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Post by firerob on Nov 6, 2023 4:11:34 GMT -8
The firewood we were using so far is leftover from last winter and was stacked dry in the sun all year, so this is maybe very dry and could possibly be too dry, which could mean that it's producing too much gases too fast for the heater to burn with the given oxygen; which maybe could result in this pulsing (small explosions of surplus gas whenever there's enough oxygen present). I know, there's a lot of "maybes" and "coulds" ...  That is why today's 3/4 load consists of only half of the super dry wood. When this was going well and the system was up to temp, then I added the second half of fresher wood (which is kinda almost ready for burning plus it's still kinda wettish from recent rain). There wasn't a pulse (!) but the system temp also stayed 25/35°C below "normal" and there was some smoke out of the flue. Now I think that half of the fresher wood was too much; I should have just put one or two pieces on top to lower the temp a bit. We analyzed the system this morning b4 the fire. We couldn't find anything that looked out of the ordinary. We just wanted to make sure that there isn't anything like for example superwool came off the riser and hanging down or some other thing that causes constriction. Although there was nothing I'm still adding a few video captures here since I have them anyway and maybe it's interesting for someone. These two images are shots up the heat riser inside the bell (the bell wall w/out ledge is the back wall)   These two are up the flue with closed and open damper (the bell is on the right side)   This is up the riser  |
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Post by firerob on Nov 6, 2023 4:35:11 GMT -8
Apart from that,I am not convinced that roof is the cause of that pulse, so there should be something else in my opinion. Analising the build by the pictures is not easy, but all the boxes seems to be ticked. Bar one: the main air inlet is under a brick, forming a small tunnel just behind the door. In effect, the air inlet is in reality not at the front but about 10 cm behind the door. There's no example and no drawing from the website that shows this. Also, it is mentioned somewhere to keep the fuel at least 5 cm away from the door/air inlet. I am assuming you are loading the fuel in the firebox right against the air inlet inside, maybe even on top of that brick. In case that assumtion is correct, cut the fuel much shorter than that, keep it away from the air inlet AND the rear wall of the firebox and try again. When this won't cure the pulse, make the small "air tunnel" in the shape of a funnel, much wider at the inside to the sides and up. It might be necessary to dismount the door. Again, the fuel should be away from the air inlet in all circumstances. The primary air inlet is indeed a small tunnel 6cm deep. The firewood is always a minimum of 12-15cm away from this tunnel though. The firebox is 52cm deep measured from the inside of this primary air inlet to the back wall (measured from inside door it's 58cm to back wall). The firewood is cut about 33cm. So, we truly are always keeping the 5cm distance to the port and at the prim. air inlet we definetely have a min of 12cm distance at all times. So, that's not it - unfortunately! Here's a detailed picture. (The door is 4cm deep)  If you don't want to redo things on the heater because cold is setting in, run it with the door open a crack. More air is flowing over the fuel then and will solve the tunnel venturi effect at the base of the fire. Just a thought: having the door open a bit and the air inlet blocked completely shouldn't result in a slow pulse. In that case, here's your "kink". Ok, I will try this next time it's pulsing and report back. With "air inlet blocked" you mean "primary air inlet", I assume (secondary air stays open).
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Post by peterberg on Nov 6, 2023 9:07:04 GMT -8
With "air inlet blocked" you mean "primary air inlet", I assume (secondary air stays open). Yes. |
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Post by firerob on Nov 7, 2023 6:38:21 GMT -8
If you don't want to redo things on the heater because cold is setting in, run it with the door open a crack. More air is flowing over the fuel then and will solve the tunnel venturi effect at the base of the fire. Just a thought: having the door open a bit and the air inlet blocked completely shouldn't result in a slow pulse. In that case, here's your "kink". Today I used again just the dry wood and the pulse occured but, of course, not as strong as usual. It seemed that the pulsing was always just at the verge of starting, I don't know why. I cracked the door open and closed the primary air and the pulsing was gone. But, like I posted before, the pulsing always stops when I either open the front door or the maintenance door in the back. ------ Last night I studied more about Bernoulli's principle and Venturi effect. This is how I understand it: The draft in the heater creates low pressure in the firebox (P fb). The atmospheric pressure (P atm) outside becomes high pressure in relation to the low pressure in the firebox: P atm > P fbThis causes air flow and in this streamline the velocity of the flow inside the primary air tunnel (V pat) increases because of the constriction and hence the pressure (P pat) decreases (according to Bernoulli's principle). So far I understand, now here starts my assuming: I guess that this is what should be (1): P atm > P pat > P fband this is what is in my system (2): P atm > P fb >= P patThis would explain why the pulsing stops when I open the front door: then V pat decreases (because the flow takes the path of least resistance (through door) and hence P pat increases), which causes the situation described under (1). It does NOT explain though why the pulsing stops when I open the door in the back: then the draft decreases (air gets pulled from the room into the draft stream instead from the firebox) and hence P fb increases, which again causes the ("bad") situation described under (2). Is this all correct? Or does this just sound too complicated to describe in a post? The big questions I have are these: 1. Why does the situation described under (2) create pulsing? 2. Why do the pressures behave so different just because the primary air tunnel is 6cm deep instead of...I don't know... what do others have here: 1mm, 1cm, 2cm,... in the end they are all just bigger or smaller tunnels, arent't they? I apologize for all of this rambling, but I would really like to understand this problem first and know for sure that my air tunnel is in fact the cause for the pulsing before I go to the length of removing the door and somehow(?) changing this air tunnel. Thanks a lot for y'all patience  |
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Post by fiedia on Nov 8, 2023 8:14:52 GMT -8
before I go to the length of removing the door and somehow(?) changing this air tunnel.
