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Post by Solomon on Jan 16, 2021 13:21:07 GMT -8
I decided to build my 8" Walker Superhot J out of foam board first just to make sure everything fit together and I discovered some issues. Firstly, there is a gap in the floor between the bricks and the second layer of the floor of the burn tunnel. Is this an ash pit? Typo? Should I put another brick in there? If it's not an ash pit, I would probably put another brick in there, just for extra protection. But I could see how it might be an ash pit. Second: The sides of the burn tunnel are about 1/4" too long, meaning the bricks don't fit. Easy fix, just trim 1/4" off the sides, I guess. Third: The back of the burn tunnel is too short. You can see some issues that show up even in the drawing. drive.google.com/uc?export=view&id=
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Post by josephcrawley on Jan 16, 2021 17:56:53 GMT -8
If you've purchased plans from Matt you should reach out to him directly. I helped on a build based on his plans and he was very helpful and responsive.
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Post by Solomon on Jan 16, 2021 21:55:25 GMT -8
Yes, I've done that, and he has. Turns out the plans have not had much development as his focus is on the six inch model. Now I'm thinking I should reengineer them. I've already made some changes.
How important is the burn tunnel length? The internal length in the mockup is 29.5 inches. Is that a problem? Can I have the riser a little more centered in the barrel?
How important is it to keep a consistent cross sectional area? Can the tunnel be a little bigger? Maybe for ash capacity?
How useful would it be to make the entire floor of fire brick splits? There will be 3 inches of ceramic fiber board under the splits and concrete under that.
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Post by belgiangulch on Jan 17, 2021 5:43:40 GMT -8
Hi Solomon; About your floor. Adding one more split to fill the hole would be fine, it is not an ash pit. Making your entire burn tunnel floor from bricks is counter productive. Those brick's hold heat, you want your heat going thru the core and up the riser. The bricks are there for wood abrasion purpose's only. Making the entire floor with bricks has no value in a ceramic core build, only detriment.
Core dimension's should not be changed. I have seen an 8" burn tunnel be made 1/2" deeper to allow some ash build up. So that would be 7.5" wide by 8" deep. But that should be the only change of dimension's. Do not lengthen the burn tunnel. That is a big mistake. (I know because I did it) Then I got to rebuild in the middle of winter... not fun! Centering the barrel is not important, it can be much closer to one side without any problems. The J tube dimension's were extensively tested with different sizes and shapes. If you modify them, your core will hopefully work but it will not be performing as it should. You will have excessive ash buildup and overall not be happy. If your not happy then you will tell others your not happy. We want happy RMH builders to spread the good word.
If Matts plan is a little off Please let him and Luke know. They will want to correct the problem.
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Post by Solomon on Jan 17, 2021 10:04:31 GMT -8
Matt is telling me the dimensions are good, just a scaled up version of his 6". I am needing to make adjustments however, due to gaps and that the measurements are in decimal inches.
At this point, the changes I'm planning on making are to reduce the total height just a hair to bring the top piece down flush with the bricks. I think I will leave the internal tunnel length at 29.5". I thought about moving the sides in a little bit to eliminate gaps around the bricks, but that would mess up the shape of the riser port and the feed port.
I have already added a third brick on the floor. I don't think extra bricks on the floor will be much of an issue, considering that excessive insulation is beneath them. May take a minute or two extra to warm up, but the bricks are not wicking the heat away to anywhere once they're warmed up. Plus, they'll be covered in ash anyway, an additional layer of insulation.
I'm not trying to lengthen the burn tunnel, that's the way the design came. Matt says he prefers the longer burn tunnel as he believes it to be more efficient. Evans' book says the burn tunnel can be longer, it just needs to be better insulated. A longer tunnel just means a fraction more friction loss and heat loss before the burning gases get to the riser. But in this case, we're working with a fiber board core, which is going to be far and away more insulative and lower mass than a brick tunnel. Add to that, I'm going to be using an extra tall 2" thick ceramic fiber riser sleeve. So I believe I have built in some safety factor.
What I meant by centering the barrel was more in terms of actually putting the riser toward the "rear" of the barrel. I want as little heat as possible going toward the wall. Evans says in whichever way the barrel is eccentric, that's the direction more heat will radiate. I have not found much testing done with the ceramic fiber board 8 inch core, however, Matt tells me that CFB cores are much more reliable and efficient than brick.
Thinking of the claims of the Dragon Heater, they say for that core, you need two barrels to handle the radiant heat. My system, because of the tall riser and using a barrel as a manifold, I'm going to be using right about two barrels. Fortunately I have a nine foot ceiling so that's helpful, but I'm expecting to need heat shielding on the ceiling as well as the wall. The core is going to be sitting on a 2" sheet of CFB directly on concrete.
Matt has removed the 8" plans from his site and refunded my purchase. I didn't ask for either of those, just to be clear.
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