Here's the cracks at the back of the fireplace

Here is a link that might be useful: fireplace cracks

Hey htnspz,

Somethings up with your picture links. Have another go at uploading the photos and hopefully someone can help.

Trying again:

this try:
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That is a zero clearance fireplace. I had one at my previous home. Sucked all the heat out that you put in. I ended up sliding in a lopi wood stove and lining the chimney. Worked great.

That probably wasn't much help. Looks like that one may be a little small to insert a wood stove. From what I remember, those "bricks" are laminate stone on a metal backing. The advice you received may work provided there are not any leaks in the box or chimney itself.
I am not a fan of the zero clearance.

You can buy an "insert" for that zero clearance fireplace.

With no glass doors on your current fireplace, you are losing a lot of heat up the chimney, even with the damper closed.

You can post your question on www.Hearth.com, an outstanding online resource.

Good luck.