Mystery Damp on internal wall

[Migmag26]

HI,
Some context first - moved into the mid-terrace house in July 2021, house was built in 1875. No heating throughout the summer, only hot water coming on twice daily. No issues.
Heating has been turned on recently twice daily and in the last month a the damp patch pictured has appeared.

You'll notice the picture of the exterior - the damp begins internally about 0.5m upwards, where I'd say the floorboard internally begin and the damp patch extends up as pictured about 0.8m from the skirting. The internal wall is not wet to touch, visible damp, and the meter is telling me it's about 25%. I've had a fan heater on it and it looks to be drying slowly.

IMG-20211122-WA0000.jpg (250.8 KiB) Viewed 1237 times

IMG-20211118-WA0001.jpg (108.84 KiB) Viewed 1237 times

To date:
I've had a boiler engineer come and service the boiler with no real issues other than the flue needing replacing as it's old and the seals on the joins have "gone", BUT it's not leaking etc.. He said for condensation to escape into the wall, it would need to get through two pipes essentially, is the flue is made up of the water vapour pipe and gases one.

Had a damp guy over to check briefly and give a remedial solution of cutting out the plaster and installing a damp proof membrane (akin to tanking) and making good. However, that doesn't solve the mystery - he reckons the historic damp course has failed. possible, I guess.

Possible theories:
1. The exterior mortar around the flue/flue seal, which seems a bit old and cracking is allowing cold air (or from else where) into the gap between the wall, warmth from the boiler/flue is causing condensation on the wall. basically, the boiler room is in a tanked out basement conversion room below the damp patch, sub level.

2. Rising damp - but from where?

That's all i've got. Any theories welcome! :)

[wes56]

You seem to have strap pointing which, besides looking terrible, will hide any DPC, and allow water to be trapped.
Low down the wall you have what - splodges of pointing mortar for what purpose? They too will catch rain water.
The entrance steps rise higher than any DPC, and are therefore bridging the DPC.

Nowadays a common response to damp problems such as yours is to remove the skirting, and all plaster to 300 above the last damp signs, and then render in sand and lime. Doing this, its claimed, will give you another 20 yrs or so damp free surfaces. No tanking, no injections
Is your flooring and joists rot/damp free?

Where are the air bricks?
You need another lower first step - 71/2"should be the max for a riser.
You could have penetrating damp or rising damp or both?

[Migmag26]

Is your flooring and joists rot/damp free?

Hi, thanks for replying.
Is your flooring and joists rot/damp free? YES, the brick below the skirting , under and around the flue is all bone dry.
Where are the air bricks? There's an air brick on the right side of the window (looking at the exterior picture)

You could have penetrating damp or rising damp or both? We've been here 6 months already and had some mega downpours, more so then than now, and nothing appeared. Again, this has all started since the heating being turned on in the last month, twice a day. It's a coincidence, but a safe one to make.