There are two possibilities; since you didn’t describe that hole in detail it could be a combustion air supply or an ash dump cleanout for your fireplace.
A combustion air
intake for a fireplace is usually located in a lower portion of the fireplace burning chamber / hearth (side or rear wall), an opening to exterior that provides fresh air for the burning process.
It was (and still is in many jurisdictions) required and meant to support clean burning process and to prevent depressurizing of a home which could result in creating hazardous conditions like Carbon Monoxide spillage into the living area / backdrafting.
R1006.2 Exterior air intake.
The exterior air intake shall be capable of supplying all combustion air from the exterior of the dwelling or from spaces within the dwelling ventilated with outside air such as non-mechanically ventilated crawl or attic spaces. The exterior air intake shall not be located within the garage or basement of the dwelling nor shall the air intake be located at an elevation higher than the firebox. The exterior air intake shall be covered with a corrosion-resistant screen of 1/4-inch (6 mm) mesh.
The combustion air passageway shall be a minimum of 6 square inches (3870 mm2) and not more than 55 square inches (0.035 m2), except that combustion air systems for listed fireplaces shall be constructed according to the fireplace manufacturer’s instructions.
Locating the exterior air outlet in the back or sides of the firebox chamber or within 24 inches (610 mm) of the firebox opening on or near the floor is permitted. The outlet shall be closable and designed to prevent burning material from dropping into concealed combustible spaces.
What’s puzzling is that it appears that the above IRC articles are based on the assumption, not a research that such installation is a good strategy to reduce spillage susceptibility. If you are in Canada, the mandatory requirements for outdoor air supplies for fireplaces have been entirely removed from the NBC (National Building Code) in 1995. This was done based on Canada Mortgage and Housing Corporation (CMHC) research that concluded:
“The susceptibility to combustion spillage due to room depressurization is not affected in a predictable way by the presence or absence of air supplied from outdoors, whether supplied to the combustion chamber or indirectly through a supply duct terminating near the fireplace.”
Also, the supply of outdoor air for masonry fireplaces was made non-mandatory because the CMHC research showed that outdoor air supplies to be ineffective, and that direct-to-combustion chamber supplies could be hazardous because of the potential for wind-induced reverse flow of combustion gases through the supply duct.
The second possible purpose for the hole in your question is an ash dump cleanout for the fireplace... that makes ash removal process cleaner.
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