing would collapse!

Nah it'll be fine, you could post some photos if you want more reassurance.