Air brick in chimney breast?

[NWMark]

We are getting a damp patch on the plaster on the chimney breast upstairs. starts about 12" up from the floor and is about 18" high

There is a gas fire downstairs still plumbed into the chimney, although it doesnt get used. Upstairs i dont know if there ever was an opening for a fire as it was like this when we moved in and the breast is less deep than the one downstairs, maybe 12"+ downstairs and 6" upstairs.

We have a shared chimney with next door, with 4 pots on, is each pot attached to a separate 'chamber'?? if so im not sure which pots our ours and which are his, they are all open ended, no 'caps' on.

Ive read i need to install an air brick, is it as simple as remove some plaster, take a brick out and replace with an air brick, or is there more to it than that?

Cheers

[NWMark]

Thanks for the replies.

House was built 1930-ish, ill get up into the loft tomorrow and have apoke around.

Think ill remove some of the plaster board and see what is behind there, house has been completely reskimmed and painted before we bought it, so hard to tell what was there in the past.

So if i turn the fire on downstairs, on a cold morning i should in theory see heat coming out of one pot only?