Installation of cassette in existing fireplace

[Karlia]

It sounds very reasonable with changed effect.

 

[MetteKson]

To answer my own question. Just insert in open fireplace is stated as an essential change of hearth. BFS 2017:4 VÄS 3.

Then I guess it's because an insert in an open fireplace changes the output? I.e., it could affect the chimney or the structure of the chimney?

Yes, insert can mean other stresses and to keep track of, among other things, fire safety, the municipality requires notification.

Imagine you're installing an insert in a hearth in an old cabin. If you don't know what you're doing there, it can go really bad.