Question about building bell for 6" batchbox heater

Hopefully my last question for a while.  My plan was to make my entire bell from 3 hole bricks.  Now I'm wondering if that is a mistake and I need to use fire brick for say, the last top third of the bricks up to the cover of the riser?  Obviously firebrick is much more expensive, but I don't want to build this and have the heat destroy my bricks, forcing me to build part of this over again.

Hi Trace;
Were you intending on a single-skin bell?
If so, then yes, you want to use firebrick above the riser.  
On the roof, and down the sides perhaps 3-4 layers of brick.

Your three-hole bricks will become solid clay bricks after you fill the holes with clay.

Got it, thanks. I toyed with the idea of a double skin bell.  I'm still not certain which is the better way to go, so I thought I would just do a single bell now.

Trace, per guidance from Thomas and Peter... I transitioned to refractory brick at the level equal to the top of the riser and the gap from riser top to bottom of the roof was approx 12". My roof is double skinned with refractory on the inside, then a blanket of SW, then a layer of clay brick for the outside. I embedded a thermocouple in the mortar joint between the 1st and 2nd courses of ref brick on the wall and so far my peak temp is just under 600F. The actual temp of the internal surface of the wall is somewhat higher.

Trace Oswald wrote:Got it, thanks. I toyed with the idea of a double skin bell.  I'm still not certain which is the better way to go, so I thought I would just do a single bell now.

I just saw your comment above after posting my reply. Note that I am heating a very large area so I'll typically burn a batch down to coals and reload once or even twice which gets my inner skin up in the 500F range. With just a single burn I'm in the 300F range. There are 2 key reasons that I can see for a double skin. First, is that the mass has been doubled so you have that much more thermal battery storage for carryover until your next burn. The second is the sealing aspect and higher assurance that no gasses are escaping through marginal mortar joints.

Note that my roof is completely dry stacked. So if the need arises the prospect of removing the roof for maintenance purposes is greatly simplified vs dealing with mortar. I cut all bricks to fit quite snug and the SW blanket between the two layers aids in sealing. I've placed a CO detector on the top of the roof and with the exception of the first few burns while everything was curing, I never had it scream at me. Regardless of a single or double skin bell I would suggest you consider a double skin roof. Curious to hear comments from others with experience.

Trace;
In my opinion, if your floor can support the weight, I would build a double-skin bell.
Heating with bricks is what we are trying to do, so twice as many bricks are twice as much heat-holding power.

Trace Oswald wrote:Hopefully my last question for a while.  My plan was to make my entire bell from 3 hole bricks.  Now I'm wondering if that is a mistake and I need to use fire brick for say, the last top third of the bricks up to the cover of the riser?  Obviously firebrick is much more expensive, but I don't want to build this and have the heat destroy my bricks, forcing me to build part of this over again.

Ironically(?), the landscape supply place nearby where I found my firebrick was clueless as to where I might find three hole bricks and claimed they weren't in demand any more. If you end up with a surplus, I might be interested in them...

Hey Coydon;
Three-hole bricks were never going to be found at a landscape store.
They are intended for brick home construction, and rebar is inserted through those holes.
You find 3 hole bricks at a Masonry supply store, or better yet Trace's neighbor has thousands for sale.

thomas rubino wrote:Hey Coydon;
Three-hole bricks were never going to be found at a landscape store.
They are intended for brick home construction, and rebar is inserted through those holes.
You find 3 hole bricks at a Masonry supply store

That's where I found the irony- given the cost I would think firebrick would be the last masonry item for a landscape company to carry, unless maybe they were opening a store on a recently formed pacific island...

The guy was also part of a business network that includes masonry stores, but still only had a guess as to who might carry the 3-holes. He's also a kind of "working for the weekend" type dude though.

thomas rubino wrote:
Your three-hole bricks will become solid clay bricks after you fill the holes with clay.

I feel like I've missed the key head on this Hydra like discussion. Why three-holers for the RMH and why fill the holes with clay?

Well, that is simple.
They are cheap and just down the road from Trace's house.
Normally solid clay bricks are used.
If using the 3 holers without filling the holes they would be insulating the bell.
Filling them with clay is easy and cheap, Trace has clay pockets on his land.

My rocket vocabulary is failing me. What is a SW blanket?

Morgan SuperWool Blanket

thomas rubino wrote:Well, that is simple.
They are cheap and just down the road from Trace's house.
Normally solid clay bricks are used.
If using the 3 holers without filling the holes they would be insulating the bell.
Filling them with clay is easy and cheap, Trace has clay pockets on his land.

Well I use the three holers for the outer wall on my Bamm-Bamm style RMH. It worked pretty well, seems to be a good match for yurt/tipi environments as it can be broken down without as much effort as mortar or cob. I still have a surplus of old fashioned pure clay chimney bricks, maybe Trace is interested in a swap?

Coydon Wallham wrote:

thomas rubino wrote:Well, that is simple.
They are cheap and just down the road from Trace's house.
Normally solid clay bricks are used.
If using the 3 holers without filling the holes they would be insulating the bell.
Filling them with clay is easy and cheap, Trace has clay pockets on his land.

Well I use the three holers for the outer wall on my Bamm-Bamm style RMH. It worked pretty well, seems to be a good match for yurt/tipi environments as it can be broken down without as much effort as mortar or cob. I still have a surplus of old fashioned pure clay chimney bricks, maybe Trace is interested in a swap?

I'd be great with that, but by the time you figured gas and time from Wisconsin, I could buy the chimney blocks here for less money.

Trace;
Is there a Home Depot nearby?
I buy all my solid clay bricks at HD, and they were .48 each, now they are up to .65 each... still a great deal.
Not all HD carry them but it is certainly worth checking.

thomas rubino wrote:Trace;
Is there a Home Depot nearby?
I buy all my solid clay bricks at HD, and they were .48 each, now they are up to .65 each... still a great deal.
Not all HD carry them but it is certainly worth checking.

There is, but ours is out of stock and won't even allow online orders for them.  I checked all within 100 miles and haven't found any.

Bricks with holes in them will never hold as much heat as solid bricks, think of a thermos flask and how that works by using an air gap to insulate the internal coffee.
If you fill the brick with a dense medium like clay or cement it will be fine but if you want the very best effect then solid heavy dense brick will be the best.

Trace Oswald wrote:
I'd be great with that, but by the time you figured gas and time from Wisconsin, I could buy the chimney blocks here for less money.

Right now I'm planning to drive through LaCrosse next weekend, depending on Friday/Saturday night accommodations. I can bring whatever fits in my 5 foot bed (and hopefully won't break the transmission) with me no problem. More weight for icy road traction...