Chimney ventilation

[ntl142]

I've got a chimney breast that's been blocked up at some point in the past with no visible ventilation. There were some damp patches but the dehumidifer is slowly drying them out. As a result, some salts are now appearing through the plaster where the damp patches were. I need to add some ventilation, but don't particularly want a visible vent in the wall (ugly, and will leak precious warm air into the chimney void).

Q1. The floor is suspended wood with about a 12" gap underneath it. Is there any reason I can't use the underfloor airflow to also go up the chimney? Are there any ways in which this would normally be done? Basically make a hole in the hearth that connects the chimney void with the underfloor void. It's on a party wall so can't vent from outside unfortunately.

Q2. Will the salts ever stop forming/stay away when brushed off, or will I need to replaster anyway?

 

[bobasd]

if you post a photo of the c/breast we can see whats goin on?
have a look at all roundthe c/breast and the back walls for damp stains.

youve probly got soot salts from the unswept flue coming through the c/breast bricks. brushin it off wont make any feiffrence.
all contaminated plaster will need knockin off and done back up with a sand andlime render.
the flue needs opening up and sweeping. any other flues in the house probly need sweeping to..
vents must be fitted to any blocked over fire openings in any room
you can go through the side of the c/breast to fix a vent.
flues need venting top at terminal and bottom.
i'd stay away from going below floor venting or messing with the hearth.

 

[ntl142]

you can go through the side of the c/breast to fix a vent.
flues need venting top at terminal and bottom.
i'd stay away from going below floor venting or messing with the hearth.

Thanks for your reply. Yes I'm aware of venting in the side of the chimney breast. Two reasons I'd like to avoid this though: Thicker brickwork to fight my way through, and still leaks heated air from the house straight up the chimney.

You said you'd stay away from below floor venting. Why is this?

 

[Nige F]

Regarding the hearth - you might get away with digging a channel and laying a 110mm plastic pipe to the underfloor void and make good . Or you might have to dig the lot out and reinstate the floor with timber spanning the void. Possibly building dwarf wall to support floor joist.