Hi folks,
The plywood forms were never removed from under the hearth when the home was built
, and major demolition would be required to access the ash pit to remove the plywood. (The pit is roughly 2x13 feet of floor area and a couple feet high, making it almost impossible to reach the top except in one or two areas.)
I want to make the fireplace safe to use and ultimately install an insert. The original construction was 12" concrete followed by two courses of CMU (no mortar), a layer of common brick, mortar, and then the firebricks. To prevent embers from igniting the wood, or the construction debris left in the pit that I can't easily reach, I placed a CMU cap at the bottom of the ash dump chute and poured all the way up to the base of the common bricks with solid concrete--about 28" deep.
In hindsight, it probably would have been better to use gravel or sand--something with a lot of entrained air--for part of the fill to improve the R-value. If I simply replace the existing firebricks and ash dump door, there's a good 2" air gap above the concrete. If I were to install a firebrick instead, I'd need to fill up those last few inches. I have plenty of common bricks and quick-setting cement as well as some Rutland Fireplace Patch mortar. Other possibilities include sand or pea gravel which would provide more of a thermal barrier between the firebrick and concrete. The easiest thing to do would be to stick a brick in there and slather on some mortar on top, then place the final firebrick.
Is there anything I can do to improve the safety of this setup as I finish? I have no idea how hot the base firebricks get in operation. Does using an insert improve the picture by removing the need to have the entire system up-to-code as an operational fireplace? (An insulated liner attached directly to the insert exhaust port removes the need to restore the surrounding masonry in the chimney, so do I need to fully bring the firebox up-to-code to use an insert?) #1 concern is making sure sufficient heat can never get down to that plywood to start a fire. I am overthinking this? (Wouldn't be the first time.)
The plywood forms were never removed from under the hearth when the home was built
, and major demolition would be required to access the ash pit to remove the plywood. (The pit is roughly 2x13 feet of floor area and a couple feet high, making it almost impossible to reach the top except in one or two areas.)I want to make the fireplace safe to use and ultimately install an insert. The original construction was 12" concrete followed by two courses of CMU (no mortar), a layer of common brick, mortar, and then the firebricks. To prevent embers from igniting the wood, or the construction debris left in the pit that I can't easily reach, I placed a CMU cap at the bottom of the ash dump chute and poured all the way up to the base of the common bricks with solid concrete--about 28" deep.
In hindsight, it probably would have been better to use gravel or sand--something with a lot of entrained air--for part of the fill to improve the R-value. If I simply replace the existing firebricks and ash dump door, there's a good 2" air gap above the concrete. If I were to install a firebrick instead, I'd need to fill up those last few inches. I have plenty of common bricks and quick-setting cement as well as some Rutland Fireplace Patch mortar. Other possibilities include sand or pea gravel which would provide more of a thermal barrier between the firebrick and concrete. The easiest thing to do would be to stick a brick in there and slather on some mortar on top, then place the final firebrick.
Is there anything I can do to improve the safety of this setup as I finish? I have no idea how hot the base firebricks get in operation. Does using an insert improve the picture by removing the need to have the entire system up-to-code as an operational fireplace? (An insulated liner attached directly to the insert exhaust port removes the need to restore the surrounding masonry in the chimney, so do I need to fully bring the firebox up-to-code to use an insert?) #1 concern is making sure sufficient heat can never get down to that plywood to start a fire. I am overthinking this? (Wouldn't be the first time.)
