Not able to get full fire brick

I am attempting my first mass heater.  It will be on a wood floor in my home.  I see the common plan is fir two layers of full fire brick under the fire box.  I have half brick fire brick, a couple sheets of cement board, and a bunch of 2" thick pavers.  Would that work for under the half bricks to insulate the floor under the fire box?
Would get more fire brick, but $ is now a major issue and really need to get this done so we can shut off the electric heaters.
Thank you for any advice.

Hi Thomas ;  Welcome to Permies.
Lets start with your floor.  Is it strong enough to support a rmh? We will assume that it is, or will be.
Next we need to protect the wood floor. Normal procedure is to use plain clay brick, laid flat with spacing for airflow in between. Your 2" thick pavers could work but the larger surface area would transfer more heat down into your wood. I recommend using the clay brick (home depot) and not using your pavers until the top of your mass, where they will work much better
Your concrete board goes on top of this. I recommend a double layer of 1/2" hardy board.
With only split firebrick to work with. I recommend 4" of insulated (straw) cob under the firebrick core. Hold that cob in place with a run of bricks glued (construction adhesive) around the perimeter of your cement board. If your mass is going up to a wall it should have an air gap to keep your heat indoors , not outdoors.

As to floor strength...I will be adding 4x4 posts on footing blocks under the floor joists where the stove is to be built. (Thank you for your concern)
Will I be needing that air gap under the heated mass bench as well as the fire box? Or is the temperature enough lower there to do without? Also the back of the mass bench is against an outside wall.  I will try to get a pic of the area, and a layout drawing together asap so that my questions and answers are as clear as possible.

Better to have it all suspended. That way your gaining another warm air surface, and your floor is safe for sure.
Any outside wall should have air space or extra insulation.

Hey Thomas;   Any sawmills in your area ?  Most have a dry kiln for their lumber.  Full thickness firebricks will be in use if they do.    Simply asking (the right person) for surplus used  or new bricks might get you some bricks.  I did that and after talking to the owner about RMH's , looking at them on line...   He gave me several hundred FREE used bricks under the agreement he could come see my rmh in action!
What a deal!  There could be a similar deal waiting for you to go find... food for thought...

Long time since I originally posted here. I did end up getting a large supply of full fire brick. Finally getting started on the actual build.  I added a bunch of 4x4 post on block supports under my floor and then built a new, raised subfloor to get up to level with the rest of the house. Starting to set up my burn chamber now.
Pics of what I have so far.

Laying out the burn chamber.

Ok. Time for input from the more experienced of you.  Am I off base anywhere so far?

Thomas Schuessler wrote:Ok. Time for input from the more experienced of you.  Am I off base anywhere so far?

too much mass in the firebox and heat riser!

Thomas,  To elaborate on what Satamax said if I may...

Look back to the first post made by Thomas Rubino "I recommend 4" of insulated (straw) cob under the firebrick core." This insulation will help stop the heat from going down towards your wooden floor. The air gap you made is great but its always good to have extra insurance. It will also help to keep more heat into the fire which is where you want it for a more clean burn. As for the rest of the stove, I see it is designed similar to the suggestions made in the builders guide. It certainly will work, but all that mass does take a fair amount of heat away from the fire especially when it is cold. Generally speaking you want as much insulation in the firebox/heat riser and mass in the heat capturing device you choose.

Ok.  Thank you! I see what you mean.  I was thinking of the fire brick as insulation, but it is more mass... Necessary mass as it is needed to withstand the direct heat of the fire, but still needing to be insulated all around for quick heatup and better draw.
It will be a few days before I can make up some straw cob, and get that layer in. Then I will mockup again and post pics before making my firebox permanent.

Thomas, your two layers of bricks at the bottom, could be replaced by a layer of air entrained concrete slabs, and firebrick splits atop.

Ok... Time for some updates. Rebuilt the firebox after putting down 4" of cob.

My plan has been to use 6" pipe for the exhaust, but after my first test fire it is not drawing enough.  I believe that I made my burn chamber too large and so it being larger volume than my stove pipe is choking off the flow. Wondering if I can save this by going up to 8" pipe, or by adding a parallel 4" pipe to up my draw. Otherwise I will have to tear down my fire box and rebuild it with a smaller inside cross section.  I discovered late in the assembly that I was working off a design for using 8" pipe and so I narrowed it, but forgot to shorten it height wise as well. I might possibly be able to help things by adding a layer of brick to the chamber floor to reduce the inside area as well.  Anyway I am rambling ideas now and will check back fir your suggestions later. Thanks for any advice.

