Questions about firebricks and cast iron

First off.... thanks to you both for your passion for fire and warmth. It is wonderful to watch and learn from you. I'm waiting for your video with much anticipation.
A couple of questions.....Are firebricks supposed to be used for all the 'bricks' of a rocket mass heater? If not,how many are needed and where abouts in the structure?
I've read where there may be a benefit to using a flat piece of cast iron or thick metal on the top of the horizontal burn chamber,what is your experience with this?
I'm attracted to having a RMH away from an outside wall,what are your thoughts/experiences with doing this?
How successful is any sort of built-in oven in a RMH?
Thanks
Kate

Hi Kate,
Thanks for the kind words, and great questions.

First off.... thanks to you both for your passion for fire and warmth. It is wonderful to watch and learn from you. I'm waiting for your video with much anticipation.
A couple of questions.....Are firebricks supposed to be used for all the 'bricks' of a rocket mass heater? If not,how many are needed and where abouts in the structure?

We have built with both firebrick and reclaimed building brick. The important thing is to use a clay-based brick or refractory material, not a concrete paver-type material.
Firebrick handles somewhat higher temperatures, and is more regular and easier to stack. It's probably more durable (old, soft-fired or hand-formed firebrick can be even more resilient according to some builders).
But new firebrick does cost more. I'd say the most useful areas are to have firebrick for the burn tunnel sides and 'bridge' or roof; the floor pad is one of the easiest places to fudge in some smaller brick because it doesn't even have to match, and will not get as hot as the other parts of the burn tunnel or heat riser.

I've read where there may be a benefit to using a flat piece of cast iron or thick metal on the top of the horizontal burn chamber,what is your experience with this?

Not sure where you heard that. I would expect it to either
a) warp or burn out, and
b) while warping and expanding with heat, stress and de-stabilize the nearby masonry. Steel can even 'crawl' out of place with uneven heating.

We tend to use all masonry in this very hot area. Folks swapping in metal often are running shorter-cycle test systems, and have not seen the high-end temperatures from a longer or more frequent burn cycle.

I'm attracted to having a RMH away from an outside wall,what are your thoughts/experiences with doing this?

We like to locate the heater centrally in the occupied space, or slightly off-center away from the kitchen and closer to sitting/sleeping/office areas. If the heater is designed along with the house, it can potentially heat more than one room as part of a shared wall. We don't recommend burying the barrel in a wall, however - the bench, which in practical terms is 'zero-clearance', is a lot easier to poke through an interior wall with fewer safety issues.

How successful is any sort of built-in oven in a RMH?
Thanks
Kate

The easiest oven is the 'tin-foil hat' approach described in Ianto's book - an insulated dome right on top of the barrel. We are trying out a terra-cotta version, but it's a lot heavier so I may need to fit it with a door so I can get into it when Ernie's not handy.
I've seen a couple of other oven variations, notably Matt Walker's feed-area mini-oven (not a design I'd recommend for everyone due to the fiddly relationship between separate air and fuel feeds, but a great example of how to tweak things to suit yourself).

But new firebrick does cost more.

Which makes it a good idea for old brick buildings that are being demolished. Unlike the new construction bricks with three holes in them, hundred year old buildings were built with actual solid bricks. An old school from a hundred years ago was being demolished two counties over and I was able to buy bricks at 45 cents apiece. Can't do much better than that -- price wise or material wise.

John Elliott wrote:

But new firebrick does cost more.

Which makes it a good idea for old brick buildings that are being demolished. Unlike the new construction bricks with three holes in them, hundred year old buildings were built with actual solid bricks. An old school from a hundred years ago was being demolished two counties over and I was able to buy bricks at 45 cents apiece. Can't do much better than that -- price wise or material wise.

We found an aggregate place in Michigan that had mountains of old brick they were turning into driveway gravel. Cost something like 25 cents per piece, firebrick, factory stack and arch bricks, red brick, or lump of concrete and building brick. The eastern US has some great resources just waiting for reclamation. (not jealous... not jealous ... )

-Erica