Yellow stains and mold on walls adjacent to the chimney

Hi everyone,

we bought a house last year and around January we started to notice a yellow stain on a room wall adjacent to the chimney that was getting bigger. It was okay after the winter ended. Now the issue is current again and in the laundry and changing room mold has even formed under the wallpaper bordering the chimney.

I have now removed the wallpaper and the plaster, which was extremely porous and partially "swollen."

I suspect there is a chimney decay here and we will have to spend money on an appropriate renovation (pipe in the chimney?!). Has anyone had experience and can maybe conclude or exclude something based on the pictures? The chimney sweep would probably be the best contact, right?

Thanks in advance for all the answers

P.S.: We have a gas heating system, not condensing. It blows 170°C into the chimney - I had hoped that would be enough to carry all the moisture out :-o

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Was the chimney ever used for wood or coal stoves? Old creosoting is hardly repairable. You can almost only work with lining shells.

Phew, that thing is 60 years old. I think it always used oil. But it has 3 flues, maybe there used to be a stove attached.

It’s probably dampened, yes. Our chimney in the attic is like that too. Back then, a plastic pipe was installed for the gas heating and the roof was renovated. There’s really not much you can do, it will probably come through everywhere again, it’s just in the stones. In our case, after multiple repaintings it turned yellow again. You can probably try re-plastering or covering the chimney in consultation with the chimney sweep. Since it’s only the bedroom for us, we just left it as it is.

Had to deal with such spots professionally this spring. The cause there was a chimney flue for an open fireplace that was no longer in use. There was no cover on the chimney. If it rains in long enough and nothing dries out the chimney due to draft, it becomes damp and looked like that. Some chimney owners had a small roof installed on the chimney, others put newspaper at the bottom, which they regularly replaced. Both helped. I wonder if the chimney could be dried by heating, attaching a small roof, applying a barrier layer on the plaster that is still intact—broken parts must obviously be removed—and then it would be fine again?

There are blocking coatings that block this accordingly (if the cause is resolved). New plaster has to be applied anyway. Otherwise, the 170 degrees are obviously not enough.