Chimneys that are bowed, bulged, tilting generally need to be better maintained and occasionally better secured.
In older fireplaces or ones with damaged liners hydroscopic salts may be created from burning wood or other fossil fuels. These salts will crystallize in the mortar as they dry, causing the mortar joint to expand.
When there is a wind, one side of the chimney may dry out faster than the opposite side; therefore on one side of the chimney the joints may expand more than the other side. This may create a bow in the chimney.
A similar process may occur in homes near the ocean where the air has miniature salt droplets in it. Likewise, if there is a prevailing wind, then the side of the chimney facing the wind will tend to have the mortar joints expand more than the other side.
Some chimney movement overtime is normal. However, if the movement is significant, additional maintenance or repair work may be needed.
Qualified fireplace contractors and masonry contractors can evaluate whether there is a need to do additional work or to only monitor the condition for changes.
Who to consult? Fireplace contractors and inspectors.
Fireplace contractors will usually advise buyers about the seriousness and whether anything should be done, or just to leave things as they are.