Hi Thomas,   A few issues that I see with your system. First, your heat riser is pretty darn short. It should be around 4x the height of the feed tube with a minimum of gap of 2" to the top of the barrel (but more is better especially if your not using it for cooking). You could use the barrel you have now as your manifold base (since its already cut), remove its top and then put another barrel on top of that.
You could also build a brick manifold then put the barrel on top of that also if that's more to your liking.
Are you insulating the heat riser? All those jutting bricks makes it much harder to either wrap or tamp insulation around it. They could either be cut to size or be replaced with a stabilized perlite/clay riser or a Ceramic Fiber blanket "5 minute riser". The more insulation you use in the combustion unit rather than hard firebrick will give you better results - a burn that is cleaner much quicker and better draft on cold startups.
The CSA (Cross Sectional Area) as you discovered needs to be the same throughout the system. This will determine whether you go with a 6 or 8" exhaust pipe.

If you use a ratio of 1:2:4, then your riser looks like it's only 2 i.e. half the required height assuming CSA is preserved.

Finally got a semi successful burn.  Had to add fire brick inside my burn tunnel to get my CSA down to that of my 6" pipe. Then put 4 ft of vertical pipe after exiting the wall to add draw.  I burned in the box for about 2hrs, and never achieved "rocket" only reached a max barrel temp of about 350°F.  Hoping that adding my cob around the box and barrel will help, but am thinking that better insulated chimney in the barrel might be key as you say, and then adding height to chimney and barrel if the insulation isn't enough.

Thomas, i don't quite get it. Where is your chimney? Not your short heat riser, but the actual chimney?

I currently have temporary  pipes hooked up for exhaust.  There will be a mass bench along the wall that I will run the pipe through.

Once again, temporary set up for fire box testing purposes.

Ok. So I went and extended my chimney another 5' up past the roof where I will want it permanently, and now am getting more draw and heat and am finally getting some"rocket" effect!

Since we have achieved rocket effect we have started adding the cob aroun the fire box and barrel.

OK. What is your top gap between riser and barrel?

What is your side gap. How that side pipe is implemented?

My top gap is 2".  No idea what my side gap is, but it is big.  I have my half brick riser inside the barrel and nothing else.  I had never come across any mention of the side gap being critical.

Top gap at two inches is too small imho.

Side gap is important, at the flue exit. As in the tube's projection.

http://donkey32.proboards.com/thread/1406/calculating-ring-circumference-projection-gap

But your riser is too short anyway. And you have no real chimney  outtside.

It will not function as it is now.

Satamax Antone wrote:Top gap at two inches is too small imho.

Side gap is important, at the flue exit. As in the tube's projection.

http://donkey32.proboards.com/thread/1406/calculating-ring-circumference-projection-gap

But your riser is too short anyway. And you have no real chimney  outtside.

It will not function as it is now.

Sorry, i didn't see your previous post with the extended chimney. That's better. Try to insulate that outside part. It will work better.


Or even better, keep the chimney inside as much as you can. And put mass around it.

As long as you're not at 60C° exiting the chimney, at mid burn, that means you can extract a smidge more heat.

Thank you all for your help.  I have insulated pipe for the outside chimney, just needed to get an immediate solution up for now. The insulated pipe is heavy and I will need to put in some support for that weight that I haven't time for just yet. As for the top gap and side gap, I will be looking into those issues as soon as I can.  Life and too many projects needing attention slows progress on all of the projects.  At this time, the stove does draw and we have been adding cob to the firebox as I have time. I will next be concentrating on the mass bench next, and will also be able to pry off the barrel lid and see what's what in there. We are able to achieve about 500-600°F on the barrel lid after about 1-2 hrs of burning. The stove does draw, but not enough to truly "rocket", and without the mass for heat storage, we have to keep a fire burning to keep getting heat, so nit achieving the really high barrel temps is kind of helpful as we are not cooking ourselves out of the house.  Once I have the mass for heat storage, then improvement to chimney and barrel will become more critical.
Thanks to all your advice, I at least know where to look and what to fix! The fixes seem to be relatively simple, and I will continue to update this thread as I work to finish and improve this build.  I am excited to build my second heater in my shop now that I have made my mistakes and know what do better the second time.