Can't quite get it right

[wintergrow]

Hi,
From Dunstable Mass, Greetings. I have built a RMH in my greenhouse and have succesfully pushed smoke through 45 ft of a very substantial raised bed. This was two nights ago. Yesterday morning I found that the mortar between the bricks right at the feed tube cracked. I am guessing that it is from expansion and contraction and think i will add another layer of bricks around it to keep it a more constant temp. I am using Fire Stop II, refractory cement.
Also, I bought Mr Evans book and read it through but aparently left out an important part. I didn't extend the ring of space between the heat exchange barrel and insulation container(lettered H in the book's Dimetia and Proportia) all the way down to the bottom most part so the gas hasn't maximum amount of time to cool. Is this critical? Is this why I am having some draft problems?

[bryan]

I also have been in the process of building an RMH. Having trouble getting sufficient draft as well, but as I fiddle around with dimensions it is slowly improving.

Mainly replying to comment on your mortar. I bought fire stop II as well and was not impressed with it. It had some odd tendencies like getting more wet after it set for a while and slightly weeping, and also seemed like it was little low on actual cement in the mix. But I had bought a whole bag so I kept at it. Had lots of cracking problems though and ended up taking wire mesh and regular cement and covering the outside of the brick. Sort of like stucco'ing it. Made a very strong casing and think the outside surface temps are low enough that standard cement will be ok. Also bought a small pail of Rutland Refractory cement and found it to be much harder after it set than the heat stop.

Bryan

[Donkey]

Man..
IF you guys have good clay rich soil around, I'd counsel you to abandon the cement based solutions!
I have yet to be satisfied with ANYTHING store-bought except for fireclay and one castable refractory product. Honestly, I don't much use those anymore as I find that the stuff I can dig up in my yard does the job. It does the job and is ABSOLUTELY FREE!! .... Free is good..
If you have clay, try it out on a test stove outside where it doesn't matter and see how it performs. If it's good, go ahead and move it inside.

To mortar: Make ALL of your mortar joints AS THIN AS POSSIBLE. If I can, I'll usually just dip the faces (of the brick) in a bucket of clay slip and stick 'em down. REMEMBER, mortar is for HOLDING THE BRICKS APART, to make up for little un-even-nesses and keep them from rocking around, so use as little as possible. Any sand that you use in the mortar should be NOT crystalline, unless you are using silica sand. Quartz sand WILL crack the mortar and should not be used.

[bryan]

Good advice. I have been working with light weight refractory bricks (ceramic). Went that route for 3 reasons:
1) The RMH is for a yurt--wood floors,cloth walls. So controlling temps is REALLY important. The ceramic bricks are 5 to 8x more insulating than clay bricks.
2) The yurt is a ways out so doing the preliminary construction in town and want a unit that is not too heavy to move -- refractory bricks are only 2 lbs vs 8 for regular.
3) I'm an engineer dork and like working high tech stuff.

That said I have had many issues with the bricks -- they are fragile, have very different expansion behavior than clay (so clay 'mortar' would crack), they insulate so well that the temp differentials are pretty extreme (can fire it for 30 minutes and the outside is dead cold but touch a stick to the inside wall and it starts burning in less than a second! ), and they suck up water like crazy so are hard to mortar. The one great thing about them is you can cut them or carve them super easily, so can make nice shapes.

So the mortar comment was based on finding firestop to be weak, and rutland refractory cement to be much harder. With clay bricks a clay mortar makes lots of sense, especially if it is given time to dry and then fired up slowly the first time.

[josephjcole]

I've used soft brick before for building and they do work just fine with a clay mortar. I can't speak for what you might find in your yard, as clay differs everywhere. They work perfectly with a 1:1 mixture of fireclay and silica sand. Mix up the mortar extra thin in a 5 gallon bucket. The brick will absorb enough to form a good bond when dipped in the bucket. Commercial mortars are expensive, have a limited shelf life, and make work impossible to tear apart at a latter date if you ever make a mistake or change your mind. You shouldn't have any trouble using clay mortar with your insulating bricks.
Joe

[bryan]

By 'fireclay' do you mean native clay soil that is good, or bought fireclay? I tried soils and was unsuccessful and then moved to mortar. Maybe should have tried buying fireclay, but at the time the bricks are pricey enough that didn't want to iterate too much. After having paid thru the nose for mortar though I think I should have experimented more. But patience is not my strong suit

[josephjcole]

Yep,
purchased fireclay. Hawthorne Bond is the brand I usually use, though there are several others. Get something with a larger mesh size. I think you'll be happier once you give this a try... it will actually "set" pretty fast with soft brick due to the porus nature of softbrick (set is maybe a misleading term... dry might be better). You can still pry them apart though and scrape the mortar into a bucket and reuse it if you need to make a change later.
Joe