I think you're okay here -- the code requirement about solid or fully-grouted blocks applies to the masonry supporting the flue liner. What you're looking at in the photo is a decorative brick wrap around the structural portion of the chimney. My main concern would be to ensure that this area is properly capped and sealed so that critters and rainwater can't get into the cavity and find their way further into your home.
The left side flue was appropriate when it was built with a closed chase supporting the flue tiles. The right side has no flue tiles and would only be acceptable like a 100 years ago.
That's true if the right side is a flue, but with neither fire brick nor flue liner in evidence, I suspect it's purely decorative -- I'm interpreting OP's text description to mean that we're looking at one of two lined flues, and one of two unexpected void spaces he found when removing the cap. I don't think the void is actually connected to anything, and instead is just decorative, but if I'm wrong and you're correct, then yeah -- there are more serious issues to deal with here
Sorry, I suppose I should have clarified that. The right side is the "gap". Both flues are lined with clay tiles. The left flue is not visible in the picture.
Does the wall dividing the gap and the flue tiles visible from the top go all the way down or does it stop after a few bricks and then just has the flue stack naked in the chase?
So it is basically a closed chase with brick surrounding the flue. That isn't how it would be built today, but very common on most older homes. What you really don't want is naked flue tiles in an open chase with no brick supporting them because they will be prone to serious failures like the tile stack collapsing.
You don't really need to worry about that cavity. They want the brick cores filled and adequate mortar joints in the brick, not dry set(no mortar).
Construct a new crown to seal off the chase and extend the flue tiles by about 6" off the top of the new crown. There is a chance of a cross breeze pulling smoke down the adjacent flue, but there are a few factors that can cause that and I would need a few photos of the whole picture to have a decent idea if that could be an issue.
That makes me feel better. There was no crown initially. The flue tiles terminated flush with the brick inside the chimney cap. The old cap made cleaning and inspecting the flues difficult. It also had a fair bit of water damage.
I would use a grinder to put rebar down over the gap, set into the bricks, put cement board on top of that which has a 1/2" overhang onto the brick. I am extra and would do a rebar every brick across the short side. From there you can built a mold to pour a rebar and concrete slab with a 2" overhang, or no overhang if your rain cap can accommodate the tile extensions. The overhang will help to protect the mortar joints of the brick, so if your cap has less drip edge, I would just pour with the overhang. You do want to make sure the concrete is shaped to allow for water runoff, water sitting on the crown will fuck shit up faster if it sits. Use some foam seal around the tiles to give some relief for expansion and contraction. Seal over the foam with an exterior polyurethane sealant so water will have a harder time acting on the joint between the tile and the concrete.
If you elect to have the tiles with their own individual rain caps, I would suggest adding a sand cap over the concrete to act as a sacrificial wash. You'll want to slip a tuck pointing trowel up against the tile to give some expansion room. Seal that wash to the tile with polyurethane as well. You don't need more than an inch thickness, tapering to 1/2" at the edge. You will have to replace/patch that sand cap every so often, so make sure to check on it. Most of the time you just need to patch it with the leftover sand cap thinned out and applied with a brush, but you can use mortar as well.
I hadn't thought of it that way. I suppose the decorative brick could be just considered a chase. I haven't been able to find any similar examples so I wasn't quite sure what I was looking at.
