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Post by pancakes on Oct 27, 2023 15:33:46 GMT -8
Hi all, New here and trying to design a replacement RMH for my wood stove and I'd like to get some advice on my current plans. I'll post my pictures/sketchup first to give you a visual idea of what I'm thinking and then try to explain myself haha. Also don't mind the unfinished core in the skp.    drive folder: drive.google.com/drive/folders/1cNG030CbIbg2L1gg3REG-tKGOclBHrNB?usp=drive_linkskp: drive.google.com/uc?export=download&id=1GcKz6Y-c5Br_sQMmUJ9mIy0RgKuk0AKbI've got a an existing brick pad and wall section for the heater to sit on. The brick pad/wall section are centered and away from other combustibles. Floor - 48"W x 57"L Wall - 48"W x 84"H Ceiling slopes upwards from where it meets the wall Existing 6" chimney port is 54" up the wall. It's not a ton of space so I'd like to use as much of it as possible. My second constraint is that I'd like to use the rest of a cut-up 120 gallon propane tank as part of my bell. The pictures don't include the domed top of the tank as I'm not great at sketchup (plus it'd just get in the way of looking at the riser/channels). The tank is pretty big and leaving it centered prevents a 6" exhaust from connecting up to the existing chimney port. So my idea was to have two 4" plungers that would rise at the back corners, parallel to the tank, and then curve over and Y into the 6" chimney. The ISA of the tank/brick including the brick channels is about 5.3 m^2. A little short of ideal but I also don't mind decreasing the CSA a bit to match. Might be a good idea anyway to help with the smaller total area of the 4" plungers? Note that the tank sits over the channels in the sides of the brick part of the bell as well as a smaller gap in the rear. The idea was to have most of the gasses travel around the sides, but still have some take a more direct route to the plungers to help with draw as it's already restricted. If that's not sound thinking, I can always cover the back gap or follow some other suggestion haha. So my main question are: 1. Will two plungers work? It just seems like such a satisfying solution to my space problem  2. Is there enough space in the channels for gasses to flow from tank->channels->plungers? 3. I've mostly seen 55 gallon drum builds. The 120 gallon tank is 1/4". Will that act as a hybrid thermal mass/radiator? Or just be bad at both? 4. Is there enough mass here to be effective? Hoping for 6-8 hours of release. Don't know if there are any formulas for that one :/ 5. What would you do different/what would you critique? Auxiliary question: 1. Will high temp stove paint work on the tank? I think it'd be cool to use chalk to decorate it with how much surface area it's got. Thanks for looking and for all the cool inspiration in this community! |
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Post by fiedia on Oct 28, 2023 0:46:00 GMT -8
I tried your link but I get just one pic (the top one). Can you share the skp file ?
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Post by pancakes on Oct 28, 2023 3:02:45 GMT -8
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Post by fiedia on Oct 28, 2023 9:46:13 GMT -8
1. you have a problem as the csa of your two plungers is smaller than the csa of your system size: 2*pi*(4"/2)^2 < pi*(6"/2)^2. to work well you need at least the same csa. 2. I would not put such channels. Inside a bell, hot fumes stay at the top. The main heat source at your bell bottom is the firebox. Your channels will prevent radiating heat exchange between the firebox and the bell walls. 3. I have no experience here but it will surely act as a good radiator. 4. I saw a rule of thumb: 100kg = 1h release. Does your bell weight 600 to 800 kg?
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Post by pancakes on Oct 28, 2023 15:25:03 GMT -8
Thanks for the input fiedia. Looking at the model again I have room for the plungers to be up to 6". This wasn't the case in my first iteration. I guess that means I don't need two of them, but I like the symmetry so I think I'll try it unless someone's got some advice otherwise. Can always remove one.
And good to know about the condensation. I've got some easy space on the sides of the riser to put some more mass so that will hopefully help with keeping the bell hot enough, long enough to prevent condensation if I understood why that was happening in ericaus's build.
I can get rid of the channels easy enough, but I'd think I'd still like some support pillars for the sides of the tank. Pretty sure I'm still coming up between 1/2 - 1 m^2 short of recommended ISA though.
