Edit: Resolved. Decorative brick is actually a chase and doesn't need to be completely filled in. Thanks to @Death_Equity@lemmy.world for advice on rebuilding the crown.
I have a double sided wood burning brick fireplace which is honestly just a big collection of code violations. It was converted to propane shortly after -- I'm assuming -- the people who built the house 50 years ago discovered that it smoked badly in the house. I fixed all the major issues with the firebox and smoke chamber and converted it back to wood burning this past winter.
I had a new steel cap fabricated to replace the old brick and concrete cap, which I removed. During this process, I discovered that the brick is basically just a facade. There are huge gaps on either side of the flues which are encased in 4" cinder blocks. My understanding of fire code (at least in the US) for masonry fireplaces is that the material surrounding the flue is not permitted to have any gaps in it. NFPA says all gaps in the brick should be filled with mortar. However, I'm not sure what to do with a gap this large. I'm not sure if filling it with mortar or concrete is an option. I've considered installing a steel flue liner but those are expensive and there are two flues. I'm also concerned about steel liners changing the draft characteristics since the chimney is on the short side.
If money were no object, I would tear this thing out and put a more efficient steel insert in but that is cost prohibitive.
Anyone have any experience fixing something like this?
I'm certain neither flue would meet modern safety standards, and that is just from what I can see in that picture with very little information. There are some grandfathering of some issues.
Both flue almost certainly has deteriorated mortar joints or damaged tiles. You would need to have the flue camera inspected(level 2 inspection).
~~The right flue is fully fucked and should not be used. The tiles are gone and there is masonry that has fallen and is obstructing the flue.~~ Edit: Not a flue, is open chase cavity.
To determine if the flue will be able to have a stainless liner, you need the measurements of the firebox width, depth, and rear wall width. You then can put those measurements into an online calculator to determine liner size. The next problem is how much space you have in the chase(chimney structure surrounding the liner). If your chase has an opening of 10"x10" and you need an 11" liner, that is a problem.
The flue may be able to be coated with a masonry product like Fireguard or Fluesaver. That would depend on the condition of the flue tiles and the exact system used to reline.
The top of the chimney should not be closed off with a cap like you have there. It should have the tiles or a liner extending past the crown(top) of the chimney, the crown would close off the chase and keep water, debris, animals, etc. from being able to enter the chase. Water and products of combustion acting on the mortar joints of tiles and brick will destroy the structure. Just putting that cap on there is better than nothing, but it is more of a way to buy time so you can actually fix it properly. The crown used to be covered and sealed properly, but has deteriorated to the state it is in now.
Get a certified chimney sweep out there for a level 2 inspection of both flue and they will be able to give you a better understanding of what all needs to be done and you can go from there. Use a company that does the inspections and the work. If they don't do the work, then you will have to deal with inaccurate quotes and a bigger headache. Just do an inspection on the right one, no sweep. They should be swept with the inspection to get a good idea of the tile and mortar joint condition.
Edit: fixed some stuff based on further explanation of the situation.