RMH Foundation on Wood Floor

I'm designing my home mass heater, a 6 inch batch system in a brick bell. Core will be elevated from the floor of the bell.  I estimate it will all weigh about one ton.

I see where folks leave air gaps in their brickwork on the floor to promote airflow and keep the floor cool.
Looking for an easier way, I came across these cinder blocks with holes through them.
I figure if I sandwich a layer of these blocks with cement board, I might just have a fireproof foundation that stays cool with air flowing in through the "tubes" on the floor and up behind the tin heat shield that protects the wall. A constant flow driven by the heat of the RMH.

What do you think? Would that be enough air flow to counteract the heat conduction downward through the cement board and blocks? I figure it would be more flow than I would get with brick tunnels.

Hi Matt;
I think that will work great for you as long as your core is up away from the floor.
There is only minimal heat that reaches the floor of a bell and then fine fly ash covers the bottom and adds insulation.  
Around the core and the roof over the riser are the hot spots.
By the way, if you are building a 6" batch I suggest building an extended-length box to allow using readily available precut firewood.
With a stock length 6",  you will want your wood cut short to 14" or less.

thomas rubino wrote:
By the way, if you are building a 6" batch I suggest building an extended-length box to allow using readily available precut firewood.
With a stock length 6",  you will want your wood cut short to 14" or less.

Luckily I have a Van Den Berg approved modified DSR3 design with 18 inch firebox and all the materials to build it! Now if I could just find the time...

Handy reminders!
-The base of a bell doesn't have to host the core.
-A raised firebox is easier on the back.
-The standard batch box length can be stretched by -how much again?


What if core was sitting on the bottom of the bell?
I suppose the blocks,  being only 4" or so thick, would not be a safe air gap?
What about a standard size hollow brick?

Since we are talking about them, are these blocks  too porous and not heat resistant enough to use in any part of a bell?

William Bronson wrote:
-The standard batch box length can be stretched by -how much again?

25% longer is the official stance on that question.

William Bronson wrote:
What if core was sitting on the bottom of the bell?
I suppose the blocks,  being only 4" or so thick, would not be a safe air gap?
What about a standard size hollow brick?
Are these blocks  too porous and not heat resistant enough to use in any part of a bell?

If the core was sitting on the bottom of the bell, it would conduct heat down into the floor. Would big hollow blocks be enough to keep the wood floor from being damaged? IMO, likely because of the sizeable air gap. But would those big hollow blocks be damaged by the direct heat conduction from the core above? Also maybe.

Depends on if the core was insulated on the bottom at all, which you could  do with ceramic fiber board or vermiculite board (since ceramic blanket would compress under weight.) Safe building practices like elevating the core help eliminate the maybes since nobody wants to find out the hard way!

William Bronson wrote:
Are these blocks  too porous and not heat resistant enough to use in any part of a bell?

From all my reading up it would seem that you COULD use cement blocks in the lower parts of a bell. But they are not the best material since they do not store heat like clay brick. And even then may not stand up to the thermal cycling over time. Solid cement blocks being much better than hollow but still not great.  You could NOT use them above the core exhaust unless you are using the appropriate "double skin" method where the inner layer of the bell above the core is refractory material like fire brick.
I know you know these things but I'm playing along with idea of discussing it out loud for others to read :)