Thermal expansion and contraction due to temp change. When you ran your fireplace materials expanded from the heat and when cooled down they contracted and caused the crack... Most likely HardieBacker board was installed too tight against adjoining material without leaving any gap for expansion.

The rule of thumb is to leave a 1/4" gap from the adjoining material and a 1/8" gap between the HardieBacker boards for expansion.

"The wall area above the fireplace was hot to touch."

Is the Hardie fastented to wood or metal studs please? You may have a larger problem than a crack. What does your appliance manufacturer require?

@GN Builders L.L.C, The hardiebacker probably was installed too tight, I'll see if I can find photos from before the mud.

@Joseph Corlett, LLC, metal studs because the appliance required non combustible materials. It's not a zero clearance insert.

@GN Builders L.L.C, don't seem to have photos from just the hardiebacker stage. Do I need to have all the hardiebacker removed and reinstalled? I'm anticipating my GC is going to try patching it first.

I concur that your issue is thermal expansion/contraction. I'm not optimistic you can eliminate cracking without installing control joints.

Cement board is brittle and is more prone to cracking from thermal expansion. Why was this material used instead of sheetrock?

Thanks @GN Builders L.L.C & @Charles Ross Homes!

@mike_home, sheetrock is considered combustible, the company that provided my insert originally told me they'd refuse to sell me a new decorative surround if the remodel was done with drywall.

@User, The surrounding material is all non combustible and the insert has significant gaps all around the firebox, the metal surround covers them for aesthetics.

USG claims their Sheetrock product can withstand a temperature of 125 F degrees. I am not an expert in this area. Did the installer think this temperature rating would be exceeded?

A crack developed after using the fireplace a few times. This seems it could be an ongoing problem and beyond a simple crack repair.

Judging from the surface temperatures produced when my gas fireplace is operating, there's a good chance the surface temperature above the OP's firebox (i.e., surface temperature of the drywall joint compound) exceeds 125 F if the fireplace has been operating for 30 minutes or more. There appears to be no provision for thermal expansion (i.e., no control/expansion joint.) In addition, there are different coefficients of thermal expansion for the various components that make up the assembly: steel studs, concrete backer board and drywall joint compound. Expansion cracks are pretty much, well, baked into the design.

@Charles Ross Homes, you're 100% right about the temperature. I put a food thermometer against the wall to get a rough idea and it hits 200F right above the fireplace where the crack appeared when the unit has been running for a while without the fan.

I removed the metal decorative surround today and took a look, no expansion gaps between the hardiebacker boards, so that must be it.

@mike_home, originally my GC also wanted to use a fire rated drywall but I insisted on hardiebacker from what I saw on other threads here. I guess he didn't know how to install it properly for this application.

@GN Builders L.L.C, @Charles Ross Homes, I have another question if you don't mind sharing more wisdom :)

My sound bar is in a recessed cavity about 15" above the top of the fireplace opening, but the temperature is still very hot to the touch in there while the unit is running without the fan on.

The TV is in a cavity above that again and never gets hot, but the soundbar will absolutely fry unless I do something to protect it. Any recommendations? I've attached a photo so you can see the layout.

They make a special fireplace for this application which vents the heat above the TV. I've only installed one of those which was in a master bedroom. They are pricey.

You'll need to deflect heat away from the soundbar. The best way to do it given what you have is to install a mantle above the fireplace. A floating black metal shelf would be the option I suggest. Consult with your fireplace installer to ensure whatever you do meets local code.

Marina, there is a minimum code requirement that needs to be maintained when it comes to fireplace clearances. It looks like you have that.

Not sure what size fireplace you have, I also have a gas fireplace 4' X 3' installed in the corner with tile surround and I have about 19" from glass to the shelf on top (which is the entire corner above with 3/4" projection over the tile.

I also have a glass heat shield as you see in the picture

which is part of the fireplace and being there nothing above gets hot and in the winter I run the fireplace for hours...

