What is this by the chimney?

[nedde]

Started renovating at home and discovered there was an extra wall by the chimney, thought it was part of it but it's not. What's marked with a red dot on the picture, I have a picture from an earlier inspection where you can see the boards going almost up to the outer roof. Filled with sawdust down to the floor on the upper level, at the top you can see up to the roof beams. What could it be?

 

[Eld]

Expansion tank probably. Is there a vent line running straight up through the roof from it? Or is it disconnected and replaced with a pressure expansion tank? It is insulated so it won't freeze and burst.

 

[nedde]

Could it have been? As I said, there's nothing in there except sawdust. They had an electric boiler before but replaced it with ground source heat pump. Now that I've opened up here, does it cause any problems in the attic? Or is it just a matter of insulating and not worrying about it?

 

[mlkjhr]

Just as Eld writes, it is with 99% probability the remains of a space for an expansion tank. If it doesn't bother you, just leave it be.

 

[nedde]

Now I have broken it up and checked. It was an old expansion vessel that was there. Thanks for the responses! There will be an attic hatch there now instead as I didn't have one before.