Bell elevation

Hello everyone!
I've crept on these forums long enough, I am grateful for all the inspiration found here.
I am in the process of building a rocket mass heater Jtube with a bell/bench.  I will update with pictures.
I have a few questions I hope to get some clarity on before I proceed.
The first question is in regards to heat flow into the bell.  I have dug down to put in insulation and have some flexibility in terms of elevation for placing the core in relation to the bell.
I thought I remember something in this regard but with all the information I've absorbed I can't recall if... is there any benefit to having the core higher or lower to the stratification bench?

My second question is about the core.  I already purchased enough fire brick to build the entire Jtube core, base, and riser which will be wrapped in 1" ceramic wool.  Since I already have the firebrick is it worth making a 5 min riser instead, and if so what would be the best use of the remaining firebrick that would have made the riser? The manifold and bench will be full red brick with a 55G steel drum over the riser.

Last question, for the base insulation.  I currently have 2x4's of XPS 2.5" foam/cement insulation boards (2" foam + 1/2" concrete.)   Do you think this is adequate to insulate under the bench and/or core? (Core will have full base layer in firebrick as shown in the book.) Or do you think I need to do a perlite concrete screed underneath as well (I have enough glass wine bottles as an option to incorporate into the cement as well if need be.)  Note: this is inside my greenhouse over a hard packed (tamped) clay floor.

Thank you for your time and consideration.

Hello Medrec. Big Welcome to Permies and glad you are joining in on the inspiration.

I can't see any advantage to having the transition from the core to the bench higher or lower (to a point). The one thing you do want to make sure of though is that this transition is plenty big (150% as a minimum size) and follows a smooth path with no sharp turns. This is one of the biggest bottle necks in a lot of stoves and if you already know about this, its not too late to hear it again.

The more you can replace mass with insulation in the burn zone the better. So yes, I would definitely make a 5 minute riser. You won't be sorry.
With the rest of your brick, I would save it for your next stove!
Not long from now when your grinning at how well your stove works, your going to want a sauna, another space to heat, an outdoor cookstove etc. and you'll be glad to have those bricks.

You'll never go wrong with insulating more under the core. In your case, its to keep all the heat going upwards rather than into the Earth which will suck up as much as it can get and rob it from complete combustion.

Look forward to seeing your photos of your new Green house Dragon under construction.

Thanks for you reply Gerry.
After further thought I believe my misunderstanding in terms of the elevation may have been in regards to raising the elevation of the core for better insulation air gap to the earth below. I probably wouldn't want the core so close to the xps concrete foam underneath (might melt it) and wonder if it is possible to get away with using cement blocks on top of the foam/concrete sheet and beneath the ceramic fiber blanket and base firebrick layer.   I'm a bit uncertain what temperatures I should expect there.
I also noticed today that there is moisture buildup in that area, so I will need to come up with something to prevent any water reaching the ceramic blanket.  I recall mention of avoiding getting the ceramic wet which would essentially destroy it.

On another note, I was originally planning on using urbanite and cob for the bench until I scored all the free bricks.  This requires me to rethink how I will be incorporating my use of water mass for thermal capture.
A year ago in preparation for this project I picked up a good quantity of copper alum fin baseboard.  I plan on using it to help radiate heat on the opposite side of my greenhouse and I'm wondering if I can't also use it in inverse - inside the bell.

Cheers

Temperatures below the core will get very hot especially if you do an all day burn. Any air gaps (like in the cement blocks) would help greatly to break the thermal bridge and keep the heat from transferring down further.
A small trench filled with pea gravel would make for a good base. Because your working on top of clay, it may be wise to also make a drainage ditch away from the trench to keep water from pooling.

I'm not quite following you on having your baseboard located inside your bell. The experiment  sounds like fun but also seems like its making things much more complex than they need to be.
Hard to beat the simplicity of a bell / stratification chamber. Just make a high mass enclosed space any shape you want and your done....let gravity and draft do the rest!

So I really hoped I would be further along at this point but at least I'm making some good progress.  These are the cement foam boards I was referring to earlier, it's to protect the vinyl siding from melting.  I didn't want to remove the siding so it's easier to return the wall to normal when I move. It also, along with wire mesh (more to be added,) is to help keep out the critters.  Before I transformed this side of my garage into a lean-to hybrid greenhouse in late fall, a large otter had access tunnels from where that barrel is buried.
That barrel currently has a a half barrel temporarily on top of it for now, but will be connected into another water barrel that will be elevated for proper thermosiphon.  Since I had to dig down for critter prevention I figured I might as well take advantage of the extra foot height for the bell.  

The corner square will house the core and will be using cement blocks filled with perlite (maybe a clay perlite mix.) The plan is to put down hardi board over the blocks, then ceramic fiber, and finally the base of firebrick.
This is at the north wall and on the other side I will make a proper drainage ditch since it is the lowest point of the greenhouse roof.