Looking at your fireplace, hard to tell from the picture I see something projecting is that something that supposes to reflect the heat?

I would contact the installers who provided and installed your gas insert fireplace and see if there should be a glass heat shield or if one is available. Most gas fireplaces have them because glass radiates a huge amount of heat and something should be there... I mean 200F you can bake something there.

Here's why I think it cracked, and is only the first of cracks over time: You said drywall joint compound was used. Setting compound hardens via chemical reaction; joint compound hardens via drying. Do not use joint compound over cementious backerboard. The backer absorbs the water, which leads to cracking. You can use setting compound (e.g. Durabond 45) if the cementious backer is first "primed" per manuf reqmts to seal it. In any case the correct tape and screws have to be used, they are different than for regular drywall. So it seems to me the gypsum compound is thick, it moves differently than the cementious board with temp/moisture changes, and had unvisible stress cracks from initial installation. A better solution is to parge with a cementious layer rather than any gypsum based product.

For the soundbar, create a larger space for the tv and move it up there into one single opening (check your fireplace instructions for minimum "mantel" clearance). The soundbar opening doesn't look aligned with the fireplace anyway, and the entire massing is visually more complicated with (3) different sized openings within the white background. Also one future consideration about the tv is the replacement (not if, but when) will have slightly different dimensions and the opening is perfectly tight to this current tv. You can paint the insert black so you don't notice a gap or the soundbar size compared to the tv.

I'd get a full-length noncombustible mantle below that soundbar at a minimum.

Apologies for the delay, I ran a few tests based on your suggestions. @GN Builders L.L.C, I hadn't noticed the built-in heat shield in the fireplace before but after your post I took a look. Sure enough there is one but it's very shallow. I went to a hardware store, picked up a heat shield-shaped aluminized steel baking sheet and put that in as a test. Set it back as far as I could until it was against the ducting. Worked! Ran the insert for over an hour at full heat without the fan and the wall only got as far as 100F. Half the temperature it was before!

Based on that test, when ordering a new surround, i'll specify a deeper and slightly more projecting heat shield than the one that came with the fireplace originally. The baking sheet obviously sticks out way too far but was a good test.

@3onthetree, I'll talk to my contractor about the mix of materials. With a more effective heat shield, I'm hoping we'll get away with patching the cracks.

@Joseph Corlett, LLC, If I have to after getting a new heat shield then I guess I will, but I prefer the look without a mantle if the heat shield does enough.

I don't know if one is available for your model, but Heatilator makes a blower that you may be able to retrofit which throws heat into the room from the underside of the fireplace.

@Charles Ross Homes I think the one you talking about comes with a grill on top and the bottom of the unit where the bottom grill pulls the air in and sends warmed air out the top.

I wonder where is the pilot light on her unit, hard to tell from the pictures, usually there is access from the bottom and a keyed shut-off valve on the side.

The units I'm familiar with have the electronic controls accessible behind a removable grill at the bottom. That's where they are pre-wired for an optional fan which comes on about 10 to 15 minutes after start-up and shuts off 10 to 15 minutes after the unit is turned off. I had one retrofit in my home. It definite reduces the amount of heat at the top of the fireplace.

Mine is the Heat & Glo i35, it has a fan that pulls air in underneath and back out the top. I just can't be certain the fireplace will always be used with the fan running, so need to make sure it's safe without it too. My fan is controlled with a remote, but there are a bunch of controls accessible from the bottom right of the unit e.g. battery backup.

Marina, take a look at this Heat shield please, it's surface mounted and it's adjustable width maybe this will work much better by attaching it right above the frame. Check some of the reviews this might work much better.

https://amzn.to/3I22Pvk

Good luck

thanks, I will definitely get that! I also need something that goes back behind the fireplace to stop heat going up inside the wall, but I guess I could use any piece of steel sheet metal for that and use this on the outside.

Thank you all! Learned so much from this thread

i need to hire marina to diagnose my fireplace issues please.