Finally put a bulb in the bottom of the chimney and looked down from the roof.
Found that the device to keep the damper open that was installed by the gas fireplace people doesn't work.
Soot & probably creosote, yes, visible.
Anyway, here's a quote from a 2009 patent application for a device to re-mortar the joints from within, in a chimney lined with clay tiles (like mine).
"It has been estimated that nearly half of all masonry chimneys that are lined with sections of clay flue tile have at least one joint (the juncture between the each section) that has deteriorated and needs to be replaced. This can create a potentially hazardous condition for the occupants. The purpose of a chimney is to contain and convey the products of combustion to the outside. If the chimneys liner contains gaps and voids, it can no longer be relied on to function for its intended purpose.
All nationally recognized building codes (National Fire Protection Association 211 Standard for Chimneys & International Residential Building Code-R1001.9 Flue lining installation) call for flue liners to be joined together and sealed with a non-water-soluble refractory cement that meets ASTM C199 medium duty classification. Unfortunately, it has been found that during the original construction, builders often use standard mortar to lay or seal flue tile sections. Because standard mortar is water soluble and not meant for high heat applications, joints constructed with standard mortar may deteriorate more rapidly than sections joined together and sealed with refractory mortar.
Because most residential chimney flues are long narrow tubes, it is nearly impossible to repair chimneys that are found to have this condition. Until now the only reliable method has been to tear down the chimney and rebuild it or to line the existing chimney flue with stainless steel pipe."
Only 1/4th of the tile perimeter abuts the house and the gases still have to go through brick seams. I think the fire danger from open tile seams is minimal because of our layout, and we have CO detectors. We've never had a CO alarm nor had symptoms.
Anybody here see a problem with wait & see? Some scenario I haven't thought of?