Matt Todd

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since Apr 25, 2019
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Always a backyard gardener, now expanding into permaculture!
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Recent posts by Matt Todd

Andy Bhill wrote:Yeah those pages had me going back and forth. I don’t have all the 3D stuff but a quick sketch is something like this.

Edit. Pic needs turned 90 degrees. Haven’t figured this site out yet

Ok, I guess that's kinda how I did imagine it. That should work just fine... but it makes the structure more awkward in needing support and you lose the ability to use the top box for an oven or just a window. And seeing the secondary fire burn is half the magic of this design

The Sketchup 3D program was too difficult for me to make anything decent with. But I did find it possible to use for measuring and scaling up Peters existing designs and verifying my calculations.  

Andy Bhill wrote:
Yessir. I just found the shoebox threads last night and read them. My thoughts was the same as yours with a insert slid in. I’m curious as to the outside measurements of your core. I was thinking after seeing yours if the firebox could be partially outside and the shoebox area turned at 180 degrees to be centered inside the tank because the tank I have is about 32 inches. Thank you for posting this!!!

Ohh boy, those 31 pages of DSR2 thread have a lot of juicy nuggets. I got to where I just started copying and pasting notes out so I didn't have to keep going back to it!

My 7inch system outside dimension are 35.5 inches long (the actual length of a 36 inch sheet of CFB), 14 wide but more like 14 & 1/8 to 14 & 1/4 actual. And 29 tall. Height was a pain because I had to add to full sheets that come as 24 inches, which meant cutting 3 inch strips to make a 2 piece wall.  

I'm having trouble picturing what you mean about turning the top box around 180 degrees.
The top exhaust of the shoebox goes 1/4 the length of the box from the front, so with a 32 inch diameter tank you'd still be able to fit. Mine only sticks out so much because I put the flue pipe behind the core. It could be beside it, or even outside on an elbow if you feel cool enough to cut a round hole in a curved surface (which I did not.) I should note that the 7inch system is designed with a 20 inch firebox, but I extended to 24 inches to match CFB size (firebox plus stub.)

Andy Bhill wrote:I’m glad I scrolled down and found this.... exactly what I have in mind but on a lil smaller scale.

My scale was going to be smaller too. Two stacked 55 gallon drums was the original plan... and it wasn't until my foam mock-up that I realized the 7" core would never fit! Turns out the guy who designed this core (Peter Van Den Berg) was running a little 4 inch core for his tests in 55 gallon drums. Guess I could have still used drums, but I would have had to figure out how to mount them above the exhaust port on top of the core.
I am pleased to share my journey building "The Sherman Tank." Because builds need fun names. Watching how others have built heaters has helped me immensely, so now I'll share!

Goal- Make a powerful heater with a removable core for an poorly insulated shop. Able to be moved. Able to add mass that can be fine tuned.  

Core choice- Peter Van Den Bergs DSR2 (Double Shoebox Rocket version2) Selected because the shape lends itself to my build, it's shape is such that it can be removed, it has a fun window which could become a door and oven, it's super efficient, and it can be ran as a simple open system (no door or secondary air tube, BUT those can be added later in this design.)

Material choice- Ceramic Fiber Board for the core because it is light weight, lending itself to removal. And because it's easier to work with than castables or firebrick. A 300 gallon steel tank for the housing/bell because I had the big ugly thing in my yard already and it fits the bill of an inclusive shell that can be moved, rather than built in place from things like brick and/or cob.

Sizing- I chose a 7 inch system to use with 8 inch flue pipe for better guarantee on proper draw. Also liked 7 inch because it fit my firewood and my tank. The one measurement you can adjust in a core is the firebox length, which I took from 20 inches to 24 inches. This also matched the longest edge of the CFB sheets. Someday I will move a system into my home which has an existing chimney with 8 inch flue.

Problems- Despite the best advise not to, I ordered CFB online, shipped to me. It was damaged. I was able to get a partial refund, but freight or in-person buying is the correct way to go.
There is bad CFB out there, and that's what I got from Phoenix Supply. It did not take the flame well. Spraying the pieces with sodium silicate saved the day. Now instead of fraying, the CFB just gets harder. I did have to water down the thick 40% sodium silicate solution by a third to be able to spray it. A little blue dye helps you see where you've covered.

I'll tell the build story mostly in pictures, but here are a few videos also:
-The initial CFB material test results. This was rather disheartening
-The smokeback at the beginning of the burn, likely caused by me still figuring out how to properly light the thing! I was able to fan it back to start the proper burn path
-The roaring takeoff of secondary combustion. A great pleasure to behold in real life
-Even smokeless burning at about the 20 minute mark. I bet it would have been sooner if I had lit properly (plus the tank residue)

Using an infrared temp gun, I got the following readings:
500 to 550 degree F Max top temp right above the exit port of the core. Otherwise tank top was 400 to 500.
200 degrees at flue base (where it meets the tank top) but 180 above that.
Towards the end of the burn the whole tank read more evenly around 300 degrees.