Coul you easily remove the top brick of the air entry without removing the door ?
It would be a simple way to see if you get better results.
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Post by firerob on Nov 8, 2023 10:46:25 GMT -8
before I go to the length of removing the door and somehow(?) changing this air tunnel.
Coul you easily remove the top brick of the air entry without removing the door ?
It would be a simple way to see if you get better results.
No, I would need to take the door off, which isn't difficult at all. I just cannot take out the brick on top of the air inlet because then the air inlet would be bigger than the recommended 50.4cm². Besides it's mortared in. I could and would do all this but it's not easy and I would just like to know for sure that that's the problem. I would rather try to chip away / saw / sand / chisel the air inlet tunnel into a funnel shape, like Peter suggested |
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Post by firerob on Feb 12, 2024 8:28:25 GMT -8
So, 3 months later: This Batch Box Rocket Mass Heater is the best stove I have ever experienced! The warmth spreads evenly in the whole house, even to the farthest corner. With outside temps down to -16C the BBR warmed the inside to 20-22C EASY with only 2 / 2.5 fires daily; and there was still room for harsher winters. The left and right wall of the firebox cracked vertically somewhere in the middle and the mortar connections between sidewalls and backwall also cracked in places but all of this didn't change the performance in the least. => next time I would use 6cm thick firebricks EVERYWHERE (no split bricks; I should have listened to Peter in this also). The pulsing phenomena is still the same (I didn't work on the problem at all) and we just work around it with the door cracked open and the prim. air inlet ~60% open. The aluminium flue didn't cause any problems; even the damper, which is also made from alu, performed well even when I accidentally forgot to open it during a full firing. No condensation problems at all. The flue outside the house is NOT insulated. All in all, excellent stove!! We couldn't have asked for anything better!!!! Peter, thank you very much again for the invaluable help you provided! The only thing I would like to mention for any future builders who stumble across this post: Research into "Fresh air inlet" into the house. I gave it to little importance during the designing and building phase; and then had to improvise during the coldest temperature phase; because that is when the lack of fresh air inlet shows up clearest; when ice cold air gets drawn into the house from all available holes and openings and this ice cold air streams towards the stove. Don't underestimate the importance of this - like I did. Thank you all for the incredible wealth of information all over this forum!
Soon, I'll post a list of all the used materials.
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Post by firerob on Feb 12, 2024 10:06:23 GMT -8
These are my records regarding...
DATA and MATERIALS of our Batch Box Rocket Stove:
180mm System to heat the space of 105m³ (38m²)
FIREBOX:
bottom and sidewalls: 22 pieces of split firebrick
backwall: 6 pieces of full firebrick
top: 1 board/plate of Vermiculite
is raised by ca. 40cm off the ground and supported by L-profile
HEAT RISER: (127.7cm high) 37 pieces of full firebrick
BELL:
ISA: ( 2x (0.97m x 2.04m)) + (( 2x (0.68m x 2.04m)) + ( 0.97m x 0.68m ) = 7.39m²
(2 sidewalls + front and back + ceiling)
ca. 400 pieces of normal bricks
mortar: mix of thick slip, sand, screened ash (by volume: 2:1:0.5) this mix was very good, I would use it again! Use gloves b'caue of ash!
Around METAL PARTS I used mortar WITHOUT ASH though! thick slip, sand (1:1)
Without the ash is good too, but with ash it hardens much faster! (building time!)
1 front door
1 back door (maintenance)
1 p-channel
some meters of L-profile (40mm x 40mm, 3mm thick)
4m of aluminium flue (180mm)
top part of bell (from the level of top of heat riser up, including ceiling): double skin
inner skin: 42 pieces of split firebrick
outer skin: normal bricks
insulation in between: superwool
bell ceiling: double skin (see above) supported by L-profile
top gap: (distance between top of heat riser and ceiling) 38cm
MATERIAL DIMENSIONS:
normal bricks (used) 12.5cm x 25cm x 6cm
split firebricks: 12.5cm x 25cm x 4cm
full firebricks: 12.5cm x 25cm x 6.5cm
mortar for firebricks ~11kg (dry) (rated 1200°C)
superwool (12.5mm thick, 61cm wide): ca. 10m
to insulate the firebox, the heat riser and the double-skin part of the bell
(above the level of top of heat riser)
Firebricks, Vermiculite and superwool are rated to 1200°C |
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Post by phyrman on Feb 14, 2024 7:25:01 GMT -8
Bravo Firerob! Congratulations on your successful build. I want to thank both you and Peter and the other contributors to this thread for all of the great information. I'm in the process of building a similar design and this thread helped me a lot with some key details. Here is my progress so far. I'm hopeful that I will have the level of success that you achieved. [img src=" ![]() ibb.co/cQGTQ3z" style="max-width:100%;" src="https://ibb.co/nbW0Lyc" alt=""]  For some reason I'm having issues again inserting images. My build thread is viewable at Permies at this link |
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Post by Vortex on Feb 14, 2024 13:22:14 GMT -8
The picture hosting site is hiding the actual url to try and prevent you from embedding images. If you click your image link above, then when the page opens with the image, right click on the image and click 'copy link location' then you'll get the actual image link that you can post here: [img]https://i.ibb.co/89W89sF/IMG-20240212-164130.jpg[/img] |
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