About using the baseboard, my line of thinking was that since I already have a bunch of it and it is good at radiating heat out, then why not use it in reverse. I pictured running it width wise at some height across the interior of the bench bell.  The problem is I'm not sure how it could impact the performance or calculate that into ISA for the bell from my 8" Jtube.  Since the back wall of the bench will be insulated inside with ceramic fiber, hence taken out of the ISA equation, I thought the radiator could compensate, but it will probably overcompensate.  I think it wise to play with that after getting the standard bell performing well.

This is my first "serious" build so please tell me if I am way off here, missing something, or need to clarify anything.  I just can't wait to get to playing with the bricks!

If the idea of the cement faced styrofoam is to protect the vinyl siding from heat, then the styrofoam needs to be protected from heat too. It will melt at possibly lower temperatures than vinyl, and burn regardless of cement coating. You need both a noncombustible insulation, and some air space so heat can escape from the back of that insulation before heating up the styrofoam or vinyl.

Medrec,    With the use of a bypass, you won't need to be too concerned about the ISA of your bell. Your greenhouse space doesn't look that big so you could maximise all the space with as large a bell as you want.
As Glenn suggested, your going to need a heat shield if you go with a barrel (or other radiating device) over your heat riser. Many ways to do this. You can line the inside of the barrel (just the side facing the house walls) with ceramic fiber blanket or cob the one side of your barrel, you could use a sheet metal heat shield with a gap for air flow, make a brick wall... etc. You could also just eliminate the barrel altogether and make a brick housing instead. This would mean no immediate heat into the space but lots more slow release heat.

Progress! Tomorrow I can finally start (p)laying bricks.  Just dry stack tested and everything tightly fits into place thankfully.
What you see  here is 2 levels of cement block filled with perlite and a sheet of 1/4" hardiboard on top.
Found a great deal on used 10" ducting that I can use for both the riser (with ceramic fiber blanket inside) and chimney flu.
Opted to use corrugated metal roof in front of cement board to protect the vinyl siding.
I have some 2" thick 2'x2' flagstone squares for the bench top and possibly a backrest.
Clay slip perlite mix will be added across the entire floor area on top of the gravel.
More to come.

Quick question, how low from the floor can I get away putting the exhaust flu? I figured 2 - 3 inches but the lower the better. Even though this is an 8" system the flu is 10" and will shrink down to 8" (as it exits the masonry and enters the room) with the addition of the an inner liner of ceramic blanket through the wall and outside. Thanks.

I am no expert but assuming the blocks are for storing heat, am not sure if filling  them with insulation is a good idear?

Medrec Martin wrote:Quick question, how low from the floor can I get away putting the exhaust flu? I figured 2 - 3 inches but the lower the better. Even though this is an 8" system the flu is 10" and will shrink down to 8" (as it exits the masonry and enters the room) with the addition of the an inner liner of ceramic blanket through the wall and outside. Thanks.

Hi Medrec,   Scots is correct. Filling the brick holes with perlite is taking away at least half of your thermal mass and ability to store heat. It would be much better to fill with a sand/clay mix (and even stuff small rocks in there). The walls won't get overly warm so as long as you leave a gap between the blocks and the vinyl wall, you should be fine.

The exhaust entrance should be low as you mentioned. A small lip is helpful to allow ash buildup over time and not block the entrance.

Scots John wrote:I am no expert but assuming the blocks are for storing heat, am not sure if filling  them with insulation is a good idear?

Hey Scots, thanks for bringing up that point. The cement blocks are mostly a pedestal to bring it up to floor level and act as a thermal break.  
I knew that I wanted perlite for the wall on the right (against garage vinyl) just to err on the side of caution.
I thought about using different material in the rest of the base blocks. I was uncertain as it's below ground and  I'm building without a bypass.

Gerry Parent wrote:
Hi Medrec,   Scots is correct. Filling the brick holes with perlite is taking away at least half of your thermal mass and ability to store heat. It would be much better to fill with a sand/clay mix (and even stuff small rocks in there). The walls won't get overly warm so as long as you leave a gap between the blocks and the vinyl wall, you should be fine.

The exhaust entrance should be low as you mentioned. A small lip is helpful to allow ash buildup over time and not block the entrance.

Thanks Gerry, here I was thinking how many inches of perlite I should put down on the bottom. Does the cement block being underground as I describe make any sense or have I misunderstood something about thermal bridging?
I'm already building without a damper.  

Below grade...gotcha Medrec.
The floor of the bell will not get that hot. In Peter van den Bergs ISA calculations, the floor is excluded because of this fact. It will also collect ash over time which will also add to the insulation value.
With that said, a few inches of stabilized perlite should suffice.

You will still get thermal bridging off the sides of the cement block walls as there is continuous cement from the inside to the outside in many places. Being at the bottom of the bell, temperatures are not that high so it would take a while before heat migrates outwards. Some of which is going into your floor so that's not too bad anyways. A thin layer of insulation on the outside of the bricks would be extra insurance for maximum heat retention but certainly not a show stopper.