To remove the fireplace and the lower section of the duct, you will need to take down the wall in front. Before removing the duct, make sure it is properly braced/strapped. Once the duct is removed, the firebox can then be removed. It is usually nailed to the framing. The opening “header” will need to be reframed to accommodate the new height. A wood-framed platform or masonry block-raised platform for the fireplace can then be created.

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Flashing is required around the edges of any wood-framed platform. This flashing should be used where concrete floor infill meets the frame. Secure the platform to the floor by making sure it is level. You can then reinstall the firebox at the new height and add a shorter section of the duct. Your unit may need minor electrical or gas lines to be changed.

You likely mean a manufactured fireplace and not a brick fireplace. A masonry fireplace can be a minefield that I wouldn’t want to open. Clear: even though you may need to cut the chimney to get it out, you can’t use a CUT part of class A chimney (or gas vent pipe) to make a fireplace. Legally, at least.

Or safely. To be able to remove the vent pieces from the fireplace, you will need to know how to do this. This can vary depending on whether the chimney pipe is sealed with Millpak, twist-locks, or bayonet tabs.

It is necessary to take out any sections that are blocking your path and then replace them with the appropriately sized ones. There are adjustable sections for gas fireplaces. Some sections can be adjusted to fit a wood-burning fireplace.

Fireplaces can pose dangers. Proper construction of the firebox, and the hearth, is crucial for the safe introduction of fire to homes with many combustible materials. Some people choose to elevate the hearth from the floor for this reason and others. This allows the fireplace to be further away from any wood or fiber flooring.