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We have an old fireplace which has been plasterboarded and plastered over. In front of this we have a wood burning stove, with a pipe which goes up to the original chimney in the roof. (outside the wall up until the chimney in the attic).
The plaster behind the stove is cracking.

From tapping on the wall it sounds hollow behind, so I think they have literally put plasterboard over the hole but not bricked up the old fireplace. I think the air is getting heated and expanding, pushing the plaster out as there is no longer a link to the chimney from the air gap.

I want to cut out the plasterboard where it is cracking (the crack forms an actual outline of the board - approx a meter by half a meter), brick up the hole and then plasterboard over the brick to match the room.

From reading around, when you brick up a fireplace you should put in an air block. Is this needed if there is no route to the chimney anymore?

If it is, can the plasterboard go over the air brick, or does it have to be left open to the room?

Would I have to brick the hole, or could I fill it with, eg expanding foam or similar?

What is the best way to remove the plasterboard so I can reuse it? (it is heatproof plasterboard so expensive)
 
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Here is a photo of the setup, and a close up of the cracking
 

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Is there is an external chimney breast that has plaster board over the original fireplace opening?
Are there any other fireplace openings in the room(s) above?

The stove flue pipe goes up to a larger sized pipe - does it go into a reducing bushing? Can you post a pic of the top of the flue at the ceiling? Where does the flue pipe go after it leaves the photo?

There are various Regs connected to a wood burning stove installation - At some stage perhaps you should call in a HETAS for an opinion on your installation?

FWIW: do you have fire and CO alarms
 
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The larger pipe is a double walled insulated pipe so it doesn't get too hit. The insulated portion of the flue goes through the ceiling, straight through the bedroom above, and then into the attic, where it then exits through the chimney stack. The stack was completely removed from the bedroom and living room by previous owners, and plastered flush to the wall.
The guy who did the install for the stove was Hetas registered.
We have fire and Co Alarms in place.

There is no external chimney beast, the wall is the party wall between semi-detached houses. I was assuming this when they removed the chimney beast and fireplace they left a cavity behind (even if small) and just put a plasterboard over the cavity. If so, the top of the cavity would be closed off (no opening into the bedroom or loft above it) meaning any air expansion would have to push the plasterboard out. Does this make sense?
 
I think the air is getting heated and expanding, pushing the plaster out as there is no longer a link to the chimney from the air gap.
Not likely. The wall is simply getting hot and expanding / flexing, which is straining the joints. You need some kind of heat shield behind the stove (i.e. big sheet of metal). Or just fill the gap and paint over, periodically.
 
What about tiling the wall behind the stove, or a big piece of stone/slate mounted on top of the plaster?
 

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