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Post by pancakes on Oct 28, 2023 16:07:21 GMT -8
Something like this look better? I wouldn't arrange the bricks next to the riser like that, but just wanted to show the extra mass. Note the 6" plungers now too.  I also did a rough estimate on the amount of mass and I should get in the ballpark of 800 kg so I'm quite happy about that. |
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Post by masonryrocketstove on Oct 28, 2023 20:25:05 GMT -8
Thanks for the input fiedia. Looking at the model again I have room for the plungers to be up to 6". This wasn't the case in my first iteration. I guess that means I don't need two of them, but I like the symmetry so I think I'll try it unless someone's got some advice otherwise. Can always remove one. It’ll definitely draw better with just one 6” ascending pipe, which I think is what you’re calling “plungers” here. Seems counterintuitive, but hot gasses don’t like to rise equally and evenly when you divide them into multiple channels. (But they will fall more or less evenly as they cool, given a confined space like in a bell, or a Finnish contraflow heater.) Gas buoyancy is really a function of displacement.. hot gas doesn’t really “rise” so much as cold surrounding gas falls (because it’s denser and heavier) and that cold gas pulled by gravity to the bottom is what pushes hot gas upward. Doesn’t sound like a huge difference, but when you divide ascending flue channels into two, those stack pipes will basically be competing with each other to draw the flue gasses into your chimney. (Ascending hot gas flue channels don’t work in tandem the way hydrostatic water pressure does, like when a confined space is filled with uniform-temperature water.) The hot gas will inevitably start to draw up slightly more in one of those channels than the other.. which heats that channel more.. causing a faster and faster rate of draw in that one side of your divided pipes (“plungers”). Sometimes the draw can favor one side so much that it actually pulls semi-warm gas back down out of the other pipe and creates a thermosyphon feedback loop between the two sides, (rather than pulling enough gas fast enough from the bottom of the bell,) and will counterintuitively slow the rate of draw on the firebox.. even though you technically have double the total cross-sectional area of exhaust stack pipe. that recirculating thermosyphon/draft feedback loop between the two channels is more and more dramatic the taller your two competing ascending pipes are, the farther apart they are from each other, and the farther they are from the heat source. All of those things allows a greater heat differential between the two ascending pipes, with the one (rising) side allowed to uptake more and more heat, while the other (falling) side continuously cools the gas passing back down the length of it. If the temperature differential between the two becomes big enough with the falling side cooling gasses rapidly enough, it can actually create a sort of slip-stream in your chimney, where the falling-side pipe is pulling gas down the chimney into the bell, with the rising-side pipe pushing some hot gas up, but not enough to make it all the way past your roofline. …when that happens, you have a stalled chimney draft that will either smother the fire or push flue gas / smoke out of your firebox into the living space. |
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Post by pancakes on Oct 29, 2023 9:27:45 GMT -8
Thanks masonryrocketstove. This is really helpful! I'll plan for one to just be aesthetic then. Eventually, I'd like to add a bench off the right side so it would have to go down to one pipe (are they really not called plungers? lol, I don't know where I got that term from haha) at that point anyway.
Would it then make sense to have the burn core lifted off the floor? To let gasses flow to the single ascending pipe? Or is it better to have the mass of brick underneath?
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Post by rakettimuurari on Oct 30, 2023 1:12:16 GMT -8
Thanks masonryrocketstove. This is really helpful! I'll plan for one to just be aesthetic then. Eventually, I'd like to add a bench off the right side so it would have to go down to one pipe (are they really not called plungers? lol, I don't know where I got that term from haha) at that point anyway. Would it then make sense to have the burn core lifted off the floor? To let gasses flow to the single ascending pipe? Or is it better to have the mass of brick underneath? Great answer by masonryrocketstove... I've been sometimes pondering this during recent years - this is one of the best worded explanations I have come by - thanks! - Also relating to a problem op seeks advice for; if I remember correctly the rule of thumb - 90 degree turns in channels would be better to be at least 150% of the chimney/ system CSA, U- turns need 200%... This summer got to see it in practice in 100 years old finnish masonry stove which was taken down... |
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Post by pancakes on Oct 30, 2023 22:24:11 GMT -8
Finished modeling up a core and made a cut list to see if they'd all fit on the boards I have available. Saved a bit by making the inner box out of split firebrick. I have four 24"x36x1" CFBs and got just about 92% yield from them with kerf cutout accounted for. Also had an idea of moving the tank further back and running a single ascension pipe through the tank. I'd imagine I'd have to insulate it to not re-heat the exhaust gasses as well as make a very complicated cut in the top of the tank. Anyone have experience with putting an exhaust pipe within the bell? Is that just not a good idea? skp: drive.google.com/uc?export=download&id=1ksYdYlryXlZJDVXYx7jlwvRAnnkMdUAs |
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Post by Vortex on Oct 31, 2023 5:41:24 GMT -8
Also had an idea of moving the tank further back and running a single ascension pipe through the tank. I'd imagine I'd have to insulate it to not re-heat the exhaust gasses as well as make a very complicated cut in the top of the tank. Anyone have experience with putting an exhaust pipe within the bell? Is that just not a good idea? Yes, works well. I wrapped mine in ceramic blanket and then a layer of the sticky backed foil for sealing ducting:  |
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Post by fiedia on Nov 1, 2023 10:48:43 GMT -8
Have you thought about adding some mass along the back wall? It may be easier to insert the flue inside such a wall than inside your bell. Such a wall would also store some heat radiated from your metal bell.
You could also partially bury the back of your metal bell in a brick bell.
My main concern is that your metal bell may not store heat that well. I am not sure that you get the 6 to 8 hours heating per burn that you expect.
Another question: Are you sure that your soil will accept the mass of a RMH ?
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Post by pancakes on Nov 1, 2023 17:59:26 GMT -8
I thought of putting a brick flue to make the cut in the tank dome a bit easier but it didn't look great and I didn't like the idea of trying to seal it. So I moved the tank forward and the whole heater backwards to be flush with the wall to make room for the flue behind the tank. I'll have to think about a brick flue outside the tank. My current thought is to reuse my 90 elbow and section of stove pipe so I only have a 7-9" tolerance from the wall to make that work. The back wall is already brick so it should help with heat storage a bit. The section of floor the RMH will be on is brick on top of slab so it should be good to hold the weight. I plan on adding mass with whatever bricks i have left over after what I have in the skp is built. Also note the slab the tank is sitting on. I plan on casting some concrete (multiple sections to prevent cracking hopefully) Current skp: drive.google.com/uc?export=download&id=1UvvP7ddntRoW5CEMDJawYTe8eL1dx2cz I also cut and roughly tacked together the core today (minus some of the splits). I've officially started the build!   |
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Post by Vortex on Nov 2, 2023 2:41:39 GMT -8
Is that white board ceramic fiber board? Looks more like calcium silicate.
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Post by pancakes on Nov 2, 2023 7:04:23 GMT -8
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