What's causing this efflorescence on internal chimney stack?

[MacLaverty]

In 2023 I had a chimney leak where the chimney stack below the roof line was getting extremely damp to about 1/3rd of the way down the stack below the roof line. In Jan 2024 we had the chimney sorted; all new lead, chimney pots and cowl all checked, new concrete flaunching and all mortar checked. I think they applied a stone sealer as well but unsure.

However, 3 months later there is a lot of efflorescence forming on a few of the exposed bricks in the loft but only about 1/2 way down to top 1/3rd of the chimney stack, nothing at the top where the roof line is. Bricks and timber bone dry even during rain but efflorescence still forming. I have included pictures of the bricks, I have wiped the efflorescence off but the entire brick was white with thick efflorescence on the bits outlined in red. Picture of chimney also attached. Oddly it is also forming on bricks that are part of the dividing wall between properties so not even in direct contact with the chimney stack.

No staining or salts coming through on the plastered chimney breast in the converted attic bedroom, picture attached.

There is a gas fire underneath on the ground floor with a flue and the fireplace in the middle bedroom has been sealed. No other signs of issues on the chimney apart from this efflorescence on the exposed bricks in the loft.

What could be causing this? I don't think it's an external leak as surely the top rows of bricks would be suffering as well? Just weird the efflorescence is forming on the bricks mid way up the stack and no plaster staining unless it is just the bricks drying out gradually as I imagine they were saturated for some time before I noticed and got the chimney sorted.

 

[ree]

OP,
Was the chimney flue swept before it was sealed off at the bedroom fireplace?
Was there a bottom vent fixed into the fireplace blocking?
The "efflorescence" - (if it is efflorescence) - appears to be minor. Maybe its not efflorescence but Hygroscopic salts leaking thro from the flues?

 

[MacLaverty]

It was sealed a long time ago, probably at least 10 years+ and only lived in the house for 2 years so no idea. No bottom vent fixed and radiator in front of the old fireplace opening but no issues at all on the chimney stack apart from these salts. The bricks were completely white with salts before I wiped it all away hence doesn't look much in the pictures.

 

[^woody^]

Once brickwork gets saturated and the salts which cause efflorescence activated, the salts tend to remain active from the slightest humidity change, and can then reappear for years.

 

[JohnD]

Are both the flues ventilated top and bottom?