Chimney smoke chamber design

[2level]

While cleaning out the chimney (masonry) the other day, I noticed the smoke shelf only goes about 1/4th of the way back. Then it's open air. There is a big void between the back of the firebox and the chimney outer wall.

Did the mason screw up the design or do a incomplete job? I'm thinking about filling the void with sand ( probably 2-300lbs.) and topping with a layer of mason mix.

Has anyone seen a smoke chamber built like that?

 

[aj3006]

It's hard to know without seeing it but I wouldn't fill with sand you
could collapse the back of the fire box.
How old is the house? things were done differently many years ago.
I would ask whats holding up the flu pipe it should be on masonry all 4 sides.
Here is a sight for you to visit the detail may help
www.masoncontractors.org when you get in there go to technical notes
then go to technical note 19A this may help explain

 

[2level]

Built in 1950. The flue is held up by masonry on all 4 sides.
Thanks for the link, but it wouldn't allow tech access without signing up.
And when signing up, I got "site not available".
Do you have cliff notes for tech note 19A?

 

[aj3006]

well it sounds like your throat was built correctly. So more than likely they
just left the area behind the damper open instead of filling in the smoke shelf.
I have seen that often on fireplaces of that era.
without seeing it it person it sounds like it's fine.
we used to have an old mason that worked for us who always built his fireplaces this way even though we wanted him to fill that area in but we
never had any issues with the fireplaces he built.
I would however leave and not fill it in.
If your worried have someone inspect it

 

[2level]

The problems are cleaning the soot out of the void is a PITA and rain water makes its way down there and leaves a moldy smell.

 

[aj3006]

can you put a top hat or spark arrester on the chimney thats what I
would recommend it's likely 12"x12" or 13"x13" flu home depot has them

 

[2level]

I have a spark arrestor, but water still gets in. The bigger issue is soot/ creosote= fire risk/ cleanup PITA.