I do have it insulated, did it just like the book said, just dont have that pic up... I had it at 2" from top of heat tube and raised it an inch, actually seems to work a bit better raised, I also increased the gap at the exhaust port to give it more flow..
Ok I wasn't sure, I wanted to try obvious things first. Any way, I am not an expert as some of the people who post here are. I am feeling my way through. I would like to build one of these soon myself, but I am warmer than you so it is not as urgent for me.
I am assuming your fire bricks are 9x4.5x2.5, So you feed tube and tunnel are 6.5x6.5 inch (9-2.5 is 6.5) for a CSA of 42.25 sqin, but your heat riser is 8inch and pi x 4squared is 50.3 sqin CSA. I could be wrong, but I thought the whole "J" was supposed to be constant CSA. maybe try a 7inch heat riser for 38.5 sqin or redesign the feed. There is such a thing as 7 inch stove pipe (just looked it up) so that may be the easier route. I don't have the book in hand, but got this from referring to some of the Aprovecho documents.
The insulator pipe is about 20", left a 2" gab all the way around the 24" 55gal barrel. Should I make the insulator pipe smaller? Say 12-14"?
CSA wise you should be fine. The documentation I have shows as little as 1.25in gap. Some one with more hands on would probably have a better answer. That one is harder to test than the too big heat riser. It would be a lot of work to change I would think, so it would be nice to have a better opinion than mine. Look for the "Portable rocket mass heater" thread and look though it, there may be something there that helps.
paul wheaton wrote:
I built it.
I think it might be too much pipe with too many turns.
I bypassed most of the pipe and it worked much better, but the combustion didn't seem as rockety as I remember. And the exhaust seemed rather smoky.
So .... a six inch duct has a cross of (pi*r*r) -> (pi*9): 28.26 square inches
I think the burn tunnel is 7.1 inches high and 4.1 inches wide.
The intersection between the two is .... less. Maybe 20% less. I suppose that could be problematic, but I didn't think it would be this problematic.
Critical dimensions are in a word "Critical" Cross sectional area cannot be reduced by 20%. the air volume is to low and the laminar flow coefficient is too high.
If you want to stay with 8" ID riser with a cross section of 50sqin, you need to increase the size of your tunnel and feed. Remember, you are already partly blocking the feed with fuel. Instead of 6.5 inches, you need to go to at least 7inches square, but it is not so easy to change the height and besides you want the interface to the riser to be big. Another quote from the same place...
you dont need much to make the square work with the bricks; the bridge bricks just need to lap about 1/8th over the sides.
So, leaving the height at 7 inches (one brick on it's side and one on edge) move the sides out 1/2 inch on each side. This will leave you 3/4 inch on each side for the tunnel top to rest on. I am assuming the hole under the riser is the same as the feed hole (6.5 inches square?) it should be wider and/or longer. At 6.5 inches wide it would have to be 7.75 long to match the CSA of your 8in riser, but you are going from square to round so corners and sides will be blocked. If you widen the whole base to 7.5 inches and set the length at 7.5 in as well, your riser will still be supported on 4 sides and the corners can be blocked with clay. With the height of 7in and a width of 7.5 your tunnel is now the same CSA as your riser, good. Your feed is also wider at the same 7.5 inches and so just needs to be lengthened to 6.75 to be the same all the way through... I think I would make it 7.5 inches square to look right.
If this is too much work, switch your riser to 7 in inside diameter.
Both feed tube hole and exhaust hole are just a little under 7.5" sq just as you say....
I'm out of ideas then. Thank you for sharing your experience. I will know to test whatever I build in the back yard first, or be willing to try and tear down a few times. In am disappointed that none of the people with more experience has said anything, but not everyone spends all there time at home or typing
Just one or two things to try temporarily.... I don't know if your flu has a cleanout at the bottom... try leaving that open to exhaust at that point (maybe the rocket doesn't have enough push for the height of your flu) I have seen a lot of RMH that exhaust just out a window and have heard that getting it to exhaust up a flu is harder and some times takes time for the flu to get warm enough for the draft to work. Some people have put a small fire hole right at the flu to get the draft going.
