Burning perlite? How hot

I've been using this rocket mass heater in a 12'x24' cabin for three years now. It needed some maintenance so I took it apart. Got a surprise.

After taking out the layer of firebrick I noticed that the brick-deep layer of perlite was burned.

After taking out the perlite, I noticed the hardy board under it was cracked and falling apart.

After taking out the hardy board, I noticed the aluminum flashing was discolored and distorted.

After taking out the flashing, I noticed the OSB subfloor was burned.

And underneath the OSB subfloor, the insulation was also burned.

I think this RMH got pretty hot. Once we repair the floor we're replacing it with a pellet stove.

[Let's see if the pictures I attached show up.]

Whoa. Good thing you tore that puppy down when you did.

Thanks for sharing this. This could be important info for other builders. I'm wondering if some heat got past the fire bricks at one of the seems. Did you mortar your fire brick? If so, was there any signs of cracked mortar?

It think there is an important lesson here for those building over wood subfloors. You need to have a backup incase burn chamber containment is breached. Perlite is not going to be good enough.

Mike Dinsmoor wrote:Thanks for sharing this. This could be important info for other builders. I'm wondering if some heat got past the fire bricks at one of the seems. Did you mortar your fire brick? If so, was there any signs of cracked mortar?

It think there is an important lesson here for those building over wood subfloors. You need to have a backup incase burn chamber containment is breached. Perlite is not going to be good enough.

It was mortared, but when I took it apart it came apart easily.

We were using firemuffins that we make, to get the fire started. They are mostly old candles melted and mixed with sawdust. Maybe the wax was melting and going into crevices, weakening the mortar over time and eventually making its way down.

Good catch Ron, you saved your cabin.  This is why it is suggested to raise your core and mass on a base of supporting bricks when building on a wood subfloor.  2.5 " airspace under your RMH and your floor would never have gotten this hot. Sorry to hear your going with a pellet stove.  

Wow. Rocket mass China syndrome...

I think these days that should be  Rocket Mass Japan syndrome ....fukishima and all you know...

thomas rubino wrote:Good catch Ron, you saved your cabin.  This is why it is suggested to raise your core and mass on a base of supporting bricks when building on a wood subfloor.  2.5 " airspace under your RMH and your floor would never have gotten this hot. Sorry to hear your going with a pellet stove.  

Is it necessary to raise a bench also? I was planning on just raising the core and not the bench. I was hoping to have a raised area under that core that could be wood storage/drying area. Under the bench I was going to have a 1.5" GFRC cement pad with fireproof insulation rockwool pad cast into the under part of the pad. It will be on a reinforced wood subfloor.

Hi Mike; It is recommended (common) to raise both core and mass.  I suspect that it would take many years, but your transition area will at times get  400-500 F temps.  Wood burns at 456 F  ... below that temp,  pyrolysis happens , a slow change in the chemical composition of the wood which  lower's  the burn temp down possibly burning your wood floor.    1.5" cement pad with rockwool "sounds" like it could handle that just fine... but i wonder?
If it were me I would put that cement board up on flat bricks and space the whole thing up ... never have to worry that way.

Yep, insulation, no matter how good, does not prevent heat transmission, just slows it down. If you do not have an air gap to let heat dissipate before it gets to combustible material, the combustible material will get hot over time. Any combustion core on a solid base over wood will eventually cause a fire and is a fatal mistake. I hope the OP will consider rebuilding with safe clearances and air gaps and not let this experience frighten him away from an effective, efficient heat source.

thomas rubino wrote:Hi Mike; It is recommended (common) to raise both core and mass.  I suspect that it would take many years, but your transition area will at times get  400-500 F temps.  Wood burns at 456 F  ... below that temp,  pyrolysis happens , a slow change in the chemical composition of the wood which  lower's  the burn temp down possibly burning your wood floor.    1.5" cement pad with rockwool "sounds" like it could handle that just fine... but i wonder?
If it were me I would put that cement board up on flat bricks and space the whole thing up ... never have to worry that way.

At this point I am changing my design to include an air gap under the bench as well as under the firebox. Im not sure if I even want to store wood under the fire box at this point LOL. I am assuming that this air gap needs to be open to the room and not covered up for aesthetics??? I'm glad this post came up. Question now is, how to support the cob up off the ground. I'm guessing everyone is using a cement slab??? Wish I was building on a slab, reinforcing the sub floor is a big project on its own.