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Kiva Fireplace Firebox Expansion Question

5085 Views 4 Replies 3 Participants Last post by  dom-mas
I am currently designing a kiva fireplace that will be built into an adobe home, and have run into a possible issue that I can't seem to find any online guidance on. The issue concerns the expansion of the firebox (clay firebrick) and how that may affect the curved, plastered, front face of the fireplace. Because there is very little documentation online for kiva FP construction, I've had to try and adapt standard FP code and directions intto my design. I've seen a lot of advice for leaving airspace between the backup wall and the firebox to allow for expansion. I've incorporated this into my design, but because the front face is tied into, and bonded with the firebox - as well as the backup wall (refer to attached pics), will thermal expansion of the firebox cause enough movement to affect the front of the FP - i.e. cracks in the plaster, mortar joint failure? I saw where someone suggested just filling the cavity between the firebox and backup wall completely with mortar - making a solid, composite wall, and then incorporate the airspace between that and the adjacent masonry - which in this case will be the adobe brick. But I'm concerned if I do that, any thermal movement within that whole structure could cause mortar joint failure. Any insight into any of this will be greatly appreciated.

Note about the illustrations - there will be an additional four courses above what is pictured for enclosure of the smoke chamber. The dark red bricks are clay firebrick and the pink is standard brick.

Thanks in advance,
Frank Williams


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Here is what I suggest you follow if you can.

Email Jim Buckly if you have more questions, he loves this stuff. Not that we dont, but I would like to see his throat system used to prevent any movement like you are mentioning. Short of that you need to build a throat with a bond break in it somewhere to ensure it wont move.
Thanks for the suggestion on the Rumford. I would prefer not go down that road, though I may have to if I can't find a workable solution. I'd like to retain the "all brick" aspect of my design if possible.

The tough part of this will be separating the throat structure from the face. Is it possible to leave a mortar gap that's open only to the cavity between the firebox and backup wall, as long as both sides are supported by a sufficient structure?
If you could use the Rumford throat it would solve your problems. The throat could be trimmed to fit if needed. The print says to use a 30" throat piece
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Have you tried going to

Many heater builders just accept cracks as part of the natural course of building a masonry heater
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