The fact that your mass heating pipe work (at least in the pictures) is still open and not covered with mass may have some impact... like maybe the flu gas is so cool by the time it gets to the flu there can be no draft.... what does the pipe feel like just before entering the flu? The standard minimum flu temperature for a masonry mass heater is over 200 degrees and many of them have a mass bypass damper that is open for starting and closed once the draft is going (they measure the flu gas in the center of flow, so a measurement of the pipe itself might be lower). On the off chance you have already put in the mass... maybe it needs to dry still (clutching at straws here). Have you tried running it with the end brick (farthest from the barrel) of the feed hole removed? This might have the effect of lowering the height of the feed tube and giving the rocket more pushing power.
I am sorry I can't be more helpful. But I need to "play in the mud" before I can say any more.
I too am really new at this but I'v done quite a bit of video watching. Just an observation here. I count 5 brick from ur burn chamber to your feed hole. Try taking away 1 brick to move ur feed hole closer to ur burn chamber. Good Luck.
I can see I would be looking at the same problem as I have a tall flu through two stories and a attic. I will be using the tank from a water heater in mine, so I should be able to have a taller riser. I hope to start with 48 inches and see if that keeps blowing the fire out I think I will get some fire brick to play with out back.
Keep it up, sound like your close.
Thanks for your help, at least someone is trying to lend a hand.
No problem, I do what I can and besides I learned a lot too. As I said I plan on doing my own soon. We heat with hydro right now, which is better than gas, at least cost wise.... but our hydro is two stage, low rate for the first bit and higher rate for the rest. Anything I can do to add heat will help. I have gotten my house hold used to cooler air in here and we only heat any one area when we expect someone to actually be there. However, a higher base temp in the house would help. I am going to try going off grid in the city here to see how far I can get. My two projects right now are heat and to get my freezer off grid and onto solar... next spring it's gardening... I hope to do better this year, but I have never done gardening before... I'm learning. What I got tasted good though.
The riser tube has to be insulated in order for the hot air to travel up and push the cooler air down. You said it works good when first lit and then stops working. I"m thinking that when you light it up, it's cold so it starts a nice draft but then the chamber heats up and equalizes and you get blow-back.
Just another thought. We'll figure it out yet.
For the exit gas I mad a 3rd chamber the same size as the feed tube, it doubled as a cleanout. The 8" pipe came out the side and started its run, for some reason (and I will figure the science of it out and post it) the gases didn't like that chamber. I think it was possible that because it share a wall with the heat riser chamber it would create heat and cause a sort of bottleneck from the heat rising.
Could be. That makes sense. What doesn't, is the pictures I have seen where they have used fire bricks stacked to make the riser, unless they use insulating fire brick. Hmm fire brick $1.50... insulating fire brick $6.00 for a split at that (what I can get around here). The other thing that I find strange is the drawing showing only 1.25 inches gap from barrel to insulation of riser. Sure there is the required CSA going down from top to bottom,,, but then there is the exit 50sqin pipe but only 32sqin access to the pipe (less actually because the bottom part of the pipe is right next to the floor) That may be why things got a little better when you made the insulation only 12 inches around instead of 20. Also raising it means the gas has access from the sides too, not just the top. Assuming 3/4 of the way around the exit pipe gas access, there would still need to be about 2.75 inches clearance from the barrel to the riser. The way you were doing it... you would need (assuming the hole in the brickwork was 8 inches wide though it sounds like it may be only 7.5 if it is the same as feed) at least a 6.5 inch gap. With your setup, you only had 2" and then 6" with the 12" riser, still not enough at 1/2" too small. If it was the same as your feed... it would be 1.5 inches too small. That is if what you have done and what I see in my head are the same thing. So far I've missed a few times on that....
So here is what I did...... I plugged the exit hole and tore apart the rear of the chamber, I put an elbow on the pipe shoved it under the barrel and then cobbed it to form a funnel affect,
That should get you much closer to your 50sqin. I will remember to pay very close attention to that interface!
Now on to the next RMH!
Too wet to play outside in the mud for me for now.... Glad you got it going.
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