matt walkers cast core.

been messing about with the bricks a bit and seem to have the dimensions about right now. so i want to move on to a cast core like matt walkers one. i am going to be using a 6 inch riser rather than 8 inch as the rmh will be heating a 16foot yurt insulated with celotex. the sq footage is around 1200 so from what i have read 6 inch will be plenty.
his video is for a 8 inch riser so i need to reduce everything by about 40% or so. trying to get an idea of how much fire clay to order. he recons around 92 pounds total weight so if i keep the insulation at 4 inches thick should i assume around 60 pounds or one 25kg bag of fire clay. i have 100 litres of perlite but no fire cement as yet.
would rockwool mixed in with the mix work as well as fibreglass matting? i have both but a lot more rockwool. i know it isnt a precise art but does that sound about right?
its either 25kgs or 50kgs of fire clay.
i got a sheet of aquaboard today similar to hadiebacker board. i am going to try and add a trip wire and the ski slope at the back if i can as well. plus the p channel just to keep right up date. i thought i may try to mould the tripwire into the aquaboard separately cook it in the oven and then fit it on the top of the mould before i finish casting the rest of the mould. not sure if it will work or not.

Hi Bob; when i built my 8" core using matts plan's, I had 2, 50 # bags of fireclay on hand and I didn't have quite enough! I needed 3 bags ! Even on a 6" I would have no less than 2 bags. I'm sure that rockwool will work fine as a binder. Plywood will work as well as hardy board or aquaboard, what ever you use will burn out quickly. Also matt has plans for a cast riser with the same ingredients , You will need a bag of fireclay for this also . Matt used a piece of stove pipe for his sacrificial inner form , I used a piece of (soni tube) cardboard concrete form as my inner form and it worked great! Good luck Tom

thomas rubino wrote:Hi Bob; when i built my 8" core using matts plan's, I had 2, 50 # bags of fireclay on hand and I didn't have quite enough! I needed 3 bags ! Even on a 6" I would have no less than 2 bags. I'm sure that rockwool will work fine as a binder. Plywood will work as well as hardy board or aquaboard, what ever you use will burn out quickly. Also matt has plans for a cast riser with the same ingredients , You will need a bag of fireclay for this also . Matt used a piece of stove pipe for his sacrificial inner form , I used a piece of (soni tube) cardboard concrete form as my inner form and it worked great! Good luck Tom

thanks thomas, i have ordered one bag for now to make the riser with so i have some idea of quanities. i am going to use cardboard tube and paint it with cement slip as suggested by someone either here or on donkeys site.
should i make the riser wall 4 inches as well? i have some oil drums 11 inches diameter, but i could easily extend them up to 14 inches if needed. i have plenty of those so easioer to do that than find a 14 inch drum the right height.

Bob ; The barrel i used was 14" dia. x 27" tall . My inner liner was close to a foot taller. with an 8" system i only had 3" wall thickness. I'm sure that 4" could only be better. Your 11' barrels would only give you 2.5", but most likely would work just fine . After a long winter of keeping my greenhouse warm the cast riser is in perfect condition !

Bob, I think your plan will work out just fine. One heads up about reducing the measurements of the core; mock up your build with outer barrel and riser included and make sure your core is long enough that the outer barrel doesn't cover part of the feed when it's all assembled. With the 6" core casts I've done, I've kept the burn tunnel length the same as my 8" core so there is plenty of room for the outer barrel.

With regards to dimensions, I find it easier to think about the size of the openings and passages rather than a percent of what I did for the 8". For a 6" you'll want 5.5" square for those as opposed to the 7 3/8"or whatever it was for the 8". That way you can keep the burn tunnel length and core/feed/riser stub height the same, which I find to work very well.

Tom, I read somewhere about your feed getting banged up, and I wanted to let you know that is totally part of the trade off of this type of core, you did nothing wrong. Light highly insulating properties and hard abrasion resistance are properties that are kinda at odds, so I go with the function I want and deal with the wear. I don't find it too bad, I use care when loading and try to just be patient when the fuel is burning down low so I don't go in there and bang it all around trying to put a new load in too soon. I've tried a lot of stuff to make it more durable and always come back to the softer core mix for the feed since the performance is so good. At this point, I have pretty much settled in to patching my feed a couple times a winter, and I find it totally acceptable.