Mark Dumont

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since Jan 10, 2020
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Recent posts by Mark Dumont

Thanks so much for the replies and links!!  It's a huge help to be able to bounce these ideas off of fellow RMH builders (the rest of the world is convinced we're crazy).

Below are a few more photos of my latest mock up, for clarification.  I plan to lay down 1" of CFB to protect the wooden floor below from high temps, and then 1 layer of brick as mass to buffer the heat on the floor and provide some thermal mass storage.  Any comments?  Is the layer of brick on the floor a good idea?  Unnecessary?

Gerry, your comment on ISA pertains to my next thought - If I were to use 6" ducting instead of a split barrel bench my ISA would be about 2,800 square inches, (about 140" of pipe) as opposed to 2,046 square inches - the exposed area of barrel (minus the brick/ mass covering the lowest sides).  The duct would give me more ISA, but I like the idea of the barrel chamber better, it seems the self-regulating stratification allows the mass to extract more heat from the flue gasses before escaping, so maybe the ISA of pipe and bell chamber is comparing apples and oranges?  

Another advantage of the pipe, though, is that it allows for a lower bench design, without the warm strip at the top.

Gerry, the thought crossed my mind of insulating the mass above the peak of the barrel to avoid the warm strip.  Given my design goals of keeping the bench low and comfortable, i have a total of 4 inches above the peak of the bench.  If I were to place 1" of CFB here that would leave only 3" for cob, E & E Wisner recommend a minimum of 4" for structure...  hmmm....

Another detail - I cut the slot on the inlet pipe thinking I wanted to deliver the incoming heat as high as possible in the bell (and withdraw the exhaust as low as possible).  Now I'm thinking perhaps it would be better to rotate that slot underneath, and get the heat delivered just a little further into the bell.  
6 hours ago
Plus, I have a clarification question - Can the base of the split barrel bench be lower than the base of the combustion unit?  My understanding is that it can, as long as the inlet from the combustion unit is high and the exhaust draw up the chimney is low. i.e.on the floor.  I found this thread from Donkey's forum: Perter van de Berg says that the base of the combustion unit and bench bell must be even, but I recall reading somewhere (??) that the heat can be downdrafted considerably...  
9 hours ago
By the way, it;s a J-tube, kiln shelf is covering the combustion chamber in the photo...
20 hours ago
I'm wondering how much mass to put above and below my split barrel bell/ stratification chamber in my RMH cob bench?  It will be sitting on a raised platform over a wooden floor.  See photos of mock ups below.  The  challenge I'm facing is how to get enough mass under and over the split barrels without raising the bench up too high and making it uncomfortable to sit on. Comfortable bench height is considered to be 18"  I'd like to keep it to  20 - 21".   The platform eats up 3.75", a split barrel sits 12" at the highest point, In the Rocket Mass Heater Builders Guide, E &E Wisner recommend 4-5" under  and 4-6" over stove pipe buried in mass.  Add it up (using the lower numbers) and you get 23.75", too high for a comfortable bench.  Options I'm considering:

*Cut some off of my barrel split to lower the peak height.
*Fill the floor of the split barrel in with some mass (in my case, bricks or cob, which is kinda similar to the first option  I lean towards bricks, for a flatter surface that's easier to clean of ash).   I These two options reduce the internal surface area a bit.  
*Put down a layer of insulation (CFB or Clay-perlite) under the barrel split in order to protect the wooden floor beneath , then cover with a few inches of cob or brick.

Does a bell / stratification system require the same mass over and under as a stove pipe system?  I'm wondering if the temps on the floor of the barrel splits will be somewhat cooler than a pipe, so I could get away with a little less mass?  Any guess what temps I can expect in the split barrel intake and exhaust?  This is a critical design decision i don't want to mess up - don't want the base to get so hot that it damages the floor, not do I want the bench so high that it's uncomfortable to use.  

Any input welcome!  Thanks!
20 hours ago
thanks for the detailed links Satamax, these are great!!  Just what I was looking for!

The post by Konstantin Kirsch mentions experiencing smoke in the room from the feed tube, is this pretty common w/ J-tubes?  can it be mitigated with a sliding door/ bricks, or even a bot of water like he uses?  My wife, who is all in on the RMH, is sensitive to smoke, so it's gonna be an important issue.
3 days ago
Hey Folks!  Looking for more details on a P-channel for a J-tube RMH - I'm familiar with the basic concept, i.e, an extra piece of metal in the feed tube to improve air flow and prevent the fuel from blocking air. but I can't seem to find any details on design, fabrication, installing.  Any info and/ or links would be much appreciated!!

3 days ago
Thanks for the replies, and for your explanations Gerry, very helpful!!
6 days ago
A mason heater friend turned me on to, as a source for stove doors suitable for a batch box RMH at a very reasonable price.  The owner, Max Edelson, builds masonry stoves and rocket mass heaters, so he gets it and can give some basic advice.   I was stoked to find this!  Photos fo some doors below, you can download a catalog and price list at the website.

6 days ago

So Gerry, I've been mulling over your response:  

As far as how hot you burn, if the temperature isn't high enough, certain gasses will not combust to give a complete burn and you might as well just be building a regular wood stove that burns much cooler (and often dirtier).

While I understand what you're saying in the first half of the sentence, about the significance of the high temperature, I'm still questioning the second part, is a batch box built with full, uninsulated bricks just as inefficient as a regular wood stove? - my design started with s J-tube, and then evolved into a batch box.  the J-tube is already much more efficient than a regular wood stove due to the tall riser and the thermal mass.  By converting to a batch box I'm adding the improvements of a secondary air port and increased turbulence.  Are these added features non-functional in the absence insulation?  While the fire will take longer to heat up, and while that escaped heat may be "robbing" the fire, it is also heating the mass and my house, which is my prime objective (this is the point mason heaters are making to me).  As I've mentioned, I'm building this in my living room, and really wanting to avoid regular tear downs and rebuilds, so I'm wary of insulative materials such as CFB that appear to be much shorter lived than fire brick.  This trade-off is a fascinating question to me and I'm surprised I don't see it elucidated elsewhere on this forum, or maybe it is and I've missed it.  

So I guess I'm still on the fence about whether to use CFB in my design or not, I'm going to consult with a mason heater today and will report back my findings.  Meanwhile thanks for helping me puzzle through all this.
1 week ago
Great feedback, thanks so much!  We took the bottom off the drum today and will install it w/ removeable lid up!

Thanks again!!
1 week ago