Conclusion- Well over a year ago, I started studying rocket mass heaters. And to build one, it takes lots of studying! You have to determine your goals, your style, your materials, then run into obstacles until you hammer out something that will then need a bunch more refinements. Special thanks to Peter Van Den Berg, Matt Walker, Thomas Rubio, Gerry Parent, and all the folks on Donkey's board. Most of this core came from studying the 31 pages of Peters DSR2 build here:

Next up- I'll do an update post when I move it into the shop. This will include how I'll add thermal mass, and how the chimney will interface.
Any comments, criticisms, and questions are welcome!

5 days ago

John F Dean wrote:Let's go back to the original post. Why can't you change the pitch of the porch roof?

I mean... I could. But that would mean disassembling and rebuilding the 54 foot length of it, which is way more work that I want to do or pay for. Especially since it only leaks in 1 or 2 places. Granted, I have fantasized about just chopping a few inches off the uprights above the porch rail and re-securing them :)

Currently what I'm looking at trying is foam backer rods. Closed cell foam sticks to tuck into that gap and see if it works before attempting any more permanent solutions.
5 days ago

thomas rubino wrote:Ha Ha Matt;   Wait???   Are you drinking this morning???  How could you wait after all that work??? That dragon needs to stretch her wings NOW or they will atrophy... then your pucked!

I build my fire the same way I always have... (did I say different?)  two chunks one on each side paper and kindling in the middle ... all the rest of the wood on top of that and light her off, rite back at the door... It burns great for me this way.  No startup issues.

Between the red-flag no-burn warning and the core not quite fitting (and me being tired of the angle grinder), it'll be tonight or Friday before I light her up.

But anyhow, you mean to tell me that you just light your fire at the door end of the box and what, it migrates back to the port because that is where the magic happens?!
I mean, I believe you. It just sounds too easy to be true so I'll be happy to try it. I struggled with J-tube lighting and figured I'd have the same issues with a batch box.
6 days ago
I put the finishing touches on my 7" DSR2 core last night and I'm stupid excited to slide it into the bell and get a test burn... but crying because it will be super windy today and I should probably wait!
The 20 inch fire box of a 7" system got stretched out to 24 inches in my design to match the standard length of the CFB. But all the cut wood I have currently was done before I had this design finalized so I'm left hoping the majority of it will fit. I think it's mostly cut to 20 inches (I hope anyway.)

So that brings up a question: How do you start a fire? Because in my head it's lighting some kindling mid-box and pushing it back as you load and then loading on top. But if your material is sized to the box that doesn't leave much room. Have you ever done or seen a video on lighting/loading a batch box?  

1 week ago

Eliot Mason wrote:Matt:

The C intersection should be dry, and filling it up with silicone or other sealant violates a cardinal rule of roofing - ALWAYS let the water go.  If some water finds its way through a nail hole or something and into that cavity and it can drain away - or vent away - then you can create real problems!  

So ... why isn't the porch roof draining?  Unless it is completely flat that water should go away, especially as water coming off the pitched roof has energy to help sweep any water off.  I'd look at your gutters and any trim details on the roof edge.

HA, I figured the cardinal rule of roofing would be more like "keep stuff under the roof dry" :)
The porch roof is almost totally flat. I keep the gutters clean. Too much water comes through to be acceptable. I believe that current flashing would work, but the seam ridges keep it suspended when it should actually conform around them. It wasn't there originally. This was a roofers slap dash solution to the leak that was even worse before ("fixing" the porch roof was a condition we made for buying the house, and it was done as cheaply as possible it would seem.) They stopped coming back after I bothered them too many times after every heavy rain.    
1 week ago
Not strictly a greenhouse/permaculture question... but this is the roof on a porch I want to glaze to become my greenhouse and I don't want it to leak!

When a heavy rain hits the house roof (A) and porch roof (C), it backs up under the flashing (B) and leaks in various places under the porch roof. The porch roof does not have enough pitch to carry away water fast enough. Since I cannot change the pitch of the porch roof, I believe I should somehow seal the gap between the flashing and the porch roof (B and C) to block water entry. I have used a silicone caulk in one spot and it does block the water, but it’s not practical to do the whole porch this way (40+ feet at one inch tall is too much caulk gunning.) What would you do here? Flexible adhesive flashing? Some sort of extruded product? Thanks.  
1 week ago
They really help my complexion.... look good on Facebook live with their pleasant golden light! Otherwise meh. I mean they are a nice safe lamp too since the electronics are housed inside a big block of mineral.
1 week ago