Wood-burning insert--firebox condition--conflicting advice

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skeller1

New Member
Oct 15, 2025
2
Michigan
Hi. I have a 100-year-old home with a brick chimney and firebox. I want to install a wood-burning insert (Osburn Matrix or similar).
I've had two installers inspect the fireplace and chimney and provide recommendations. Unfortunately they completely conflict, so I'm hoping for advice and information.
The firebox has gaps in the grout, but is otherwise sound.
One installer also noted that beneath the brick and concrete of the firebox floor, spanning the ash dump, there is wooden formwork that was left in place after the original construction a century ago (seems this is common). He is concerned about this--reporting that he seen cases of this wood charring and even burning as it dries out over time--and says that an insert should not be installed. What I understand from him is that--even when installing an insert--code says that the firebox must meet the conditions for an open wood fire (to protect combustibles around and below the box).
The second installer says that is crazy: all of the high-efficiency inserts he is recommending have their own fireboxes and airflow around them which offer plenty of protection. He notes that the inserts include blowers, wires, and plastic parts that are not affected by the heat, so there is zero risk to the wood separated by eight inches of the brick and concrete floor of the firebox.
Given that the risk involves the house burning down, I'd really like to know who is correct!
I've looked at the installation manuals for the inserts and they have little to say about this. There is only some general language suggesting that one consult local codes.
I would appreciate any advice and/or direction to the appropriate codes. Thanks.
 
It's not uncommon to see the form wood left in place even if wrong. There should be no wood under the hearth, by code. The second installer is looking at this from a safety standpoint, and is probably right, but in an insurance case they will be looking for code violations.
 
Thanks for the quick response. This fits my impression of the contradiction: It's likely safe but against code, which leaves me open to an insurance problem should anything ever happen. (And what does "likely" mean here? I really want that risk to be as close to zero as possible.)
The conservative installer is suggesting a PriorFire kit for the firebox, but this also comes along with more substantial work in the chimney. It is not inexpensive!